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.

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)> >;

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;

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 and .

The following example demonstrates how recursive_wrapper could be used to solve the problem presented in : 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 ); } Performance: boost::recursive_wrapper has no empty state, which makes its move constructor not very optimal. Consider using std::unique_ptr or some other safe pointer for better performance on C++11 compatible compilers.

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 ). 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 .

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 & ) const { return false; // cannot compare different types } template <typename T> bool operator()( const T & lhs, const T & rhs ) const { 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( seq1.begin(), seq1.end(), seq2.begin() , boost::apply_visitor( visitor ) ) );

Multi visitation extends the power and flexibility of visitation by allowing simultaneous visitation of the content of three and more different variant objects. Note that header for multi visitors shall be included separately. Notably this feature requires that multi visitors are incompatible with the visitor objects discussed in the tutorial above, as they must operate on same amout of arguments that was passed to apply_visitor. The following demonstrates the implementation of a multi visitor for three parameters: #include <boost/variant/multivisitors.hpp> typedef boost::variant<int, double, bool> bool_like_t; typedef boost::variant<int, double> arithmetics_t; struct if_visitor: public boost::static_visitor<arithmetics_t> { template <class T1, class T2> arithmetics_t operator()(bool b, T1 v1, T2 v2) const { if (b) { return v1; } else { return v2; } } }; As expected, the visitor is applied to three variant arguments by means of apply_visitor: bool_like_t v0(true), v1(1), v2(2.0); assert( boost::apply_visitor(if_visitor(), v0, v1, v2) == arithmetics_t(1) );