Boost: Base-from-Member Idiom Documentation

# Base-from-Member Idiom

The class template `boost::base_from_member` provides a workaround for a class that needs to initialize a base class with a member. The class template is in boost/utility/base_from_member.hpp which is included in boost/utility.hpp. The class template is forward declared in boost/utility_fwd.hpp.

There is test/example code in base_from_member_test.cpp.

## Contents

- Contents
- Rationale
- Synopsis
- Usage
- Example
- Credits
  - Contributors

## Rationale

When developing a class, sometimes a base class needs to be initialized with a member of the current class. As a naïve example:

>     #include <streambuf>  // for std::streambuf
>     #include <ostream>    // for std::ostream

>     class fdoutbuf
>         : public std::streambuf
>     {
>     public:
>         explicit fdoutbuf( int fd );
>         //...
>     };

>     class fdostream
>         : public std::ostream
>     {
>     protected:
>         fdoutbuf buf;
>     public:
>         explicit fdostream( int fd )
>             : buf( fd ), std::ostream( &buf )
>             {}
>         //...
>     };

This is undefined because C++'s initialization order mandates that the base class is initialized before the member it uses. R. Samuel Klatchko developed a way around this by using the initialization order in his favor. Base classes are intialized in order of declaration, so moving the desired member to another base class, that is initialized before the desired base class, can ensure proper initialization.

A custom base class can be made for this idiom:

>     #include <streambuf>  // for std::streambuf
>     #include <ostream>    // for std::ostream

>     class fdoutbuf
>         : public std::streambuf
>     {
>     public:
>         explicit fdoutbuf( int fd );
>         //...
>     };

>     struct fdostream_pbase
>     {
>         fdoutbuf sbuffer;

>         explicit fdostream_pbase( int fd )
>             : sbuffer( fd )
>             {}
>     };

>     class fdostream
>         : private fdostream_pbase
>         , public std::ostream
>     {
>         typedef fdostream_pbase  pbase_type;
>         typedef std::ostream     base_type;

>     public:
>         explicit fdostream( int fd )
>             : pbase_type( fd ), base_type( &sbuffer )
>             {}
>         //...
>     };

Other projects can use similar custom base classes. The technique is basic enough to make a template, with a sample template class in this library. The main template parameter is the type of the enclosed member. The template class has several (explicit) constructor member templates, which implicitly type the constructor arguments and pass them to the member. The template class uses implicit copy construction and assignment, cancelling them if the enclosed member is non-copyable.

Manually coding a base class may be better if the construction and/or copying needs are too complex for the supplied template class, or if the compiler is not advanced enough to use it.

Since base classes are unnamed, a class cannot have multiple (direct) base classes of the same type. The supplied template class has an extra template parameter, an integer, that exists solely to provide type differentiation. This parameter has a default value so a single use of a particular member type does not need to concern itself with the integer.

## Synopsis

>     template < typename MemberType, int UniqueID = 0 >
>     class boost::base_from_member
>     {
>     protected:
>         MemberType  member;

>         base_from_member();

>         template< typename T1 >
>         explicit  base_from_member( T1 x1 );

>         template< typename T1, typename T2 >
>         base_from_member( T1 x1, T2 x2 );

>         //...

>         template< typename T1, typename T2, typename T3, typename T4,
>          typename T5, typename T6, typename T7, typename T8, typename T9,
>          typename T10 >
>         base_from_member( T1 x1, T2 x2, T3 x3, T4 x4, T5 x5, T6 x6, T7 x7,
>          T8 x8, T9 x9, T10 x10 );
>     };

The class template has a first template parameter MemberType representing the type of the based-member. It has a last template parameter UniqueID, that is an `int`, to differentiate between multiple base classes that use the same based-member type. The last template parameter has a default value of zero if it is omitted. The class template has a protected data member called member that the derived class can use for later base classes (or itself).

There is a default constructor and several constructor member templates. These constructor templates can take as many arguments (currently up to ten) as possible and pass them to a constructor of the data member. Since C++ does not allow any way to explicitly state the template parameters of a templated constructor, make sure that the arguments are already close as possible to the actual type used in the data member's desired constructor.

## Usage

With the starting example, the `fdoutbuf` sub-object needs to be encapsulated in a base class that is inheirited before `std::ostream`.

>     #include <boost/utility/base_from_member.hpp>

>     #include <streambuf>  // for std::streambuf
>     #include <ostream>    // for std::ostream

>     class fdoutbuf
>         : public std::streambuf
>     {
>     public:
>         explicit fdoutbuf( int fd );
>         //...
>     };

>     class fdostream
>         : private boost::base_from_member<fdoutbuf>
>         , public std::ostream
>     {
>         // Helper typedef's
>         typedef boost::base_from_member<fdoutbuf>  pbase_type;
>         typedef std::ostream                        base_type;

>     public:
>         explicit fdostream( int fd )
>             : pbase_type( fd ), base_type( &member )
>             {}
>         //...
>     };

The base-from-member idiom is an implementation detail, so it should not be visible to the clients (or any derived classes) of `fdostream`. Due to the initialization order, the `fdoutbuf` sub-object will get initialized before the `std::ostream` sub-object does, making the former sub-object safe to use in the latter sub-object's construction. Since the `fdoutbuf` sub-object of the final type is the only sub-object with the name "member," that name can be used unqualified within the final class.

## Example

The base-from-member class templates should commonly involve only one base-from-member sub-object, usually for attaching a stream-buffer to an I/O stream. The next example demonstrates how to use multiple base-from-member sub-objects and the resulting qualification issues.

>     #include <boost/utility/base_from_member.hpp>

>     #include <cstddef>  // for NULL

>     struct an_int
>     {
>         int  y;

>         an_int( float yf );
>     };

>     class switcher
>     {
>     public:
>         switcher();
>         switcher( double, int * );
>         //...
>     };

>     class flow_regulator
>     {
>     public:
>         flow_regulator( switcher &, switcher & );
>         //...
>     };

>     template < unsigned Size >
>     class fan
>     {
>     public:
>         explicit fan( switcher );
>         //...
>     };

>     class system
>         : private boost::base_from_member<an_int>
>         , private boost::base_from_member<switcher>
>         , private boost::base_from_member<switcher, 1>
>         , private boost::base_from_member<switcher, 2>
>         , protected flow_regulator
>         , public fan<6>
>     {
>         // Helper typedef's
>         typedef boost::base_from_member<an_int>       pbase0_type;
>         typedef boost::base_from_member<switcher>     pbase1_type;
>         typedef boost::base_from_member<switcher, 1>  pbase2_type;
>         typedef boost::base_from_member<switcher, 2>  pbase3_type;

>         typedef flow_regulator  base1_type;
>         typedef fan<6>          base2_type;

>     public:
>         system( double x );
>         //...
>     };

>     system::system( double x )
>         : pbase0_type( 0.2 )
>         , pbase1_type()
>         , pbase2_type( -16, &this->pbase0_type::member )
>         , pbase3_type( x, static_cast<int *>(NULL) )
>         , base1_type( pbase3_type::member, pbase1_type::member )
>         , base2_type( pbase2_type::member )
>     {
>         //...
>     }

The final class has multiple sub-objects with the name "member," so any use of that name needs qualification by a name of the appropriate base type. (Using `typedef`s ease mentioning the base types.) However, the fix introduces a new problem when a pointer is needed. Using the address operator with a sub-object qualified with its class's name results in a pointer-to-member (here, having a type of `an_int boost::base_from_member<an_int, 0> :: *`) instead of a pointer to the member (having a type of `an_int *`). The new problem is fixed by qualifying the sub-object with "`this->`," and is needed just for pointers, and not for references or values.

There are some argument conversions in the initialization. The constructor argument for `pbase0_type` is converted from `double` to `float`. The first constructor argument for `pbase2_type` is converted from `int` to `double`. The second constructor argument for `pbase3_type` is a special case of necessary conversion; all forms of the null-pointer literal in C++ also look like compile-time integral expressions, so C++ always interprets such code as an integer when it has overloads that can take either an integer or a pointer. The last conversion is necessary for the compiler to call a constructor form with the exact pointer type used in `switcher`'s constructor.

## Credits

### Contributors

Ed Brey
  Suggested some interface changes.
R. Samuel Klatchko (rsk@moocat.org, rsk@brightmail.com)
  Invented the idiom of how to use a class member for initializing a base class.
Dietmar Kuehl
  Popularized the base-from-member idiom in his IOStream example classes.
Daryle Walker
  Started the library. Contributed the test file base_from_member_test.cpp.

Revised: 14 June 2003

Copyright 2001, 2003 Daryle Walker. Use, modification, and distribution are subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or a copy at <http://www.boost.org/LICENSE_1_0.txt>.)