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boost::apply_visitor — Allows compile-time checked type-safe application of the given visitor to the content of the given variant, ensuring that all types are handled by the visitor.
// In header: <boost/variant/apply_visitor.hpp> template<typename Visitor, typename Variant> typename Visitor::result_type apply_visitor(Visitor & visitor, Variant&& operand); template<typename Visitor, typename Variant> typename Visitor::result_type apply_visitor(const Visitor & visitor, Variant&& operand); template<typename BinaryVisitor, typename Variant1, typename Variant2> typename BinaryVisitor::result_type OR decltype(auto) apply_visitor(BinaryVisitor & visitor, Variant1&& operand1, Variant2&& operand2); template<typename BinaryVisitor, typename Variant1, typename Variant2> typename BinaryVisitor::result_type OR decltype(auto) apply_visitor(const BinaryVisitor & visitor, Variant1&& operand1, Variant2&& operand2); template<typename MultiVisitor, typename Variant1, typename Variant2, typename Variant3> typename MultiVisitor::result_type OR decltype(auto) apply_visitor(MultiVisitor & visitor, Variant1&& operand1, Variant2&& operand2, Variant3&& operand3, ... other_operands); template<typename MultiVisitor, typename Variant1, typename Variant2, typename Variant3> typename MultiVisitor::result_type OR decltype(auto) apply_visitor(const MultiVisitor & visitor, Variant1&& operand1, Variant2&& operand2, Variant3&& operand3, ... other_operands); template<typename Visitor> apply_visitor_delayed_t<Visitor> apply_visitor(Visitor & visitor); template<typename Visitor> apply_visitor_delayed_cpp14_t<Visitor> apply_visitor(Visitor & visitor);
The behavior of apply_visitor is dependent on
        the number of arguments on which it operates (i.e., other than the
        visitor). If your compiler does not support the rvalue references or reference qualifiers then all the 
        forwarding references from above degrade to non const lvalue reference. The function behaves as follows:
          
variant operand.variant
              operands.variant
              operands. Maximum amount of parameters controlled by
              BOOST_VARAINT_MAX_MULTIVIZITOR_PARAMS
              macro. Those functions are actually defined in a header boost/variant/multivisitors.hpp
              (See the section called “Header <boost/variant/multivisitors.hpp>”). That header must be manually included
              if multi visitors are meant for use.C++03 compatible generic function object
              or
                C++14 compatible generic function object
              that accepts either one, two or arbitrary count of arguments and invoke
              apply_visitor using
              these arguments and visitor, thus behaving as
              specified above. (This behavior is particularly useful, for
              example, when one needs to operate on each element of a sequence
              of variant objects using a standard library
              algorithm.)| Returns: | The overloads acccepting operands return the result of applying the given visitor to the content of the given operands. The overload accepting only a visitor return a function object, thus delaying application of the visitor to any operands. | 
| Requires: | The given visitor must fulfill the
        StaticVisitor
        concept requirements with respect to each of the bounded types of the
        given variant. | 
| Throws: | The overloads accepting operands throw only if the given visitor throws when applied. The overload accepting only a visitor will not throw. (Note, however, that the returned function object may throw when invoked.) |