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## Function apply_visitor

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.

## Synopsis

    // 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);

## Description

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:

- Overloads accepting one operand invoke the unary function call operator of the given visitor on the content of the given `variant` operand.
- Overloads accepting two operands invoke the binary function call operator of the given visitor on the content of the given `variant` operands.
- Overloads accepting three or more operands invoke the function call operator of the given visitor on the content of the given `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.
- The overloads accepting only a visitor return a `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.)