[library Boost.TypeTraits
    [copyright 2000 2005 Adobe Systems Inc, David Abrahams, Steve Cleary, 
    Beman Dawes, Aleksey Gurtovoy, Howard Hinnant, Jesse Jones, Mat Marcus, 
    Itay Maman, John Maddock, Thorsten Ottosen, Robert Ramey and Jeremy Siek]
    [purpose Meta-programming support library]
    [license
        Distributed under the Boost Software License, Version 1.0.
        (See accompanying file LICENSE_1_0.txt or copy at
        <ulink url="http://www.boost.org/LICENSE_1_0.txt">
            http://www.boost.org/LICENSE_1_0.txt
        </ulink>)
    ]
    [authors [authors, various]]
    [category template]
    [category generic]
    [last-revision $Date: 2005/08/24 16:49:32 $]
]

[def __boost_root ../../../../]
[def __tof '''<replaceable><link linkend="boost_typetraits.integral_constant">true_type</link>-or-<link linkend="boost_typetraits.integral_constant">false_type</link></replaceable>''']
[def __below '''<replaceable>see-below</replaceable>''']
[def __true_type [link boost_typetraits.integral_constant true_type]]
[def __false_type [link boost_typetraits.integral_constant false_type]]
[def __integral_constant [link boost_typetraits.integral_constant integral_constant]]
[def __inherit [*Inherits:]]
[def __std_ref [*C++ Standard Reference:]]
[def __header [*Header:]]
[def __compat [*Compiler Compatibility:]]
[def __examples [*Examples:]]
[def __type [*type:]]
[def __transform_workaround [link transform.broken_compiler_workarounds_ compiler workarounds]]
[def __intrinsics [link boost_typetraits.intrinsics intrinsics]]

[def __is_void [link boost_typetraits.is_void is_void]]
[def __is_integral [link boost_typetraits.is_integral is_integral]]
[def __is_floating_point [link boost_typetraits.is_floating_point is_floating_point]]
[def __is_pointer [link boost_typetraits.is_pointer is_pointer]]
[def __is_reference [link boost_typetraits.is_reference is_reference]]
[def __is_member_pointer [link boost_typetraits.is_member_pointer is_member_pointer]]
[def __is_array [link boost_typetraits.is_array is_array]]
[def __is_union [link boost_typetraits.is_union is_union]]
[def __is_class [link boost_typetraits.is_class is_class]]
[def __is_enum [link boost_typetraits.is_enum is_enum]]
[def __is_enum [link boost_typetraits.is_enum is_enum]]
[def __is_function [link boost_typetraits.is_function is_function]]

[def __is_arithmetic [link boost_typetraits.is_arithmetic is_arithmetic]]
[def __is_fundamental [link boost_typetraits.is_fundamental is_fundamental]]
[def __is_object [link boost_typetraits.is_object is_object]]
[def __is_scalar [link boost_typetraits.is_scalar is_scalar]]
[def __is_compound [link boost_typetraits.is_compound is_compound]]
[def __is_member_function_pointer [link boost_typetraits.is_member_function_pointer is_member_function_pointer]]
[def __is_member_object_pointer [link boost_typetraits.is_member_object_pointer is_member_object_pointer]]

[def __alignment_of [link boost_typetraits.alignment_of alignment_of]]
[def __rank [link boost_typetraits.rank rank]]
[def __extent [link boost_typetraits.extent extent]]
[def __is_empty [link boost_typetraits.is_empty is_empty]]
[def __is_const [link boost_typetraits.is_const is_const]]
[def __is_volatile [link boost_typetraits.is_volatile is_volatile]]
[def __is_abstract [link boost_typetraits.is_abstract is_abstract]]
[def __is_polymorphic [link boost_typetraits.is_polymorphic is_polymorphic]]
[def __has_virtual_destructor [link boost_typetraits.has_virtual_destructor has_virtual_destructor]]
[def __is_pod [link boost_typetraits.is_pod is_pod]]
[def __has_trivial_constructor [link boost_typetraits.has_trivial_constructor has_trivial_constructor]]
[def __has_trivial_copy [link boost_typetraits.has_trivial_copy has_trivial_copy]]
[def __has_trivial_assign [link boost_typetraits.has_trivial_assign has_trivial_assign]]
[def __has_trivial_destructor [link boost_typetraits.has_trivial_destructor has_trivial_destructor]]
[def __is_stateless [link boost_typetraits.is_stateless is_stateless]]
[def __has_nothrow_constructor [link boost_typetraits.has_nothrow_constructor has_nothrow_constructor]]
[def __has_nothrow_copy [link boost_typetraits.has_nothrow_copy has_nothrow_copy]]
[def __has_nothrow_assign [link boost_typetraits.has_nothrow_assign has_nothrow_assign]]

[def __is_base_of [link boost_typetraits.is_base_of is_base_of]]
[def __is_convertible [link boost_typetraits.is_convertible is_convertible]]
[def __is_same [link boost_typetraits.is_same is_same]]

[def __remove_const [link boost_typetraits.remove_const remove_const]]
[def __remove_volatile [link boost_typetraits.remove_volatile remove_volatile]]
[def __remove_cv [link boost_typetraits.remove_cv remove_cv]]
[def __remove_reference [link boost_typetraits.remove_reference remove_reference]]
[def __remove_extent [link boost_typetraits.remove_extent remove_extent]]
[def __remove_all_extents [link boost_typetraits.remove_all_extents remove_all_extents]]
[def __remove_pointer [link boost_typetraits.remove_pointer remove_pointer]]
[def __add_reference [link boost_typetraits.add_reference add_reference]]
[def __add_pointer [link boost_typetraits.add_pointer add_pointer]]
[def __add_const [link boost_typetraits.add_const add_const]]
[def __add_volatile [link boost_typetraits.add_volatile add_volatile]]
[def __add_cv [link boost_typetraits.add_cv add_cv]]

[def __type_with_alignment [link boost_typetraits.type_with_alignment type_with_alignment]]
[def __aligned_storage [link boost_typetraits.aligned_storage aligned_storage]]

[def __function_traits [link boost_typetraits.function_traits function_traits]]

[section:intro Introduction]

The Boost type-traits library contains a 
set of very specific traits classes, each of which encapsulate a single trait 
from the C++ type system; for example, is a type a pointer or a reference type? 
Or does a type have a trivial constructor, or a const-qualifier?
 
The type-traits classes share a unified design: each class inherits from a 
the type __true_type if the type has the specified property and inherits from
__false_type otherwise.

The type-traits library also contains a set of classes that perform a 
specific transformation on a type; for example, they can remove a 
top-level const or volatile qualifier from a type. Each class that 
performs a transformation defines a single typedef-member `type` 
that is the result of the transformation.

[endsect]

[section:background Background and Tutorial]

The following is an updated version of the article "C++ Type traits"
by John Maddock and Steve Cleary that appeared in the October 2000 
issue of [@http://www.ddj.com Dr Dobb's Journal].

Generic programming (writing code which works with any data type meeting a 
set of requirements) has become the method of choice for providing reusable code. 
However, there are times in generic programming when "generic" just isn't 
good enough - sometimes the differences between types are too large for an 
efficient generic implementation. This is when the traits technique 
becomes important - by encapsulating those properties that need to be 
considered on a type by type basis inside a traits class, we can 
minimize the amount of code that has to differ from one type to another, 
and maximize the amount of generic code.

Consider an example: when working with character strings, one common operation is 
to determine the length of a null terminated string. Clearly it's possible to 
write generic code that can do this, but it turns out that there are much more 
efficient methods available: for example, the C library functions `strlen` and 
`wcslen` are usually written in assembler, and with suitable hardware support 
can be considerably faster than a generic version written in C++. 
The authors of the C++ standard library realized this, and abstracted the 
properties of `char` and `wchar_t` into the class `char_traits`. Generic code 
that works with character strings can simply use `char_traits<>::length` to 
determine the length of a null terminated string, safe in the knowledge 
that specializations of `char_traits` will use the most appropriate method 
available to them.

[h4 Type Traits]

Class `char_traits` is a classic example of a collection of type specific 
properties wrapped up in a single class - what Nathan Myers termed a 
/baggage class/[link background.references \[1\]]. In the Boost type-traits library, we[link background.references \[2\]] have written a 
set of very specific traits classes, each of which encapsulate a single trait 
from the C++ type system; for example, is a type a pointer or a reference type? 
Or does a type have a trivial constructor, or a const-qualifier? 
The type-traits classes share a unified design: each class inherits from a 
the type __true_type if the type has the specified property and inherits from
__false_type otherwise. As we will show, these classes can be used in 
generic programming to determine the properties of a given type and introduce 
optimizations that are appropriate for that case.

The type-traits library also contains a set of classes that perform a 
specific transformation on a type; for example, they can remove a 
top-level const or volatile qualifier from a type. Each class that 
performs a transformation defines a single typedef-member `type` 
that is the result of the transformation. All of the type-traits 
classes are defined inside namespace `boost`; for brevity, namespace-qualification 
is omitted in most of the code samples given.

[h4 Implementation]

There are far too many separate classes contained in the type-traits library 
to give a full implementation here - see the source code in the Boost library 
for the full details - however, most of the implementation is fairly repetitive 
anyway, so here we will just give you a flavor for how some of the classes are 
implemented. Beginning with possibly the simplest class in the library, 
`is_void<T>` inherits from `__true_type` only if `T` is `void`.

   template <typename T> 
   struct __is_void : public __false_type{};

   template <> 
   struct __is_void<void> : public __true_type{};

Here we define a primary version of the template class `__is_void`, and 
provide a full-specialization when `T` is `void`. While full specialization 
of a template class is an important technique, sometimes we need a 
solution that is halfway between a fully generic solution, and a full 
specialization. This is exactly the situation for which the standards committee 
defined partial template-class specialization. As an example, consider the 
class `boost::is_pointer<T>`: here we needed a primary version that handles 
all the cases where T is not a pointer, and a partial specialization to 
handle all the cases where T is a pointer:

   template <typename T> 
   struct __is_pointer : public __false_type{};

   template <typename T> 
   struct __is_pointer<T*> : public __true_type{};

The syntax for partial specialization is somewhat arcane and could easily 
occupy an article in its own right; like full specialization, in order to 
write a partial specialization for a class, you must first declare the 
primary template. The partial specialization contains an extra <...> after the 
class name that contains the partial specialization parameters; these define 
the types that will bind to that partial specialization rather than the 
default template. The rules for what can appear in a partial specialization 
are somewhat convoluted, but as a rule of thumb if you can legally write two 
function overloads of the form:

   void foo(T);
   void foo(U);

Then you can also write a partial specialization of the form:

   template <typename T>
   class c{ /*details*/ };

   template <typename T>
   class c<U>{ /*details*/ };

This rule is by no means foolproof, but it is reasonably simple to remember 
and close enough to the actual rule to be useful for everyday use.

As a more complex example of partial specialization consider the class 
`remove_extent<T>`. This class defines a single typedef-member `type` that 
is the same type as T but with any top-level array bounds removed; 
this is an example of a traits class that performs a transformation on a type:

   template <typename T> 
   struct __remove_extent
   { typedef T type; };

   template <typename T, std::size_t N> 
   struct __remove_extent<T[N]>
   { typedef T type; };

The aim of `__remove_extent` is this: imagine a generic algorithm that is 
passed an array type as a template parameter, `__remove_extent` provides a 
means of determining the underlying type of the array. For example 
`remove_extent<int[4][5]>::type` would evaluate to the type `int[5]`. 
This example also shows that the number of template parameters in a 
partial specialization does not have to match the number in the 
default template. However, the number of parameters that appear after the 
class name do have to match the number and type of the parameters in the 
default template.

[h4 Optimized copy]

As an example of how the type traits classes can be used, consider the 
standard library algorithm copy:

   template<typename Iter1, typename Iter2>
   Iter2 copy(Iter1 first, Iter1 last, Iter2 out);

Obviously, there's no problem writing a generic version of copy that works 
for all iterator types `Iter1` and `Iter2`; however, there are some 
circumstances when the copy operation can best be performed by a call to 
`memcpy`. In order to implement copy in terms of `memcpy` all of the 
following conditions need to be met:

* Both of the iterator types `Iter1` and `Iter2` must be pointers. 
* Both `Iter1` and `Iter2` must point to the same type - excluding const and 
volatile-qualifiers. 
* The type pointed to by `Iter1` must have a trivial assignment operator. 

By trivial assignment operator we mean that the type is either a scalar type[link background.references \[3\]] or:

* The type has no user defined assignment operator. 
* The type does not have any data members that are references. 
* All base classes, and all data member objects must have trivial assignment operators. 

If all these conditions are met then a type can be copied using `memcpy` 
rather than using a compiler generated assignment operator. The type-traits 
library provides a class `__has_trivial_assign`, such that 
`has_trivial_assign<T>::value` is true only if T has a trivial assignment operator. 
This class "just works" for scalar types, but has to be explicitly 
specialised for class/struct types that also happen to have a trivial assignment 
operator. In other words if __has_trivial_assign gives the wrong answer, 
it will give the "safe" wrong answer - that trivial assignment is not allowable.

The code for an optimized version of copy that uses `memcpy` where appropriate is 
given in [link boost_typetraits.copy the examples]. The code begins by defining a template 
function `do_copy` that performs a "slow but safe" copy.  The last parameter passed
to this function may be either a `__true_type` or a `__false_type`. Following that 
there is an overload of do_copy that uses `memcpy`: this time the iterators are required
to actually be pointers to the same type, and the final parameter must be a 
`__true_type`.  Finally, the version of `copy` calls `do_copy`, passing 
`__has_trivial_assign<value_type>()` as the final parameter: this will dispatch
to the optimized version where appropriate, otherwise it will call the 
"slow but safe version".

[h4 Was it worth it?]

It has often been repeated in these columns that "premature optimization is the 
root of all evil" [link background.references \[4\]]. So the question must be asked: was our optimization 
premature? To put this in perspective the timings for our version of copy 
compared a conventional generic copy[link background.references \[5\]] are shown in table 1.

Clearly the optimization makes a difference in this case; but, to be fair, 
the timings are loaded to exclude cache miss effects - without this 
accurate comparison between algorithms becomes difficult. However, perhaps 
we can add a couple of caveats to the premature optimization rule:

*If you use the right algorithm for the job in the first place then optimization 
will not be required; in some cases, memcpy is the right algorithm. 
*If a component is going to be reused in many places by many people then 
optimizations may well be worthwhile where they would not be so for a single 
case - in other words, the likelihood that the optimization will be 
absolutely necessary somewhere, sometime is that much higher. 
Just as importantly the perceived value of the stock implementation will be 
higher: there is no point standardizing an algorithm if users reject it on 
the grounds that there are better, more heavily optimized versions available. 

[table Time taken to copy 1000 elements using `copy<const T*, T*>` (times in micro-seconds)

[[Version] [T] [Time]]
[["Optimized" copy] [char] [0.99]]
[[Conventional copy] [char] [8.07]]
[["Optimized" copy] [int] [2.52]]
[[Conventional copy] [int] [8.02]]
]

[h4 Pair of References]
The optimized copy example shows how type traits may be used to perform 
optimization decisions at compile-time. Another important usage of type traits 
is to allow code to compile that otherwise would not do so unless excessive 
partial specialization is used. This is possible by delegating partial 
specialization to the type traits classes. Our example for this form of 
usage is a pair that can hold references [link background.references \[6\]].

First, let us examine the definition of `std::pair`, omitting the 
comparison operators, default constructor, and template copy constructor for 
simplicity:

   template <typename T1, typename T2> 
   struct pair 
   {
   typedef T1 first_type;
   typedef T2 second_type;

   T1 first;
   T2 second;

   pair(const T1 & nfirst, const T2 & nsecond)
   :first(nfirst), second(nsecond) { }
   };

Now, this "pair" cannot hold references as it currently stands, because the 
constructor would require taking a reference to a reference, which is 
currently illegal [link background.references \[7\]]. Let us consider what the constructor's parameters 
would have to be in order to allow "pair" to hold non-reference types, 
references, and constant references:

[table Required Constructor Argument Types
[[Type of `T1`] [Type of parameter to initializing constructor]]
[[T] [const T &]]
[[T &] [T &]]
[[const T &] [const T &]]
]

A little familiarity with the type traits classes allows us to construct a 
single mapping that allows us to determine the type of parameter from the 
type of the contained class. The type traits classes provide a 
transformation __add_reference, which adds a reference to its type, 
unless it is already a reference.

[table Using add_reference to synthesize the correct constructor type
[[Type of `T1`] [Type of `const T1`] [Type of `add_reference<const T1>::type`]]
[[T] [const T] [const T &]]
[[T &] [T & \[8\]] [T &]]
[[const T &] [const T &] [const T &]]
]
 
This allows us to build a primary template definition for `pair` that can 
contain non-reference types, reference types, and constant reference types:

   template <typename T1, typename T2> 
   struct pair 
   {
   typedef T1 first_type;
   typedef T2 second_type;

   T1 first;
   T2 second;

   pair(boost::__add_reference<const T1>::type nfirst,
         boost::__add_reference<const T2>::type nsecond)
   :first(nfirst), second(nsecond) { }
   };

Add back in the standard comparison operators, default constructor, 
and template copy constructor (which are all the same), and you have a 
`std::pair` that can hold reference types!

This same extension could have been done using partial template specialization 
of `pair`, but to specialize `pair` in this way would require three partial 
specializations, plus the primary template. Type traits allows us to 
define a single primary template that adjusts itself auto-magically to 
any of these partial specializations, instead of a brute-force partial 
specialization approach. Using type traits in this fashion allows 
programmers to delegate partial specialization to the type traits classes, 
resulting in code that is easier to maintain and easier to understand.

[h4 Conclusion]

We hope that in this article we have been able to give you some idea of 
what type-traits are all about. A more complete listing of the available 
classes are in the boost documentation, along with further examples using 
type traits. Templates have enabled C++ uses to take the advantage of the 
code reuse that generic programming brings; hopefully this article has 
shown that generic programming does not have to sink to the lowest common 
denominator, and that templates can be optimal as well as generic.

[h4 Acknowledgements]

The authors would like to thank Beman Dawes and Howard Hinnant for their 
helpful comments when preparing this article.

[h4 References]

# Nathan C. Myers, C++ Report, June 1995. 
# The type traits library is based upon contributions by Steve Cleary, Beman Dawes, Howard Hinnant and John Maddock: it can be found at www.boost.org. 
# A scalar type is an arithmetic type (i.e. a built-in integer or floating point type), an enumeration type, a pointer, a pointer to member, or a const- or volatile-qualified version of one of these types. 
# This quote is from Donald Knuth, ACM Computing Surveys, December 1974, pg 268. 
# The test code is available as part of the boost utility library (see algo_opt_examples.cpp), the code was compiled with gcc 2.95 with all optimisations turned on, tests were conducted on a 400MHz Pentium II machine running Microsoft Windows 98. 
# John Maddock and Howard Hinnant have submitted a "compressed_pair" library to Boost, which uses a technique similar to the one described here to hold references. Their pair also uses type traits to determine if any of the types are empty, and will derive instead of contain to conserve space -- hence the name "compressed". 
# This is actually an issue with the C++ Core Language Working Group (issue #106), submitted by Bjarne Stroustrup. The tentative resolution is to allow a "reference to a reference to T" to mean the same thing as a "reference to T", but only in template instantiation, in a method similar to multiple cv-qualifiers. 
# For those of you who are wondering why this shouldn't be const-qualified, remember that references are always implicitly constant (for example, you can't re-assign a reference). Remember also that "const T &" is something completely different. For this reason, cv-qualifiers on template type arguments that are references are ignored.

[endsect]

[section:category Type Traits by Category]
[section:value_traits Type Traits that Describe the Properties of a Type]

These traits are all /value traits/, which is to say the traits classes all
inherit from __integral_constant, and are used to access some numerical
property of a type.  Often this is a simple true or false Boolean value, 
but in a few cases may be some other integer value (for example when dealing
with type alignments, or array bounds: see `__alignment_of`, `__rank` and `__extent`).

[section:primary Categorizing a Type]

These traits identify what "kind" of type some type `T` is.  These are split into
two groups: primary traits which are all mutually exclusive, and composite traits
that are compositions of one or more primary traits.

For any given type, exactly one primary type trait will inherit from
__true_type, and all the others will inherit from __false_type, in other
words these traits are mutually exclusive.
 
This means that `__is_integral<T>::value` and `__is_floating_point<T>::value` 
will only ever be true for built-in types; if you want to check for a 
user-defined class type that behaves "as if" it is an integral or floating point type, 
then use the `std::numeric_limits template` instead.

[*Synopsis:]

   template <class T>
   struct __is_array<T>;
     
   template <class T>
   struct __is_class<T>;
     
   template <class T>
   struct __is_enum<T>;
     
   template <class T>
   struct __is_floating_point<T>;
     
   template <class T>
   struct __is_function<T>;

   template <class T>
   struct __is_integral<T>;
     
   template <class T>
   struct __is_member_function_pointer<T>;
     
   template <class T>
   struct __is_member_object_pointer<T>;
     
   template <class T>
   struct __is_pointer<T>;
     
   template <class T>
   struct __is_reference<T>;
     
   template <class T>
   struct __is_union<T>;
     
   template <class T>
   struct __is_void<T>;
     
The following traits are made up of the union of one or more type
categorizations. A type may belong to more than one of these categories, 
in addition to one of the primary categories.

   template <class T>
   struct __is_arithmetic;
   
   template <class T>
   struct __is_compound;
   
   template <class T>
   struct __is_fundamental;
   
   template <class T>
   struct __is_member_pointer;
   
   template <class T>
   struct __is_object;
   
   template <class T>
   struct __is_scalar;
   
[endsect]

[section:properties General Type Properties]

The following templates describe the general properties of a type.

[*Synopsis:]

   template <class T>
   struct __alignment_of;

   template <class T>
   struct __has_nothrow_assign;

   template <class T>
   struct __has_nothrow_constructor;

   template <class T>
   struct __has_nothrow_copy;

   template <class T>
   struct __has_trivial_assign;

   template <class T>
   struct __has_trivial_constructor;

   template <class T>
   struct __has_trivial_copy;

   template <class T>
   struct __has_trivial_destructor;

   template <class T>
   struct __has_virtual_destructor;

   template <class T>
   struct __is_abstract;

   template <class T>
   struct __is_const;

   template <class T>
   struct __is_empty;

   template <class T>
   struct __is_stateless;

   template <class T>
   struct __is_pod;

   template <class T>
   struct __is_polymorphic;

   template <class T>
   struct __is_volatile;

   template <class T, std::size_t N = 0>
   struct __extent;

   template <class T>
   struct __rank;

[endsect]

[section:relate Relationships Between Two Types]

These templates determine the whether there is a relationship 
between two types:

[*Synopsis:]

   template <class Base, class Derived>
   struct __is_base_of;

   template <class From, class To>
   struct __is_convertible;

   template <class T, class U>
   struct __is_same;
   
[endsect]

[endsect]
[section:transform Type Traits that Transform One Type to Another]

The following templates transform one type to another, 
based upon some well-defined rule. 
Each template has a single member called `type` that is the 
result of applying the transformation to the template argument `T`.

[*Synopsis:]

   template <class T>
   struct __add_const;

   template <class T>
   struct __add_cv;
   
   template <class T>
   struct __add_pointer;

   template <class T>
   struct __add_reference;

   template <class T>
   struct __add_volatile;

   template <class T>
   struct __remove_all_extents;

   template <class T>
   struct __remove_const;

   template <class T>
   struct __remove_cv;

   template <class T>
   struct __remove_extent;

   template <class T>
   struct __remove_pointer;

   template <class T>
   struct __remove_reference;

   template <class T>
   struct __remove_volatile;

[h4 Broken Compiler Workarounds:]

For all of these templates support for partial specialization of class templates is 
required to correctly implement the transformation. 
On the other hand, practice shows that many of the templates from this 
category are very useful, and often essential for implementing some 
generic libraries. Lack of these templates is often one of the major 
limiting factors in porting those libraries to compilers that do not yet 
support this language feature. As some of these compilers are going to be 
around for a while, and at least one of them is very wide-spread, 
it was decided that the library should provide workarounds where possible. 

The basic idea behind the workaround is to manually define full 
specializations of all type transformation templates for all fundamental types, 
and all their 1st and 2nd rank cv-[un]qualified derivative pointer types, and to 
provide a user-level macro that will define all the explicit specializations needed 
for any user-defined type T. 

The first part guarantees the successful compilation of something like this:

   BOOST_STATIC_ASSERT((is_same<char, remove_reference<char&>::type>::value));
   BOOST_STATIC_ASSERT((is_same<char const, remove_reference<char const&>::type>::value));
   BOOST_STATIC_ASSERT((is_same<char volatile, remove_reference<char volatile&>::type>::value));
   BOOST_STATIC_ASSERT((is_same<char const volatile, remove_reference<char const volatile&>::type>::value));
   BOOST_STATIC_ASSERT((is_same<char*, remove_reference<char*&>::type>::value));
   BOOST_STATIC_ASSERT((is_same<char const*, remove_reference<char const*&>::type>::value));
   ...
   BOOST_STATIC_ASSERT((is_same<char const volatile* const volatile* const volatile, remove_reference<char const volatile* const volatile* const volatile&>::type>::value));
   
and the second part provides the library's users with a mechanism to make the 
above code work not only for `char`, `int` or other built-in type, 
but for they own types as well:

   namespace myspace{
      struct MyClass {};
   }
   // declare this at global scope:
   BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION(myspace::MyClass)
   // transformations on myspace::MyClass now work:
   BOOST_STATIC_ASSERT((is_same<myspace::MyClass, remove_reference<myspace::MyClass&>::type>::value));
   BOOST_STATIC_ASSERT((is_same<myspace::MyClass, remove_const<myspace::MyClass const>::type>::value));
   // etc.
   
Note that the macro BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION evaluates 
to nothing on those compilers that *do* support partial specialization.

[endsect]

[section:alignment Synthesizing Types with Specific Alignments]

Some low level memory management routines need to synthesize a POD type with
specific alignment properties.  The template `__type_with_alignment` finds the smallest
type with a specified alignment, while template `__aligned_storage` creates a type 
with a specific size and alignment.

[*Synopsis]

   template <std::size_t Align>
   struct __type_with_alignment;
   
   template <std::size_t Size, std::size_t Align>
   struct __aligned_storage;

[endsect]

[section:function Decomposing Function Types]

The class template __function_traits extracts information from function types
(see also __is_function).  This traits class allows you to tell how many arguments
a function takes, what those argument types are, and what the return type is.

[*Synopsis]

   template <std::size_t Align>
   struct __function_traits;

[endsect]

[endsect]

[section:user_defined User Defined Specializations]

Occationally the end user may need to provide their own specialization 
for one of the type traits - typically where intrinsic compiler support 
is required to implement a specific trait fully.  
These specializations should derive from boost::__true_type or boost::__false_type 
as appropriate:

   #include <boost/type_traits/is_pod.hpp>
   #include <boost/type_traits/is_class.hpp>
   #include <boost/type_traits/is_union.hpp>

   struct my_pod{};
   struct my_union
   {
      char c;
      int i;
   };

   namespace boost
   {
      template<>
      struct __is_pod<my_pod> : public __true_type{};
         
      template<>
      struct __is_pod<my_union> : public __true_type{};
      
      template<>
      struct __is_union<my_union> : public __true_type{};
      
      template<>
      struct __is_class<my_union> : public __false_type{};
   }

[endsect]

[section:intrinsics Support for Compiler Intrinsics]

There are some traits that can not be implemented within the current C++ language:
to make these traits "just work" with user defined types, some kind of additional
help from the compiler is required.  Currently (May 2005) MWCW 9 and Visual C++ 8
provide the necessary intrinsics, and other compilers will no doubt follow in due 
course.  

The Following traits classes always need compiler support to do the right thing 
for all types 
(but all have safe fallback positions if this support is unavailable):

* __is_union
* __is_pod
* __has_trivial_constructor
* __has_trivial_copy
* __has_trivial_assign
* __has_trivial_destructor
* __has_nothrow_constructor
* __has_nothrow_copy
* __has_nothrow_assign
* __has_virtual_destructor

The following traits classes can't be portably implemented in the C++ language, 
although in practice, the implementations do in fact do the right thing on all
the compilers we know about:

* __is_empty
* __is_polymorphic

The following traits classes are dependent on one or more of the above:

* __is_class
* __is_stateless

The hooks for compiler-intrinsic support are defined in 
[@../../boost/type_traits/intrinsics.hpp boost/type_traits/intrinsics.hpp], adding support for new compilers is simply
a matter of defining one of more of the following macros:

[table Macros for Compiler Intrinsics
   [[BOOST_IS_UNION(T)][Should evaluate to true if T is a union type]]
   [[BOOST_IS_POD(T)][Should evaluate to true if T is a POD type]]
   [[BOOST_IS_EMPTY(T)][Should evaluate to true if T is an empty struct or union]]
   [[BOOST_HAS_TRIVIAL_CONSTRUCTOR(T)][Should evaluate to true if the default constructor for T is trivial (i.e. has no effect)]]
   [[BOOST_HAS_TRIVIAL_COPY(T)][Should evaluate to true if T has a trivial copy constructor (and can therefore be replaced by a call to memcpy)]]
   [[BOOST_HAS_TRIVIAL_ASSIGN(T)][Should evaluate to true if T has a trivial assignment operator (and can therefore be replaced by a call to memcpy)]]
   [[BOOST_HAS_TRIVIAL_DESTRUCTOR(T)][Should evaluate to true if T has a trivial destructor (i.e. ~T() has no effect)]]
   [[BOOST_HAS_NOTHROW_CONSTRUCTOR(T)][Should evaluate to true if `T x;` can not throw]]
   [[BOOST_HAS_NOTHROW_COPY(T)][Should evaluate to true if `T(t)` can not throw]]
   [[BOOST_HAS_NOTHROW_ASSIGN(T)][Should evaluate to true if `T t, u; t = u` can not throw]]
   [[BOOST_HAS_VIRTUAL_DESTRUCTOR(T)][Should evaluate to true T has a virtual destructor]]
]

[endsect]

[section:mpl MPL Interoperability]

All the value based traits in this library conform to MPL's requirements
for an [@../../libs/mpl/doc/refmanual/integral-constant.html Integral Constant type]: that includes a number of rather intrusive
workarounds for broken compilers.  

Purely as an implementation detail, this
means that `__true_type` inherits from [@../../libs/mpl/doc/refmanual/bool.html `boost::mpl::true_`], `__false_type` inherits
from [@../../libs/mpl/doc/refmanual/bool.html `boost::mpl::false_`], and `__integral_constant<T, v>` inherits from
[@../../libs/mpl/doc/refmanual/integral-c.html `boost::mpl::integral_c<T,v>`] (provided `T` is not `bool`)

[endsect]

[section:examples Examples]

[section:copy An Optimized Version of std::copy]

Demonstrates a version of `std::copy` that uses `__has_trivial_assign` to
determine whether to use `memcpy` to optimise the copy operation 
(see [@../../libs/type_traits/examples/copy_example.cpp copy_example.cpp]):

   //
   // opt::copy
   // same semantics as std::copy
   // calls memcpy where appropriate.
   //

   namespace detail{

   template<typename I1, typename I2, bool b>
   I2 copy_imp(I1 first, I1 last, I2 out, const boost::__integral_constant<bool, b>&)
   {
      while(first != last)
      {
         *out = *first;
         ++out;
         ++first;
      }
      return out;
   }

   template<typename T>
   T* copy_imp(const T* first, const T* last, T* out, const boost::__true_type&)
   {
      memcpy(out, first, (last-first)*sizeof(T));
      return out+(last-first);
   }


   }

   template<typename I1, typename I2>
   inline I2 copy(I1 first, I1 last, I2 out)
   {
      //
      // We can copy with memcpy if T has a trivial assignment operator,
      // and if the iterator arguments are actually pointers (this last
      // requirement we detect with overload resolution):
      //
      typedef typename std::iterator_traits<I1>::value_type value_type;
      return detail::copy_imp(first, last, out, boost::__has_trivial_assign<value_type>());
   }


[endsect]

[section:fill An Optimised Version of std::fill]

Demonstrates a version of `std::fill` that uses `__has_trivial_assign` to
determine whether to use `memset` to optimise the fill operation 
(see [@../../libs/type_traits/examples/fill_example.cpp fill_example.cpp]):

   //
   // fill
   // same as std::fill, but uses memset where appropriate
   //
   namespace detail{

   template <typename I, typename T, bool b>
   void do_fill(I first, I last, const T& val, const boost::__integral_constant<bool, b>&)
   {
      while(first != last)
      {
         *first = val;
         ++first;
      }
   }

   template <typename T>
   void do_fill(T* first, T* last, const T& val, const boost::__true_type&)
   {
      std::memset(first, val, last-first);
   }

   }

   template <class I, class T>
   inline void fill(I first, I last, const T& val)
   {
      //
      // We can do an optimised fill if T has a trivial assignment 
      // operator and if it's size is one:
      //
      typedef boost::__integral_constant<bool, 
         ::boost::__has_trivial_assign<T>::value && (sizeof(T) == 1)> truth_type;
      detail::do_fill(first, last, val, truth_type());
   }


[endsect]

[section:destruct An Example that Omits Destructor Calls For Types with Trivial Destructors]

Demonstrates a simple algorithm that uses `__has_trivial_destruct` to
determine whether to destructors need to be called 
(see [@../../libs/type_traits/examples/trivial_destructor_example.cpp trivial_destructor_example.cpp]):

   //
   // algorithm destroy_array:
   // The reverse of std::unitialized_copy, takes a block of
   // initialized memory and calls destructors on all objects therein.
   //

   namespace detail{

   template <class T>
   void do_destroy_array(T* first, T* last, const boost::__false_type&)
   {
      while(first != last)
      {
         first->~T();
         ++first;
      }
   }

   template <class T>
   inline void do_destroy_array(T* first, T* last, const boost::__true_type&)
   {
   }

   } // namespace detail

   template <class T>
   inline void destroy_array(T* p1, T* p2)
   {
      detail::do_destroy_array(p1, p2, ::boost::__has_trivial_destructor<T>());
   }


[endsect]

[section:iter An improved Version of std::iter_swap]

Demonstrates a version of `std::iter_swap` that use type traits to
determine whether an it's arguments are proxying iterators or not,
if they're not then it just does a `std::swap` of it's dereferenced 
arguments (the
same as `std::iter_swap` does), however if they are proxying iterators
then takes special care over the swap to ensure that the algorithm
works correctly for both proxying iterators, and even iterators of
different types 
(see [@../../libs/type_traits/examples/iter_swap_example.cpp iter_swap_example.cpp]):

   //
   // iter_swap:
   // tests whether iterator is a proxying iterator or not, and
   // uses optimal form accordingly:
   //
   namespace detail{

   template <typename I>
   static void do_swap(I one, I two, const boost::__false_type&)
   {
      typedef typename std::iterator_traits<I>::value_type v_t;
      v_t v = *one;
      *one = *two;
      *two = v;
   }
   template <typename I>
   static void do_swap(I one, I two, const boost::__true_type&)
   {
      using std::swap;
      swap(*one, *two);
   }

   }

   template <typename I1, typename I2>
   inline void iter_swap(I1 one, I2 two)
   {
      //
      // See is both arguments are non-proxying iterators, 
      // and if both iterator the same type:
      //
      typedef typename std::iterator_traits<I1>::reference r1_t;
      typedef typename std::iterator_traits<I2>::reference r2_t;

      typedef boost::__integral_constant<bool,
         ::boost::__is_reference<r1_t>::value
         && ::boost::__is_reference<r2_t>::value
         && ::boost::__is_same<r1_t, r2_t>::value> truth_type;

      detail::do_swap(one, two, truth_type());
   }


[endsect]

[endsect]

[section:reference Alphabetical Reference]

[section:add_const add_const]

   template <class T>
   struct add_const
   {
      typedef __below type;
   };
  
__type The same type as `T const` for all `T`.  

__std_ref 3.9.3.

__compat If the compiler does not support partial specialization of class-templates
then this template will compile, but the member `type` will always be the same as 
type `T` except where __transform_workaround have been applied.

__header ` #include <boost/type_traits/add_const.hpp>` or ` #include <boost/type_traits.hpp>`

[table Examples

[ [Expression] [Result Type]]

[[`add_const<int>::type`][`int const`]]

[[`add_const<int&>::type`] [`int&`]]

[[`add_const<int*>::type`] [`int* const`]]

[[`add_const<int const>::type`] [`int const`]]

]

[endsect]

[section:add_cv add_cv]

   template <class T>
   struct add_cv
   {
      typedef __below type;
   };
  
__type The same type as `T const volatile` for all `T`.  

__std_ref 3.9.3.

__compat If the compiler does not support partial specialization of class-templates
then this template will compile, but the member `type` will always be the same as 
type `T` except where __transform_workaround have been applied.

__header ` #include <boost/type_traits/add_cv.hpp>` or ` #include <boost/type_traits.hpp>`

[table Examples

[ [Expression] [Result Type]]

[[`add_cv<int>::type`][`int const volatile`]]

[[`add_cv<int&>::type`] [`int&`]]

[[`add_cv<int*>::type`] [`int* const volatile`]]

[[`add_cv<int const>::type`] [`int const volatile`]]

]

[endsect]

[section:add_pointer add_pointer]

   template <class T>
   struct add_pointer
   {
      typedef __below type;
   };
  
__type The same type as `remove_reference<T>::type*`.  

The rationale for this template
is that it produces the same type as `TYPEOF(&t)`, 
where `t` is an object of type `T`.

__std_ref 8.3.1.

__compat If the compiler does not support partial specialization of class-templates
then this template will compile, but the member `type` will always be the same as 
type `T` except where __transform_workaround have been applied.

__header ` #include <boost/type_traits/add_pointer.hpp>` or ` #include <boost/type_traits.hpp>`

[table Examples

[ [Expression] [Result Type]]

[[`add_pointer<int>::type`][`int*`]]

[[`add_pointer<int const&>::type`] [`int const*`]]

[[`add_pointer<int*>::type`] [`int**`]]

[[`add_pointer<int*&>::type`] [`int**`]]

]

[endsect]

[section:add_reference add_reference]

   template <class T>
   struct add_reference
   {
      typedef __below type;
   };
  
__type If `T` is not a reference type then `T&`, otherwise `T`.

__std_ref 8.3.2.

__compat If the compiler does not support partial specialization of class-templates
then this template will compile, but the member `type` will always be the same as 
type `T` except where __transform_workaround have been applied.

__header ` #include <boost/type_traits/add_reference.hpp>` or ` #include <boost/type_traits.hpp>`

[table Examples

[ [Expression] [Result Type]]

[[`add_reference<int>::type`][`int&`]]

[[`add_reference<int const&>::type`] [`int const&`]]

[[`add_reference<int*>::type`] [`int*&`]]

[[`add_reference<int*&>::type`] [`int*&`]]

]

[endsect]

[section:add_volatile add_volatile]

   template <class T>
   struct add_volatile
   {
      typedef __below type;
   };
  
__type The same type as `T volatile` for all `T`.  

__std_ref 3.9.3.

__compat If the compiler does not support partial specialization of class-templates
then this template will compile, but the member `type` will always be the same as 
type `T` except where __transform_workaround have been applied.

__header ` #include <boost/type_traits/add_volatile.hpp>` or ` #include <boost/type_traits.hpp>`

[table Examples

[ [Expression] [Result Type]]

[[`add_volatile<int>::type`][`int volatile`]]

[[`add_volatile<int&>::type`] [`int&`]]

[[`add_volatile<int*>::type`] [`int* volatile`]]

[[`add_volatile<int const>::type`] [`int const volatile`]]

]

[endsect]

[section:aligned_storage aligned_storage]

   template <std::size_t Size, std::size_t Align>
   struct aligned_storage
   {
      typedef __below type;
   };
  
__type a built-in or POD type with size `Size` and an alignment 
that is a multiple of `Align`.  

__header ` #include <boost/type_traits/aligned_storage.hpp>` or ` #include <boost/type_traits.hpp>`

[endsect]

[section:alignment_of alignment_of]
   template <class T>
   struct alignment_of : public __integral_constant<std::size_t, ALIGNOF(T)> {};
  
__inherit Class template alignment_of inherits from 
`__integral_constant<std::size_t, ALIGNOF(T)>`, where `ALIGNOF(T)` is the
alignment of type T.  

['Note: strictly speaking you should only rely on 
the value of `ALIGNOF(T)` being a multiple of the true alignment of T, although
in practice it does compute the correct value in all the cases we know about.]

__header ` #include <boost/type_traits/alignment_of.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`alignment_of<int>` inherits from `__integral_constant<std::size_t, ALIGNOF(int)>`.]

[:`alignment_of<char>::type` is the type `__integral_constant<std::size_t, ALIGNOF(char)>`.]

[:`alignment_of<double>::value` is an integral constant 
expression with value `ALIGNOF(double)`.]

[:`alignment_of<T>::value_type` is the type `std::size_t`.]

[endsect]

[section:extent extent]
   template <class T, std::size_t N = 0>
   struct extent : public __integral_constant<std::size_t, EXTENT(T,N)> {};
  
__inherit Class template extent inherits from `__integral_constant<std::size_t, EXTENT(T,N)>`, 
where `EXTENT(T,N)` is the number of elements in the N'th array dimention of type `T`.

If `T` is not an array type, or if `N > __rank<T>::value`, or if the N'th array bound
is incomplete, then `EXTENT(T,N)` is zero.

__header ` #include <boost/type_traits/extent.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`extent<int[1]>` inherits from `__integral_constant<std::size_t, 0>`.]

[:`extent<double[2][3][4], 1>::type` is the type `__integral_constant<std::size_t, 3>`.]

[:`extent<int[4]>::value` is an integral constant 
expression that evaluates to /4/.]

[:`extent<int[][2]>::value` is an integral constant 
expression that evaluates to /0/.]

[:`extent<int[][2], 1>::value` is an integral constant 
expression that evaluates to /2/.]

[:`extent<int*>::value` is an integral constant 
expression that evaluates to /0/.]

[:`extent<T>::value_type` is the type `std::size_t`.]

[endsect]

[section:function_traits function_traits]
[def __argN '''arg<replaceable>N</replaceable>_type''']

   template <class T>
   struct function_traits
   {
      static const std::size_t    arity = __below;
      typedef __below           result_type;
      typedef __below           __argN; 
   };

The class template function_traits will only compile if:

* The compiler supports partial specialization of class templates.
* The template argument `T` is a /function type/, note that this ['[*is not]] 
the same thing as a /pointer to a function/.

[table Function Traits Members 
[[Member] [Description]] 
[[`function_traits<T>::arity`] 
   [An integral constant expression that gives the number of arguments accepted by the function type `F`.]]
[[`function_traits<T>::result_type`] 
   [The type returned by function type `F`.]]
[[`function_traits<T>::__argN`] 
   [The '''<replaceable>N</replaceable>th''' argument type of function type `F`, where `1 <= N <= arity` of `F`.]]
]

[table Examples
[[Expression] [Result]] 
[[`function_traits<void (void)>::arity`] [An integral constant expression that has the value 0.]]
[[`function_traits<long (int)>::arity`] [An integral constant expression that has the value 1.]]
[[`function_traits<long (int, long, double, void*)>::arity`] [An integral constant expression that has the value 4.]]
[[`function_traits<void (void)>::result_type`] [The type `void`.]]
[[`function_traits<long (int)>::result_type`] [The type `long`.]]
[[`function_traits<long (int)>::arg0_type`] [The type `int`.]]
[[`function_traits<long (int, long, double, void*)>::arg3_type`] [The type `void*`.]]
[[`function_traits<long (int, long, double, void*)>::arg4_type`] [A compiler error: there is no `arg4_type` since there are only three arguments.]]
[[`function_traits<long (*)(void)>::arity`] [A compiler error: argument type is a /function pointer/, and not a /function type/.]]

]
   
[endsect]

[section:has_nothrow_assign has_nothrow_assign]

   template <class T>
   struct has_nothrow_assign : public __tof {};
  
__inherit If T is a (possibly cv-qualified) type with a non-throwing assignment-operator
then inherits from __true_type, otherwise inherits from __false_type.  Type `T`
must be a complete type.

__compat If the compiler does not support partial-specialization of class 
templates, then this template can not be used with function types.

Without some (as yet unspecified) help from the compiler, 
`has_nothrow_assign` will never report that a class or struct has a 
non-throwing assignment-operator; this is always safe, if possibly sub-optimal.
Currently (May 2005) only Visual C++ 8 has the necessary compiler support to ensure that this
trait "just works".

__header ` #include <boost/type_traits/has_nothrow_assign.hpp>` or ` #include <boost/type_traits.hpp>`

[endsect]

[section:has_nothrow_constructor has_nothrow_constructor]
   template <class T>
   struct has_nothrow_constructor : public __tof {};
  
__inherit If T is a (possibly cv-qualified) type with a non-throwing default-constructor
then inherits from __true_type, otherwise inherits from __false_type.  Type `T`
must be a complete type.

__compat If the compiler does not support partial-specialization of class 
templates, then this template can not be used with function types.

Without some (as yet unspecified) help from the compiler, 
`has_nothrow_constructor` will never report that a class or struct has a 
non-throwing default-constructor; this is always safe, if possibly sub-optimal.
Currently (May 2005) only Visual C++ 8 has the necessary compiler __intrinsics to ensure that this
trait "just works".

__header ` #include <boost/type_traits/has_nothrow_constructor.hpp>` or ` #include <boost/type_traits.hpp>`

[endsect]

[section:has_nothrow_copy has_nothrow_copy]
   template <class T>
   struct has_nothrow_copy : public __tof {};
  
__inherit If T is a (possibly cv-qualified) type with a non-throwing copy-constructor
then inherits from __true_type, otherwise inherits from __false_type.  Type `T`
must be a complete type.

__compat If the compiler does not support partial-specialization of class 
templates, then this template can not be used with function types.

Without some (as yet unspecified) help from the compiler, 
`has_nothrow_copy` will never report that a class or struct has a 
non-throwing copy-constructor; this is always safe, if possibly sub-optimal.
Currently (May 2005) only Visual C++ 8 has the necessary compiler __intrinsics to ensure that this
trait "just works".

__header ` #include <boost/type_traits/has_nothrow_copy.hpp>` or ` #include <boost/type_traits.hpp>`

[endsect]

[section:has_trivial_assign has_trivial_assign]
   template <class T>
   struct has_trivial_assign : public __tof {};
  
__inherit If T is a (possibly cv-qualified) type with a trivial assignment-operator
then inherits from __true_type, otherwise inherits from __false_type.

If a type has a trivial assignment-operator then the operator has the same effect
as copying the bits of one object to the other: 
calls to the operator can be safely replaced with a call to `memcpy`.

__compat If the compiler does not support partial-specialization of class 
templates, then this template can not be used with function types.

Without some (as yet unspecified) help from the compiler, 
has_trivial_assign will never report that a user-defined class or struct has a 
trivial constructor; this is always safe, if possibly sub-optimal.  Currently 
(May 2005) only MWCW 9 and Visual C++ 8 have the necessary compiler __intrinsics to detect 
user-defined classes with trivial constructors.

__std_ref 12.8p11.

__header ` #include <boost/type_traits/has_trivial_assign.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`has_trivial_assign<int>` inherits from `__true_type`.]

[:`has_trivial_assign<char*>::type` is the type `__true_type`.]

[:`has_trivial_assign<int (*)(long)>::value` is an integral constant 
expression that evaluates to /true/.]

[:`has_trivial_assign<MyClass>::value` is an integral constant 
expression that evaluates to /false/.]

[:`has_trivial_assign<T>::value_type` is the type `bool`.]

[endsect]

[section:has_trivial_constructor has_trivial_constructor]
   template <class T>
   struct has_trivial_constructor : public __tof {};
  
__inherit If T is a (possibly cv-qualified) type with a trivial default-constructor
then inherits from __true_type, otherwise inherits from __false_type.

If a type has a trivial default-constructor then the constructor have no effect: 
calls to the constructor can be safely omitted.  Note that using meta-programming
to omit a call to a single trivial-constructor call is of no benefit whatsoever. 
However, if loops and/or exception handling code can also be omitted, then some
benefit in terms of code size and speed can be obtained.

__compat If the compiler does not support partial-specialization of class 
templates, then this template can not be used with function types.

Without some (as yet unspecified) help from the compiler, 
has_trivial_constructor will never report that a user-defined class or struct has a 
trivial constructor; this is always safe, if possibly sub-optimal.  Currently 
(May 2005) only MWCW 9 and Visual C++ 8 have the necessary compiler __intrinsics to detect 
user-defined classes with trivial constructors.

__std_ref 12.1p6.

__header ` #include <boost/type_traits/has_trivial_constructor.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`has_trivial_constructor<int>` inherits from `__true_type`.]

[:`has_trivial_constructor<char*>::type` is the type `__true_type`.]

[:`has_trivial_constructor<int (*)(long)>::value` is an integral constant 
expression that evaluates to /true/.]

[:`has_trivial_constructor<MyClass>::value` is an integral constant 
expression that evaluates to /false/.]

[:`has_trivial_constructor<T>::value_type` is the type `bool`.]

[endsect]

[section:has_trivial_copy has_trivial_copy]
   template <class T>
   struct has_trivial_copy : public __tof {};
  
__inherit If T is a (possibly cv-qualified) type with a trivial copy-constructor
then inherits from __true_type, otherwise inherits from __false_type.

If a type has a trivial copy-constructor then the constructor has the same effect
as copying the bits of one object to the other: 
calls to the constructor can be safely replaced with a call to `memcpy`.

__compat If the compiler does not support partial-specialization of class 
templates, then this template can not be used with function types.

Without some (as yet unspecified) help from the compiler, 
has_trivial_copy will never report that a user-defined class or struct has a 
trivial constructor; this is always safe, if possibly sub-optimal.  Currently 
(May 2005) only MWCW 9 and Visual C++ 8 have the necessary compiler __intrinsics to detect 
user-defined classes with trivial constructors.

__std_ref 12.8p6.

__header ` #include <boost/type_traits/has_trivial_copy.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`has_trivial_copy<int>` inherits from `__true_type`.]

[:`has_trivial_copy<char*>::type` is the type `__true_type`.]

[:`has_trivial_copy<int (*)(long)>::value` is an integral constant 
expression that evaluates to /true/.]

[:`has_trivial_copy<MyClass>::value` is an integral constant 
expression that evaluates to /false/.]

[:`has_trivial_copy<T>::value_type` is the type `bool`.]

[endsect]

[section:has_trivial_destructor has_trivial_destructor]
   template <class T>
   struct has_trivial_destructor : public __tof {};
  
__inherit If T is a (possibly cv-qualified) type with a trivial destructor
then inherits from __true_type, otherwise inherits from __false_type.

If a type has a trivial destructor then the destructor has no effect: 
calls to the destructor can be safely omitted.  Note that using meta-programming
to omit a call to a single trivial-constructor call is of no benefit whatsoever. 
However, if loops and/or exception handling code can also be omitted, then some
benefit in terms of code size and speed can be obtained.

__compat If the compiler does not support partial-specialization of class 
templates, then this template can not be used with function types.

Without some (as yet unspecified) help from the compiler, 
has_trivial_destructor will never report that a user-defined class or struct has a 
trivial destructor; this is always safe, if possibly sub-optimal.  Currently 
(May 2005) only MWCW 9 and Visual C++ 8 have the necessary compiler __intrinsics to detect 
user-defined classes with trivial constructors.

__std_ref 12.4p3.

__header ` #include <boost/type_traits/has_trivial_destructor.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`has_trivial_destructor<int>` inherits from `__true_type`.]

[:`has_trivial_destructor<char*>::type` is the type `__true_type`.]

[:`has_trivial_destructor<int (*)(long)>::value` is an integral constant 
expression that evaluates to /true/.]

[:`has_trivial_destructor<MyClass>::value` is an integral constant 
expression that evaluates to /false/.]

[:`has_trivial_destructor<T>::value_type` is the type `bool`.]

[endsect]

[section:has_virtual_destructor has_virtual_destructor]
   template <class T>
   struct has_virtual_destructor : public __tof {};
  
__inherit If T is a (possibly cv-qualified) type with a virtual destructor
then inherits from __true_type, otherwise inherits from __false_type.

__compat This trait is provided for completeness, since it's part of the
Technical Report on C++ Library Extensions.  However, there is currently no 
way to portably implement this trait.  The default version provided 
always inherits from __false_type, and has to be explicitly specialized for 
types with virtual destructors unless the compiler used has compiler __intrinsics
that enable the trait to do the right thing: currently (May 2005) only Visual C++
8 has the necessary __intrinsics.

__std_ref 12.4.

__header ` #include <boost/type_traits/has_virtual_destructor.hpp>` or ` #include <boost/type_traits.hpp>`

[endsect]

[section:integral_constant integral_constant]
   template <class T, T val>
   struct integral_constant
   {
      typedef integral_constant<T, val>  type;
      typedef T                          value_type;
      static const T value = val;
   };
   
   typedef integral_constant<bool, true>  true_type;
   typedef integral_constant<bool, false> false_type;

Class template `integral_constant` is the common base class for all the value-based
type traits.  The two typedef's `true_type` and `false_type` are provided for 
convenience: most of the value traits are Boolean properties and so will inherit from
one of these.

[endsect]

[section:is_abstract is_abstract]
   template <class T>
   struct is_abstract : public __tof {};
  
__inherit If T is a (possibly cv-qualified) abstract type then inherits from 
__true_type, otherwise inherits from __false_type.  

__std_ref 10.3.

__header ` #include <boost/type_traits/is_abstract.hpp>` or ` #include <boost/type_traits.hpp>`

__compat The compiler must support DR337 (as of April 2005: GCC 3.4, VC++ 7.1 (and later),
 Intel C++ 7 (and later), and Comeau 4.3.2).
Otherwise behaves the same as __is_polymorphic; 
this is the "safe fallback position" for which polymorphic types are always 
regarded as potentially abstract.  The macro BOOST_NO_IS_ABSTRACT is used to 
signify that the implementation is buggy, users should check for this in their 
own code if the "safe fallback" is not suitable for their particular use-case.

__examples

[:Given: `class abc{ virtual ~abc() = 0; };` ]

[:`is_abstract<abc>` inherits from `__true_type`.]

[:`is_abstract<abc>::type` is the type `__true_type`.]

[:`is_abstract<abc const>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_abstract<T>::value_type` is the type `bool`.]

[endsect]

[section:is_arithmetic is_arithmetic]
   template <class T>
   struct is_arithmetic : public __tof {};
  
__inherit If T is a (possibly cv-qualified) arithmetic type then inherits from 
__true_type, otherwise inherits from __false_type.  Arithmetic types include 
integral and floating point types (see also __is_integral and __is_floating_point).

__std_ref 3.9.1p8.

__header ` #include <boost/type_traits/is_arithmetic.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`is_arithmetic<int>` inherits from `__true_type`.]

[:`is_arithmetic<char>::type` is the type `__true_type`.]

[:`is_arithmetic<double>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_arithmetic<T>::value_type` is the type `bool`.]

[endsect]

[section:is_array is_array]
   template <class T>
   struct is_array : public __tof {};
  
__inherit If T is a (possibly cv-qualified) array type then inherits from __true_type, 
otherwise inherits from __false_type.

__std_ref 3.9.2 and 8.3.4.

__header ` #include <boost/type_traits/is_array.hpp>` or ` #include <boost/type_traits.hpp>`

__compat If the compiler does not support 
partial-specialization of class templates, then this template 
can give the wrong result with function types.

__examples

[:`is_array<int[2]>` inherits from `__true_type`.]

[:`is_array<char[2][3]>::type` is the type `__true_type`.]

[:`is_array<double[]>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_array<T>::value_type` is the type `bool`.]

[endsect]

[section:is_base_of is_base_of]
   template <class Base, class Derived>
   struct is_base_of : public __tof {};
  
__inherit If Base is base class of type Derived then inherits from __true_type, 
otherwise inherits from __false_type.

This template will detect non-public base classes, and ambiguous base classes.

Note that a class is not considered to be it's own base class, 
likewise, if either `Base` or `Derived` are non-class types, 
then the class will always inherit from __false_type.

Types `Base` and `Derived` must not be incomplete types.

__std_ref 10.

__header ` #include <boost/type_traits/is_base_of.hpp>` or ` #include <boost/type_traits.hpp>`

__compat If the compiler does not support partial-specialization of class templates, 
then this template can not be used with function types.  There are some older compilers
which will produce compiler errors if `Base` is a private base class of `Derived`, or if
`Base` is an ambiguous base of `Derived`.  These compilers include Borland C++, older 
versions of Sun Forte C++, Digital Mars C++, and older versions of EDG based compilers.

__examples

[:Given: ` class Base{}; class Derived : public Base{};` ]

[:`is_base_of<Base, Derived>` inherits from `__true_type`.]

[:`is_base_of<Base, Derived>::type` is the type `__true_type`.]

[:`is_base_of<Base, Derived>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_base_of<Base, Base>::value` is an integral constant 
expression that evaluates to /false/: a class is not it's own base.]

[:`is_base_of<Derived, Base>::value` is an integral constant 
expression that evaluates to /false/: the arguments are the wrong way round.]

[:`is_base_of<T>::value_type` is the type `bool`.]

[endsect]

[section:is_class is_class]
   template <class T>
   struct is_class : public __tof {};
  
__inherit If T is a (possibly cv-qualified) class type then inherits from __true_type, 
otherwise inherits from __false_type.

__std_ref 3.9.2 and 9.2.

__header ` #include <boost/type_traits/is_class.hpp>` or ` #include <boost/type_traits.hpp>`

__compat Without (some as yet unspecified) help from the compiler, 
we cannot distinguish between union and class types, as a result this type 
will erroneously inherit from __true_type for union types.  See also __is_union.  
Currently (May 2005) only Visual C++ 8 has the necessary compiler __intrinsics to
correctly identify union types, and therefore make is_class function correctly.

__examples

[:Given: `class MyClass;` then:]

[:`is_class<MyClass>` inherits from `__true_type`.]

[:`is_class<MyClass const>::type` is the type `__true_type`.]

[:`is_class<MyClass>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_class<MyClass&>::value` is an integral constant 
expression that evaluates to /false/.]

[:`is_class<MyClass*>::value` is an integral constant 
expression that evaluates to /false/.]

[:`is_class<T>::value_type` is the type `bool`.]

[endsect]

[section:is_compound is_compound]
   template <class T>
   struct is_compound : public __tof {};
  
__inherit If T is a (possibly cv-qualified) compound type then inherits from __true_type, 
otherwise inherits from __false_type.  Any type that is not a fundamental type is
a compound type (see also __is_fundamental).

__std_ref 3.9.2.

__header ` #include <boost/type_traits/is_compound.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`is_compound<MyClass>` inherits from `__true_type`.]

[:`is_compound<MyEnum>::type` is the type `__true_type`.]

[:`is_compound<int*>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_compound<int&>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_compound<int>::value` is an integral constant 
expression that evaluates to /false/.]

[:`is_compound<T>::value_type` is the type `bool`.]

[endsect]

[section:is_const is_const]
   template <class T>
   struct is_const : public __tof {};
  
__inherit If T is a (top level) const-qualified type then inherits from __true_type, 
otherwise inherits from __false_type.

__std_ref 3.9.3.

__header ` #include <boost/type_traits/is_const.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`is_const<int const>` inherits from `__true_type`.]

[:`is_const<int const volatile>::type` is the type `__true_type`.]

[:`is_const<int* const>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_const<int const*>::value` is an integral constant 
expression that evaluates to /false/: the const-qualifier is not
at the top level in this case.]

[:`is_const<int const&>::value` is an integral constant 
expression that evaluates to /false/: the const-qualifier is not
at the top level in this case.]

[:`is_const<int>::value` is an integral constant 
expression that evaluates to /false/.]

[:`is_const<T>::value_type` is the type `bool`.]

[endsect]

[section:is_convertible is_convertible]
   template <class From, class To>
   struct is_convertible : public __tof {};
  
__inherit If an imaginary lvalue of type `From` is convertible to type `To` then 
inherits from __true_type, otherwise inherits from __false_type.

Type From must not be an incomplete type.

Type To must not be an incomplete, or function type.

No types are considered to be convertible to array types or abstract-class types.

This template can not detect whether a converting-constructor is `public` or not: if
type `To` has a `private` converting constructor from type `From` then instantiating
`is_convertible<From, To>` will produce a compiler error.  For this reason `is_convertible`
can not be used to determine whether a type has a `public` copy-constructor or not.

This template will also produce compiler errors if the conversion is ambiguous, 
for example:

   struct A {};
   struct B : A {};
   struct C : A {};
   struct D : B, C {};
   // This produces a compiler error, the conversion is ambiguous:
   bool const y = boost::is_convertible<D*,A*>::value;

__std_ref 4 and 8.5.

__compat This template is currently broken with Borland C++ Builder 5 (and earlier), 
for constructor-based conversions, and for the Metrowerks 7 (and earlier) 
compiler in all cases.  If the compiler does not support `__is_abstract`, then the
template parameter `To` must not be an abstract type.

__header ` #include <boost/type_traits/is_convertible.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`is_convertible<int, double>` inherits from `__true_type`.]

[:`is_convertible<const int, double>::type` is the type `__true_type`.]

[:`is_convertible<int* const, int*>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_convertible<int const*, int*>::value` is an integral constant 
expression that evaluates to /false/: the conversion would require a `const_cast`.]

[:`is_convertible<int const&, long>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_convertible<int>::value` is an integral constant 
expression that evaluates to /false/.]

[:`is_convertible<T>::value_type` is the type `bool`.]

[endsect]

[section:is_empty is_empty]
   template <class T>
   struct is_empty : public __tof {};
  
__inherit If T is an empty class type then inherits from __true_type, 
otherwise inherits from __false_type.

__std_ref 10p5.

__header ` #include <boost/type_traits/is_empty.hpp>` or ` #include <boost/type_traits.hpp>`

__compat In order to correctly detect empty classes this trait relies on either:

* the compiler implementing zero sized empty base classes, or
* the compiler providing __intrinsics to detect empty classes. 

Can not be used with incomplete types.

Can not be used with union types, until is_union can be made to work.

If the compiler does not support partial-specialization of class templates, 
then this template can not be used with abstract types.

__examples

[:Given: `struct empty_class {};` ]

[:`is_empty<empty_class>` inherits from `__true_type`.]

[:`is_empty<empty_class const>::type` is the type `__true_type`.]

[:`is_empty<empty_class>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_empty<T>::value_type` is the type `bool`.]

[endsect]

[section:is_enum is_enum]
   template <class T>
   struct is_enum : public __tof {};
  
__inherit If T is a (possibly cv-qualified) enum type then inherits from __true_type, 
otherwise inherits from __false_type.

__std_ref 3.9.2 and 7.2.

__header ` #include <boost/type_traits/is_enum.hpp>` or ` #include <boost/type_traits.hpp>`

__compat Requires a correctly functioning __is_convertible template;
 this means that is_enum is currently broken under Borland C++ Builder 5, 
 and for the Metrowerks compiler prior to version 8, other compilers 
 should handle this template just fine.

__examples

[:Given: `enum my_enum { one, two };` ]

[:`is_enum<my_enum>` inherits from `__true_type`.]

[:`is_enum<my_enum const>::type` is the type `__true_type`.]

[:`is_enum<my_enum>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_enum<my_enum&>::value` is an integral constant 
expression that evaluates to /false/.]

[:`is_enum<my_enum*>::value` is an integral constant 
expression that evaluates to /false/.]

[:`is_enum<T>::value_type` is the type `bool`.]

[endsect]

[section:is_floating_point is_floating_point]
   template <class T>
   struct is_floating_point : public __tof {};
  
__inherit If T is a (possibly cv-qualified) floating point type then inherits from __true_type, 
otherwise inherits from __false_type.

__std_ref 3.9.1p8.

__header ` #include <boost/type_traits/is_floating_point.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`is_floating_point<float>` inherits from `__true_type`.]

[:`is_floating_point<double>::type` is the type `__true_type`.]

[:`is_floating_point<long double>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_floating_point<T>::value_type` is the type `bool`.]

[endsect]

[section:is_function is_function]
   template <class T>
   struct is_function : public __tof {};
  
__inherit If T is a (possibly cv-qualified) function type then inherits from __true_type, 
otherwise inherits from __false_type.  Note that this template does not detect /pointers
to functions/, or /references to functions/, these are detected by __is_pointer and 
__is_reference respectively:

   typedef int f1();      // f1 is of function type.
   typedef int (f2*)();   // f2 is a pointer to a function.
   typedef int (f3&)();   // f3 is a reference to a function.

__std_ref 3.9.2p1 and 8.3.5.

__header ` #include <boost/type_traits/is_function.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`is_function<int (void)>` inherits from `__true_type`.]

[:`is_function<long (double, int)>::type` is the type `__true_type`.]

[:`is_function<long (double, int)>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_function<long (*)(double, int)>::value` is an integral constant 
expression that evaluates to /false/: the argument in this case is a pointer type,
not a function type.]

[:`is_function<long (&)(double, int)>::value` is an integral constant 
expression that evaluates to /false/: the argument in this case is a 
reference to a function, not a function type.]

[:`is_function<long (MyClass::*)(double, int)>::value` is an integral constant 
expression that evaluates to /false/: the argument in this case is a pointer
to a member function.]

[:`is_function<T>::value_type` is the type `bool`.]

[endsect]

[section:is_fundamental is_fundamental]
   template <class T>
   struct is_fundamental : public __tof {};
  
__inherit If T is a (possibly cv-qualified) fundamental type then inherits from __true_type, 
otherwise inherits from __false_type.  Fundamental types include integral, floating
point and void types (see also __is_integral, __is_floating_point and __is_void)

__std_ref 3.9.1.

__header ` #include <boost/type_traits/is_fundamental.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`is_fundamental<int)>` inherits from `__true_type`.]

[:`is_fundamental<double const>::type` is the type `__true_type`.]

[:`is_fundamental<void>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_fundamental<T>::value_type` is the type `bool`.]

[endsect]

[section:is_integral is_integral]
   template <class T>
   struct is_integral : public __tof {};
  
__inherit If T is a (possibly cv-qualified) integral type then inherits from __true_type, 
otherwise inherits from __false_type.

__std_ref 3.9.1p7.

__header ` #include <boost/type_traits/is_integral.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`is_integral<int>` inherits from `__true_type`.]

[:`is_integral<const char>::type` is the type `__true_type`.]

[:`is_integral<long>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_integral<T>::value_type` is the type `bool`.]

[endsect]

[section:is_member_function_pointer is_member_function_pointer]
   template <class T>
   struct is_member_function_pointer : public __tof {};
  
__inherit If T is a (possibly cv-qualified) pointer to a member function
then inherits from __true_type, 
otherwise inherits from __false_type.

__std_ref 3.9.2 and 8.3.3.

__header ` #include <boost/type_traits/is_member_function_pointer.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`is_member_function_pointer<int (MyClass::*)(void)>` inherits from `__true_type`.]

[:`is_member_function_pointer<int (MyClass::*)(char)>::type` is the type `__true_type`.]

[:`is_member_function_pointer<int (MyClass::*)(void)const>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_member_function_pointer<int (MyClass::*)>::value` is an integral constant 
expression that evaluates to /false/: the argument in this case is a pointer to
a data member and not a member function, see __is_member_object_pointer
and __is_member_pointer]

[:`is_member_function_pointer<T>::value_type` is the type `bool`.]

[endsect]

[section:is_member_object_pointer is_member_object_pointer]
   template <class T>
   struct is_member_object_pointer : public __tof {};
  
__inherit If T is a (possibly cv-qualified) pointer to a member object (a data member)
then inherits from __true_type, 
otherwise inherits from __false_type.

__std_ref 3.9.2 and 8.3.3.

__header ` #include <boost/type_traits/is_member_object_pointer.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`is_member_object_pointer<int (MyClass::*)>` inherits from `__true_type`.]

[:`is_member_object_pointer<double (MyClass::*)>::type` is the type `__true_type`.]

[:`is_member_object_pointer<const int (MyClass::*)>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_member_object_pointer<int (MyClass::*)(void)>::value` is an integral constant 
expression that evaluates to /false/: the argument in this case is a pointer to
a member function and not a member object, see __is_member_function_pointer 
and __is_member_pointer]

[:`is_member_object_pointer<T>::value_type` is the type `bool`.]

[endsect]

[section:is_member_pointer is_member_pointer]
   template <class T>
   struct is_member_pointer : public __tof {};
  
__inherit If T is a (possibly cv-qualified) pointer to a member (either a function
or a data member) 
then inherits from __true_type, 
otherwise inherits from __false_type.

__std_ref 3.9.2 and 8.3.3.

__header ` #include <boost/type_traits/is_member_pointer.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`is_member_pointer<int (MyClass::*)>` inherits from `__true_type`.]

[:`is_member_pointer<int (MyClass::*)(char)>::type` is the type `__true_type`.]

[:`is_member_pointer<int (MyClass::*)(void)const>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_member_pointer<T>::value_type` is the type `bool`.]

[endsect]

[section:is_object is_object]
   template <class T>
   struct is_object : public __tof {};
  
__inherit If T is a (possibly cv-qualified) object type 
then inherits from __true_type, 
otherwise inherits from __false_type.  All types are object types except
references, void, and function types.

__std_ref 3.9p9.

__header ` #include <boost/type_traits/is_object.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`is_object<int>` inherits from `__true_type`.]

[:`is_object<int*>::type` is the type `__true_type`.]

[:`is_object<int (*)(void)>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_object<int (MyClass::*)(void)const>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_object<int &>::value` is an integral constant 
expression that evaluates to /false/: reference types are not objects]

[:`is_object<int (double)>::value` is an integral constant 
expression that evaluates to /false/: function types are not objects]

[:`is_object<const void>::value` is an integral constant 
expression that evaluates to /false/: void is not an object type]

[:`is_object<T>::value_type` is the type `bool`.]

[endsect]

[section:is_pod is_pod]
   template <class T>
   struct is_pod : public __tof {};
  
__inherit If T is a (possibly cv-qualified) POD type then inherits from __true_type, 
otherwise inherits from __false_type.

__std_ref 3.9p10 and 9p4.

__compat If the compiler does not support partial-specialization 
of class templates, then this template can not be used with function types.

Without some (as yet unspecified) help from the compiler, is_pod will 
never report that a class or struct is a POD; this is always safe, 
if possibly sub-optimal.  Currently (May 2005) only MWCW 9 and Visual C++ 8 have the 
necessary compiler-__intrinsics.

__header ` #include <boost/type_traits/is_pod.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`is_pod<int>` inherits from `__true_type`.]

[:`is_pod<char*>::type` is the type `__true_type`.]

[:`is_pod<int (*)(long)>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_pod<MyClass>::value` is an integral constant 
expression that evaluates to /false/.]

[:`is_pod<T>::value_type` is the type `bool`.]

[endsect]

[section:is_pointer is_pointer]
   template <class T>
   struct is_pointer : public __tof {};
  
__inherit If T is a (possibly cv-qualified) pointer type (includes function pointers, 
but excludes pointers to members) then inherits from __true_type, 
otherwise inherits from __false_type.

__std_ref 3.9.2p2 and 8.3.1.

__header ` #include <boost/type_traits/is_pointer.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`is_pointer<int*>` inherits from `__true_type`.]

[:`is_pointer<char* const>::type` is the type `__true_type`.]

[:`is_pointer<int (*)(long)>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_pointer<int (MyClass::*)(long)>::value` is an integral constant 
expression that evaluates to /false/.]

[:`is_pointer<int (MyClass::*)>::value` is an integral constant 
expression that evaluates to /false/.]

[:`is_pointer<T>::value_type` is the type `bool`.]

[endsect]

[section:is_polymorphic is_polymorphic]
   template <class T>
   struct is_polymorphic : public __tof {};
  
__inherit If T is a (possibly cv-qualified) polymorphic type 
then inherits from __true_type, 
otherwise inherits from __false_type.  Type `T` must be a complete type.

__std_ref 10.3.

__compat The implementation requires some knowledge of the compilers ABI, 
it does actually seem to work with the majority of compilers though.

__header ` #include <boost/type_traits/is_polymorphic.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[: Given: `class poly{ virtual ~poly(); };` ]

[:`is_polymorphic<poly>` inherits from `__true_type`.]

[:`is_polymorphic<poly const>::type` is the type `__true_type`.]

[:`is_polymorphic<poly>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_polymorphic<T>::value_type` is the type `bool`.]

[endsect]

[section:is_same is_same]
   template <class T, class U>
   struct is_same : public __tof {};
  
__inherit If T and U are the same types then inherits from 
__true_type, otherwise inherits from __false_type.

__header ` #include <boost/type_traits/is_same.hpp>` or ` #include <boost/type_traits.hpp>`

__compat If the compiler does not support partial-specialization of class templates, 
then this template can not be used with abstract, incomplete or function types.

__examples

[:`is_same<int, int>` inherits from `__true_type`.]

[:`is_same<int, int>::type` is the type `__true_type`.]

[:`is_same<int, int>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_same<int const, int>::value` is an integral constant 
expression that evaluates to /false/.]

[:`is_same<int&, int>::value` is an integral constant 
expression that evaluates to /false/.]

[:`is_same<T>::value_type` is the type `bool`.]

[endsect]

[section:is_scalar is_scalar]
   template <class T>
   struct is_scalar : public __tof {};
  
__inherit If T is a (possibly cv-qualified) scalar type then inherits from 
__true_type, otherwise inherits from __false_type.  Scalar types include
integral, floating point, enumeration, pointer, and pointer-to-member types.

__std_ref 3.9p10.

__header ` #include <boost/type_traits/is_scalar.hpp>` or ` #include <boost/type_traits.hpp>`

__compat If the compiler does not support partial-specialization of class templates, 
then this template can not be used with function types.

__examples

[:`is_scalar<int*>` inherits from `__true_type`.]

[:`is_scalar<int>::type` is the type `__true_type`.]

[:`is_scalar<double>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_scalar<int (*)(long)>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_scalar<int (MyClass::*)(long)>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_scalar<int (MyClass::*)>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_scalar<T>::value_type` is the type `bool`.]

[endsect]

[section:is_stateless is_stateless]
   template <class T>
   struct is_stateless : public __tof {};
  
__inherit Ff T is a stateless type then inherits from __true_type, otherwise
from __false_type.

Type T must be a complete type.

A stateless type is a type that has no storage and whose constructors and 
destructors are trivial. That means that `is_stateless` only inherits from 
__true_type if the following expression is `true`:

   ::boost::has_trivial_constructor<T>::value
   && ::boost::has_trivial_copy<T>::value
   && ::boost::has_trivial_destructor<T>::value
   && ::boost::is_class<T>::value
   && ::boost::is_empty<T>::value

__std_ref 3.9p10.

__header ` #include <boost/type_traits/is_stateless.hpp>` or ` #include <boost/type_traits.hpp>`

__compat If the compiler does not support partial-specialization of class templates, 
then this template can not be used with function types.

Without some (as yet unspecified) help from the compiler, is_stateless will never 
report that a class or struct is stateless; this is always safe, 
if possibly sub-optimal.  Currently (May 2005) only MWCW 9 and Visual C++ 8 have the necessary 
compiler __intrinsics to make this template work automatically.

[endsect]

[section:is_reference is_reference]
   template <class T>
   struct is_reference : public __tof {};
  
__inherit If T is a reference pointer type then inherits from __true_type, 
otherwise inherits from __false_type.

__std_ref 3.9.2 and 8.3.2.

__compat If the compiler does not 
support partial-specialization of class templates, 
then this template may report the wrong result for function types, 
and for types that are both const and volatile qualified.

__header ` #include <boost/type_traits/is_reference.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`is_reference<int&>` inherits from `__true_type`.]

[:`is_reference<int const&>::type` is the type `__true_type`.]

[:`is_reference<int (&)(long)>::value` is an integral constant 
expression that evaluates to /true/ (the argument in this case is
a reference to a function).]

[:`is_reference<T>::value_type` is the type `bool`.]

[endsect]

[section:is_union is_union]
   template <class T>
   struct is_union : public __tof {};
  
__inherit If T is a (possibly cv-qualified) union type then inherits from __true_type, 
otherwise inherits from __false_type.  Currently requires some kind of compiler 
support, otherwise unions are identified as classes.

__std_ref 3.9.2 and 9.5.

__compat Without (some as yet unspecified) help from the 
compiler, we cannot distinguish between union and class types using only standard C++, 
as a result this type will never inherit from __true_type, unless the user explicitly
specializes the template for their user-defined union types, or unless the compiler
supplies some unspecified intrinsic that implements this functionality.  Currently
(May 2005) only Visual C++ 8 has the necessary compiler __intrinsics to make this 
trait "just work" without user intervention.

__header ` #include <boost/type_traits/is_union.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`is_union<void>` inherits from `__true_type`.]

[:`is_union<const void>::type` is the type `__true_type`.]

[:`is_union<void>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_union<void*>::value` is an integral constant 
expression that evaluates to /false/.]

[:`is_union<T>::value_type` is the type `bool`.]

[endsect]

[section:is_void is_void]
   template <class T>
   struct is_void : public __tof {};
  
__inherit If T is a (possibly cv-qualified) void type then inherits from __true_type, 
otherwise inherits from __false_type.

__std_ref 3.9.1p9.

__header ` #include <boost/type_traits/is_void.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`is_void<void>` inherits from `__true_type`.]

[:`is_void<const void>::type` is the type `__true_type`.]

[:`is_void<void>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_void<void*>::value` is an integral constant 
expression that evaluates to /false/.]

[:`is_void<T>::value_type` is the type `bool`.]

[endsect]

[section:is_volatile is_volatile]
   template <class T>
   struct is_volatile : public __tof {};
  
__inherit If T is a (top level) volatile-qualified type then inherits from __true_type, 
otherwise inherits from __false_type.

__std_ref 3.9.3.

__header ` #include <boost/type_traits/is_volatile.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`is_volatile<volatile int>` inherits from `__true_type`.]

[:`is_volatile<const volatile int>::type` is the type `__true_type`.]

[:`is_volatile<int* volatile>::value` is an integral constant 
expression that evaluates to /true/.]

[:`is_volatile<int volatile*>::value` is an integral constant 
expression that evaluates to /false/: the volatile qualifier is not
at the top level in this case.]

[:`is_volatile<T>::value_type` is the type `bool`.]

[endsect]

[section:rank rank]
   template <class T>
   struct rank : public __integral_constant<std::size_t, RANK(T)> {};
  
__inherit Class template rank inherits from `__integral_constant<std::size_t, RANK(T)>`, 
where `RANK(T)` is the number of array dimensions in type `T`.

If `T` is not an array type, then `RANK(T)` is zero.

__header ` #include <boost/type_traits/rank.hpp>` or ` #include <boost/type_traits.hpp>`

__examples

[:`rank<int[]>` inherits from `__integral_constant<std::size_t, 1>`.]

[:`rank<double[2][3][4]>::type` is the type `__integral_constant<std::size_t, 3>`.]

[:`rank<int[1]>::value` is an integral constant 
expression that evaluates to /1/.]

[:`rank<int[][2]>::value` is an integral constant 
expression that evaluates to /2/.]

[:`rank<int*>::value` is an integral constant 
expression that evaluates to /0/.]

[:`rank<T>::value_type` is the type `std::size_t`.]

[endsect]

[section:remove_all_extents remove_all_extents]

   template <class T>
   struct remove_all_extents
   {
      typedef __below type;
   };
  
__type If `T` is an array type, then removes all of the array bounds on `T`, otherwise
leaves `T` unchanged.

__std_ref 8.3.4.

__compat If the compiler does not support partial specialization of class-templates
then this template will compile, but the member `type` will always be the same as 
type `T` except where __transform_workaround have been applied.

__header ` #include <boost/type_traits/remove_all_extents.hpp>` or ` #include <boost/type_traits.hpp>`

[table Examples

[ [Expression] [Result Type]]

[[`remove_all_extents<int>::type`][`int`]]

[[`remove_all_extents<int const[2]>::type`] [`int const`]]

[[`remove_all_extents<int[][2]>::type`] [`int`]]

[[`remove_all_extents<int[2][3][4]>::type`] [`int`]]

[[`remove_all_extents<int const*>::type`] [`int const*`]]

]

[endsect]

[section:remove_const remove_const]

   template <class T>
   struct remove_const
   {
      typedef __below type;
   };
  
__type The same type as `T`, but with any /top level/ const-qualifier removed.

__std_ref 3.9.3.

__compat If the compiler does not support partial specialization of class-templates
then this template will compile, but the member `type` will always be the same as 
type `T` except where __transform_workaround have been applied.

__header ` #include <boost/type_traits/remove_const.hpp>` or ` #include <boost/type_traits.hpp>`

[table Examples

[ [Expression] [Result Type]]

[[`remove_const<int>::type`][`int`]]

[[`remove_const<int const>::type`] [`int`]]

[[`remove_const<int const volatile>::type`] [`int volatile`]]

[[`remove_const<int const&>::type`] [`int const&`]]

[[`remove_const<int const*>::type`] [`int const*`]]

]

[endsect]

[section:remove_cv remove_cv]

   template <class T>
   struct remove_cv
   {
      typedef __below type;
   };
  
__type The same type as `T`, but with any /top level/ cv-qualifiers removed.

__std_ref 3.9.3.

__compat If the compiler does not support partial specialization of class-templates
then this template will compile, but the member `type` will always be the same as 
type `T` except where __transform_workaround have been applied.

__header ` #include <boost/type_traits/remove_cv.hpp>` or ` #include <boost/type_traits.hpp>`

[table Examples

[ [Expression] [Result Type]]

[[`remove_cv<int>::type`][`int`]]

[[`remove_cv<int const>::type`] [`int`]]

[[`remove_cv<int const volatile>::type`] [`int`]]

[[`remove_cv<int const&>::type`] [`int const&`]]

[[`remove_cv<int const*>::type`] [`int const*`]]

]

[endsect]

[section:remove_extent remove_extent]

   template <class T>
   struct remove_extent
   {
      typedef __below type;
   };
  
__type If `T` is an array type, then removes the topmost array bound, 
otherwise leaves `T` unchanged.

__std_ref 8.3.4.

__compat If the compiler does not support partial specialization of class-templates
then this template will compile, but the member `type` will always be the same as 
type `T` except where __transform_workaround have been applied.

__header ` #include <boost/type_traits/remove_extent.hpp>` or ` #include <boost/type_traits.hpp>`

[table Examples

[ [Expression] [Result Type]]

[[`remove_extent<int>::type`][`int`]]

[[`remove_extent<int const[2]>::type`] [`int const`]]

[[`remove_extent<int[2][4]>::type`] [`int[4]`]]

[[`remove_extent<int[][2]>::type`] [`int[2]`]]

[[`remove_extent<int const*>::type`] [`int const*`]]

]

[endsect]

[section:remove_pointer remove_pointer]

   template <class T>
   struct remove_pointer
   {
      typedef __below type;
   };
  
__type The same type as `T`, but with any pointer modifier removed.

__std_ref 8.3.1.

__compat If the compiler does not support partial specialization of class-templates
then this template will compile, but the member `type` will always be the same as 
type `T` except where __transform_workaround have been applied.

__header ` #include <boost/type_traits/remove_pointer.hpp>` or ` #include <boost/type_traits.hpp>`

[table Examples

[ [Expression] [Result Type]]

[[`remove_pointer<int>::type`][`int`]]

[[`remove_pointer<int const*>::type`] [`int const`]]

[[`remove_pointer<int const**>::type`] [`int const*`]]

[[`remove_pointer<int&>::type`] [`int&`]]

[[`remove_pointer<int*&>::type`] [`int*&`]]

]

[endsect]

[section:remove_reference remove_reference]

   template <class T>
   struct remove_reference
   {
      typedef __below type;
   };
  
__type The same type as `T`, but with any reference modifier removed.

__std_ref 8.3.2.

__compat If the compiler does not support partial specialization of class-templates
then this template will compile, but the member `type` will always be the same as 
type `T` except where __transform_workaround have been applied.

__header ` #include <boost/type_traits/remove_reference.hpp>` or ` #include <boost/type_traits.hpp>`

[table Examples

[ [Expression] [Result Type]]

[[`remove_reference<int>::type`][`int`]]

[[`remove_reference<int const&>::type`] [`int const`]]

[[`remove_reference<int*>::type`] [`int*`]]

[[`remove_reference<int*&>::type`] [`int*`]]

]

[endsect]

[section:remove_volatile remove_volatile]

   template <class T>
   struct remove_volatile
   {
      typedef __below type;
   };
  
__type The same type as `T`, but with any /top level/ volatile-qualifier removed.

__std_ref 3.9.3.

__compat If the compiler does not support partial specialization of class-templates
then this template will compile, but the member `type` will always be the same as 
type `T` except where __transform_workaround have been applied.

__header ` #include <boost/type_traits/remove_volatile.hpp>` or ` #include <boost/type_traits.hpp>`

[table Examples

[ [Expression] [Result Type]]

[[`remove_volatile<int>::type`][`int`]]

[[`remove_volatile<int volatile>::type`] [`int`]]

[[`remove_volatile<int const volatile>::type`] [`int const`]]

[[`remove_volatile<int volatile&>::type`] [`int const&`]]

[[`remove_volatile<int volatile*>::type`] [`int const*`]]

]

[endsect]

[section:type_with_alignment type_with_alignment]

   template <std::size_t Align>
   struct type_with_alignment
   {
      typedef __below type;
   };
  
__type a built-in or POD type with an alignment 
that is a multiple of `Align`.  

__header ` #include <boost/type_traits/type_with_alignment.hpp>` or ` #include <boost/type_traits.hpp>`

[endsect]

[endsect]

[section:credits Credits]

This documentation was pulled together by John Maddock, using 
[@../../tools/quickbook/doc/html/index.html Boost.Quickbook]
and [@boostbook.html Boost.DocBook].
 
The original version of this library was created by Steve Cleary, 
Beman Dawes, Howard Hinnant, and John Maddock.  John Maddock is the
current maintainer of the library.

This version of type traits library is based on contributions by 
Adobe Systems Inc, David Abrahams, Steve Cleary, 
Beman Dawes, Aleksey Gurtovoy, Howard Hinnant, Jesse Jones, Mat Marcus, 
Itay Maman, John Maddock, Thorsten Ottosen, Robert Ramey and Jeremy Siek.

Mat Marcus and Jesse Jones invented, and 
[@http://opensource.adobe.com/project4/project.shtml published a paper describing], 
the partial specialization workarounds used in this library.

Aleksey Gurtovoy added MPL integration to the library.

The __is_convertible template is based on code originally devised by 
Andrei Alexandrescu, see 
"[@http://www.cuj.com/experts/1810/alexandr.htm?topic=experts 
Generic<Programming>: Mappings between Types and Values]".

The latest version of this library and documentation can be found at 
[@http://www.boost.org www.boost.org]. Bugs, suggestions and discussion
should be directed to boost@lists.boost.org 
(see [@http://www.boost.org/more/mailing_lists.htm#main 
www.boost.org/more/mailing_lists.htm#main] for subscription details).

[endsect]

