Safe, generic, stack-based discriminated union container. The variant class template (inspired by Andrei Alexandrescu's class of the same name [Ale01A]) is an efficient, recursive-capable, bounded discriminated union value type capable of containing any value type (either POD or non-POD). It supports construction from any type convertible to one of its bounded types or from a source variant whose bounded types are each convertible to one of the destination variant's bounded types. As well, through apply_visitor, variant supports compile-time checked, type-safe visitation; and through get, variant supports run-time checked, type-safe value retrieval. Notes: The bounded types of the variant are exposed via the nested typedef types, which is an MPL-compatible Sequence containing the set of types that must be handled by any visitor to the variant. All members of variant satisfy at least the basic guarantee of exception-safety. That is, all operations on a variant remain defined even after previous operations have failed. Each type specified as a template argument to variant must meet the requirements of the BoundedType concept. Each type specified as a template argument to variant must be distinct after removal of qualifiers. Thus, for instance, both variant<int, int> and variant<int, const int> have undefined behavior. Conforming implementations of variant must allow at least ten types as template arguments. The exact number of allowed arguments is exposed by the preprocessor macro BOOST_VARIANT_LIMIT_TYPES. (See make_variant_over for a means to specify the bounded types of a variant by the elements of an MPL or compatible Sequence, thus overcoming this limitation.) unspecified Destroys the content of *this. Will not throw. The first bounded type of the variant (i.e., T1) must fulfill the requirements of the DefaultConstructible [20.1.4] concept. Content of *this is the default value of the first bounded type (i.e, T1). May fail with any exceptions arising from the default constructor of T1. const variant & Content of *this is a copy of the content of other. May fail with any exceptions arising from the copy constructor of other's contained type. variant && C++11 compatible compiler. Content of *this is move constructed from the content of other. May fail with any exceptions arising from the move constructor of other's contained type. T & T must be unambiguously convertible to one of the bounded types (i.e., T1, T2, etc.). Content of *this is the best conversion of operand to one of the bounded types, as determined by standard overload resolution rules. May fail with any exceptions arising from the conversion of operand to one of the bounded types. const T & Same semantics as previous constructor, but allows construction from temporaries. T && C++11 compatible compiler. Same semantics as previous constructor, but allows move construction if operand is an rvalue. variant<U1, U2, ..., UN> & Every one of U1, U2, ..., UN must have an unambiguous conversion to one of the bounded types (i.e., T1, T2, ..., TN). If variant<U1, U2, ..., UN> is itself one of the bounded types, then content of *this is a copy of operand. Otherwise, content of *this is the best conversion of the content of operand to one of the bounded types, as determined by standard overload resolution rules. If variant<U1, U2, ..., UN> is itself one of the bounded types, then may fail with any exceptions arising from the copy constructor of variant<U1, U2, ..., UN>. Otherwise, may fail with any exceptions arising from the conversion of the content of operand to one of the bounded types. const variant<U1, U2, ..., UN> & Same semantics as previous constructor, but allows construction from temporaries. C++11 compatible compiler. variant<U1, U2, ..., UN> && Same semantics as previous constructor, but allows move construction. void variant & Every bounded type must fulfill the requirements of the MoveAssignable concept. Interchanges the content of *this and other. If the contained type of other is the same as the contained type of *this, then may fail with any exceptions arising from the swap of the contents of *this and other. Otherwise, may fail with any exceptions arising from either of the move or copy constructors of the contained types. Also, in the event of insufficient memory, may fail with std::bad_alloc (why?). variant & const variant & Every bounded type must fulfill the requirements of the Assignable concept. If the contained type of rhs is the same as the contained type of *this, then assigns the content of rhs into the content of *this. Otherwise, makes the content of *this a copy of the content of rhs, destroying the previous content of *this. If the contained type of rhs is the same as the contained type of *this, then may fail with any exceptions arising from the assignment of the content of rhs into the content *this. Otherwise, may fail with any exceptions arising from the copy constructor of the contained type of rhs. Also, in the event of insufficient memory, may fail with std::bad_alloc (why?). variant & variant && C++11 compatible compiler. Every bounded type must fulfill the requirements of the MoveAssignable concept. If the contained type of rhs is the same as the contained type of *this, then move assigns the content of rhs into the content of *this. Otherwise, move constructs *this using the content of rhs, destroying the previous content of *this. If the contained type of rhs is the same as the contained type of *this, then may fail with any exceptions arising from the move assignment of the content of rhs into the content *this. Otherwise, may fail with any exceptions arising from the move constructor of the contained type of rhs. Also, in the event of insufficient memory, may fail with std::bad_alloc (why?). variant & const T & T must be unambiguously convertible to one of the bounded types (i.e., T1, T2, etc.). Every bounded type must fulfill the requirements of the Assignable concept. If the contained type of *this is T, then assigns rhs into the content of *this. Otherwise, makes the content of *this the best conversion of rhs to one of the bounded types, as determined by standard overload resolution rules, destroying the previous content of *this. If the contained type of *this is T, then may fail with any exceptions arising from the assignment of rhs into the content *this. Otherwise, may fail with any exceptions arising from the conversion of rhs to one of the bounded types. Also, in the event of insufficient memory, may fail with std::bad_alloc (why?). variant & T && C++11 compatible compiler. rhs is an rvalue. Otherwise previous operator will be used. T must be unambiguously convertible to one of the bounded types (i.e., T1, T2, etc.). Every bounded type must fulfill the requirements of the MoveAssignable concept. If the contained type of *this is T, then move assigns rhs into the content of *this. Otherwise, makes the content of *this the best conversion of rhs to one of the bounded types, as determined by standard overload resolution rules, destroying the previous content of *this(conversion is usually done via move construction). If the contained type of *this is T, then may fail with any exceptions arising from the move assignment of rhs into the content *this. Otherwise, may fail with any exceptions arising from the conversion of rhs to one of the bounded types. Also, in the event of insufficient memory, may fail with std::bad_alloc (why?). int The zero-based index into the set of bounded types of the contained type of *this. (For instance, if called on a variant<int, std::string> object containing a std::string, which() would return 1.) Will not throw. bool false: variant always contains exactly one of its bounded types. (See for more information.) Facilitates generic compatibility with boost::any. Will not throw. const std::type_info & typeid(x), where x is the the content of *this. Will not throw. Not available when BOOST_NO_TYPEID is defined. Equality comparison. bool const variant & void const U & The overload returning void exists only to prohibit implicit conversion of the operator's right-hand side to variant; thus, its use will (purposefully) result in a compile-time error. Every bounded type of the variant must fulfill the requirements of the EqualityComparable concept. true iff which() == rhs.which() and content_this == content_rhs, where content_this is the content of *this and content_rhs is the content of rhs. If which() == rhs.which() then may fail with any exceptions arising from operator==(T,T), where T is the contained type of *this. LessThan comparison. bool const variant & void const U & The overload returning void exists only to prohibit implicit conversion of the operator's right-hand side to variant; thus, its use will (purposefully) result in a compile-time error. Every bounded type of the variant must fulfill the requirements of the LessThanComparable concept. If which() == rhs.which() then: content_this < content_rhs, where content_this is the content of *this and content_rhs is the content of rhs. Otherwise: which() < rhs.which(). If which() == rhs.which() then may fail with any exceptions arising from operator<(T,T), where T is the contained type of *this. void variant<T1, T2, ..., TN> & variant<T1, T2, ..., TN> & Swaps lhs with rhs by application of variant::swap. May fail with any exception arising from variant::swap. Provides streaming output for variant types. std::basic_ostream<ElemType,Traits> & std::basic_ostream<ElemType,Traits> & const variant<T1, T2, ..., TN> & Every bounded type of the variant must fulfill the requirements of the OutputStreamable concept. Calls out << x, where x is the content of rhs. Not available when BOOST_NO_IOSTREAM is defined. Provides hashing for variant types so that boost::hash may compute hash. std::size_t const variant<T1, T2, ..., TN> & Every bounded type of the variant must fulfill the requirements of the Hashable concept. Calls boost::hash<T>()(x), where x is the content of rhs and T is its type. Actually, this function is defined in <boost/variant/detail/hash_variant.hpp> header. Exposes a variant whose bounded types are the elements of the given type sequence. variant< unspecified > type has behavior equivalent in every respect to variant< Sequence[0], Sequence[1], ... > (where Sequence[i] denotes the i-th element of Sequence), except that no upper limit is imposed on the number of types. Notes: Sequence must meet the requirements of MPL's Sequence concept. Due to standard conformance problems in several compilers, make_variant_over may not be supported on your compiler. See BOOST_VARIANT_NO_TYPE_SEQUENCE_SUPPORT for more information.