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### Tutorial

#### Basic Usage

A discriminated union container on some set of types is defined by instantiating the `boost::variant` class template with the desired types. These types are called bounded types and are subject to the requirements of the BoundedType concept. Any number of bounded types may be specified, up to some implementation-defined limit (see `BOOST_VARIANT_LIMIT_TYPES`).

For example, the following declares a discriminated union container on `int` and `std::string`:

    boost::variant< int, std::string > v;

By default, a `variant` default-constructs its first bounded type, so `v` initially contains `int(0)`. If this is not desired, or if the first bounded type is not default-constructible, a `variant` can be constructed directly from any value convertible to one of its bounded types. Similarly, a `variant` can be assigned any value convertible to one of its bounded types, as demonstrated in the following:

    v = "hello";

Now `v` contains a `std::string` equal to `"hello"`. We can demonstrate this by streaming`v` to standard output:

    std::cout << v << std::endl;

Usually though, we would like to do more with the content of a `variant` than streaming. Thus, we need some way to access the contained value. There are two ways to accomplish this: `apply_visitor`, which is safest and very powerful, and `get<T>`, which is sometimes more convenient to use.

For instance, suppose we wanted to concatenate to the string contained in `v`. With value retrieval by `get`, this may be accomplished quite simply, as seen in the following:

    std::string& str = boost::get<std::string>(v);
    str += " world! ";

As desired, the `std::string` contained by `v` now is equal to `"hello world! "`. Again, we can demonstrate this by streaming `v` to standard output:

    std::cout << v << std::endl;

While use of `get` is perfectly acceptable in this trivial example, `get` generally suffers from several significant shortcomings. For instance, if we were to write a function accepting a `variant<int, std::string>`, we would not know whether the passed `variant` contained an `int` or a `std::string`. If we insisted upon continued use of `get`, we would need to query the `variant` for its contained type. The following function, which "doubles" the content of the given `variant`, demonstrates this approach:

    void times_two( boost::variant< int, std::string > & operand )
    {
        if ( int* pi = boost::get<int>( &v ) )
            *pi *= 2;
        else if ( std::string* pstr = boost::get<std::string>( &v ) )
            *pstr += *pstr;
    }

However, such code is quite brittle, and without careful attention will likely lead to the introduction of subtle logical errors detectable only at runtime. For instance, consider if we wished to extend `times_two` to operate on a `variant` with additional bounded types. Specifically, let's add `std::complex<double>` to the set. Clearly, we would need to at least change the function declaration:

    void times_two( boost::variant< int, std::string, std::complex<double> > & operand )
    {
        // as above...?
    }

Of course, additional changes are required, for currently if the passed `variant` in fact contained a `std::complex` value, `times_two` would silently return -- without any of the desired side-effects and without any error. In this case, the fix is obvious. But in more complicated programs, it could take considerable time to identify and locate the error in the first place.

Thus, real-world use of `variant` typically demands an access mechanism more robust than `get`. For this reason, `variant` supports compile-time checked visitation via `apply_visitor`. Visitation requires that the programmer explicitly handle (or ignore) each bounded type. Failure to do so results in a compile-time error.

Visitation of a `variant` requires a visitor object. The following demonstrates one such implementation of a visitor implementating behavior identical to `times_two`:

    class times_two_visitor
        : public boost::static_visitor<>
    {
    public:

        void operator()(int & i) const
        {
            i *= 2;
        }

        void operator()(std::string & str) const
        {
            str += str;
        }

    };

With the implementation of the above visitor, we can then apply it to `v`, as seen in the following:

    boost::apply_visitor( times_two_visitor(), v );

As expected, the content of `v` is now a `std::string` equal to `"hello world! hello world! "`. (We'll skip the verification this time.)

In addition to enhanced robustness, visitation provides another important advantage over `get`: the ability to write generic visitors. For instance, the following visitor will "double" the content of any`variant` (provided its bounded types each support operator+=):

    class times_two_generic
        : public boost::static_visitor<>
    {
    public:

        template <typename T>
        void operator()( T & operand ) const
        {
            operand += operand;
        }

    };

Again, `apply_visitor` sets the wheels in motion:

    boost::apply_visitor( times_two_generic(), v );

While the initial setup costs of visitation may exceed that required for `get`, the benefits quickly become significant. Before concluding this section, we should explore one last benefit of visitation with `apply_visitor`: delayed visitation. Namely, a special form of `apply_visitor` is available that does not immediately apply the given visitor to any `variant` but rather returns a function object that operates on any `variant` given to it. This behavior is particularly useful when operating on sequences of `variant` type, as the following demonstrates:

    std::vector< boost::variant<int, std::string> > vec;
    vec.push_back( 21 );
    vec.push_back( "hello " );

    times_two_generic visitor;
    std::for_each(
          vec.begin(), vec.end()
       , boost::apply_visitor(visitor)
       );

#### Advanced Topics

This section discusses several features of the library often required for advanced uses of `variant`. Unlike in the above section, each feature presented below is largely independent of the others. Accordingly, this section is not necessarily intended to be read linearly or in its entirety.

##### Preprocessor macros

While the `variant` class template's variadic parameter list greatly simplifies use for specific instantiations of the template, it significantly complicates use for generic instantiations. For instance, while it is immediately clear how one might write a function accepting a specific `variant` instantiation, say `variant<int, std::string>`, it is less clear how one might write a function accepting any given `variant`.

Due to the lack of support for true variadic template parameter lists in the C++98 standard, the preprocessor is needed. While the Preprocessor library provides a general and powerful solution, the need to repeat `BOOST_VARIANT_LIMIT_TYPES` unnecessarily clutters otherwise simple code. Therefore, for common use-cases, this library provides its own macro `BOOST_VARIANT_ENUM_PARAMS`.

This macro simplifies for the user the process of declaring `variant` types in function templates or explicit partial specializations of class templates, as shown in the following:

    // general cases
    template <typename T> void some_func(const T &);
    template <typename T> class some_class;

    // function template overload
    template <BOOST_VARIANT_ENUM_PARAMS(typename T)>
    void some_func(const boost::variant<BOOST_VARIANT_ENUM_PARAMS(T)> &);

    // explicit partial specialization
    template <BOOST_VARIANT_ENUM_PARAMS(typename T)>
    class some_class< boost::variant<BOOST_VARIANT_ENUM_PARAMS(T)> >;

##### Using a type sequence to specify bounded types

While convenient for typical uses, the `variant` class template's variadic template parameter list is limiting in two significant dimensions. First, due to the lack of support for true variadic template parameter lists in C++, the number of parameters must be limited to some implementation-defined maximum (namely, `BOOST_VARIANT_LIMIT_TYPES`). Second, the nature of parameter lists in general makes compile-time manipulation of the lists excessively difficult.

To solve these problems, `make_variant_over< Sequence >` exposes a `variant` whose bounded types are the elements of `Sequence` (where `Sequence` is any type fulfilling the requirements of MPL's Sequence concept). For instance,

    typedef mpl::vector< std::string > types_initial;
    typedef mpl::push_front< types_initial, int >::type types;

    boost::make_variant_over< types >::type v1;

behaves equivalently to

    boost::variant< int, std::string > v2;

Portability: Unfortunately, due to standard conformance issues in several compilers, `make_variant_over` is not universally available. On these compilers the library indicates its lack of support for the syntax via the definition of the preprocessor symbol `BOOST_VARIANT_NO_TYPE_SEQUENCE_SUPPORT`.

##### Recursive `variant` types

Recursive types facilitate the construction of complex semantics from simple syntax. For instance, nearly every programmer is familiar with the canonical definition of a linked list implementation, whose simple definition allows sequences of unlimited length:

    template <typename T>
    struct list_node
    {
        T data;
        list_node * next;
    };

The nature of `variant` as a generic class template unfortunately precludes the straightforward construction of recursive `variant` types. Consider the following attempt to construct a structure for simple mathematical expressions:

    struct add;
    struct sub;
    template <typename OpTag> struct binary_op;

    typedef boost::variant<
          int
        , binary_op<add>
        , binary_op<sub>
        > expression;

    template <typename OpTag>
    struct binary_op
    {
        expression left;  // variant instantiated here...
        expression right;

        binary_op( const expression & lhs, const expression & rhs )
            : left(lhs), right(rhs)
        {
        }

    }; // ...but binary_op not complete until here!

While well-intentioned, the above approach will not compile because `binary_op` is still incomplete when the `variant` type `expression` is instantiated. Further, the approach suffers from a more significant logical flaw: even if C++ syntax were different such that the above example could be made to "work," `expression` would need to be of infinite size, which is clearly impossible.

To overcome these difficulties, `variant` includes special support for the `boost::recursive_wrapper` class template, which breaks the circular dependency at the heart of these problems. Further, `boost::make_recursive_variant` provides a more convenient syntax for declaring recursive `variant` types. Tutorials for use of these facilities is described in the section called “Recursive types with recursive_wrapper” and the section called “Recursive types with make_recursive_variant”.

###### Recursive types with `recursive_wrapper`

The following example demonstrates how `recursive_wrapper` could be used to solve the problem presented in the section called “Recursive variant types”:

    typedef boost::variant<
          int
        , boost::recursive_wrapper< binary_op<add> >
        , boost::recursive_wrapper< binary_op<sub> >
        > expression;

Because `variant` provides special support for `recursive_wrapper`, clients may treat the resultant `variant` as though the wrapper were not present. This is seen in the implementation of the following visitor, which calculates the value of an `expression` without any reference to `recursive_wrapper`:

    class calculator : public boost::static_visitor<int>
    {
    public:

        int operator()(int value) const
        {
            return value;
        }

        int operator()(const binary_op<add> & binary) const
        {
            return boost::apply_visitor( calculator(), binary.left )
                 + boost::apply_visitor( calculator(), binary.right );
        }

        int operator()(const binary_op<sub> & binary) const
        {
            return boost::apply_visitor( calculator(), binary.left )
                 - boost::apply_visitor( calculator(), binary.right );
        }

    };

Finally, we can demonstrate `expression` in action:

    void f()
    {
        // result = ((7-3)+8) = 12
        expression result(
            binary_op<add>(
                binary_op<sub>(7,3)
              , 8
              )
          );

        assert( boost::apply_visitor(calculator(),result) == 12 );
    }

###### Recursive types with `make_recursive_variant`

For some applications of recursive `variant` types, a user may be able to sacrifice the full flexibility of using `recursive_wrapper` with `variant` for the following convenient syntax:

    typedef boost::make_recursive_variant<
          int
        , std::vector< boost::recursive_variant_ >
        >::type int_tree_t;

Use of the resultant `variant` type is as expected:

    std::vector< int_tree_t > subresult;
    subresult.push_back(3);
    subresult.push_back(5);

    std::vector< int_tree_t > result;
    result.push_back(1);
    result.push_back(subresult);
    result.push_back(7);

    int_tree_t var(result);

To be clear, one might represent the resultant content of `var` as `( 1 ( 3 5 ) 7 )`.

Finally, note that a type sequence can be used to specify the bounded types of a recursive `variant` via the use of `boost::make_recursive_variant_over`, whose semantics are the same as `make_variant_over` (which is described in the section called “Using a type sequence to specify bounded types”).

Portability: Unfortunately, due to standard conformance issues in several compilers, `make_recursive_variant` is not universally supported. On these compilers the library indicates its lack of support via the definition of the preprocessor symbol `BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT`. Thus, unless working with highly-conformant compilers, maximum portability will be achieved by instead using `recursive_wrapper`, as described in the section called “Recursive types with recursive_wrapper”.

##### Binary visitation

As the tutorial above demonstrates, visitation is a powerful mechanism for manipulating `variant` content. Binary visitation further extends the power and flexibility of visitation by allowing simultaneous visitation of the content of two different `variant` objects.

Notably this feature requires that binary visitors are incompatible with the visitor objects discussed in the tutorial above, as they must operate on two arguments. The following demonstrates the implementation of a binary visitor:

    class are_strict_equals
        : public boost::static_visitor<bool>
    {
    public:

        template <typename T, typename U>
        bool operator()( const T &, const U & )
        {
            return false; // cannot compare different types
        }

        template <typename T>
        bool operator()( const T & lhs, const T & rhs )
        {
            return lhs == rhs;
        }

    };

As expected, the visitor is applied to two `variant` arguments by means of `apply_visitor`:

    boost::variant< int, std::string > v1( "hello" );

    boost::variant< double, std::string > v2( "hello" );
    assert( boost::apply_visitor(are_strict_equals(), v1, v2) );

    boost::variant< int, const char * > v3( "hello" );
    assert( !boost::apply_visitor(are_strict_equals(), v1, v3) );

Finally, we must note that the function object returned from the "delayed" form of `apply_visitor` also supports binary visitation, as the following demonstrates:

    typedef boost::variant<double, std::string> my_variant;

    std::vector< my_variant > seq1;
    seq1.push_back("pi is close to ");
    seq1.push_back(3.14);

    std::list< my_variant > seq2;
    seq2.push_back("pi is close to ");
    seq2.push_back(3.14);

    are_strict_equals visitor;
    assert( std::equal(
          v1.begin(), v1.end(), v2.begin()
        , boost::apply_visitor( visitor )
        ) );

Last revised: , at GMT | Copyright © 2002, 2003 Eric Friedman, Itay Maman

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