Provides declarations of apply_visitor for three or more ``variant`` parameters.

Controls maximum amount of ``variant`` parameters for multi visistors. Not used when `std::tuple` is available and `BOOST_VARIANT_DO_NOT_USE_VARIADIC_TEMPLATES` is not defined.

Note: Default amount is 4. Define it to bigger value before including <boost/variant/multivisitors.hpp> to increase maximum amount of ``variant`` parameters for 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. See apply_visitor for more information.

typename MultiVisitor::result_type OR decltype(auto)

MultiVisitor &
Variant1 &
Variant2 &
Variant3 &
...
typename MultiVisitor::result_type OR decltype(auto)

const MultiVisitor &
Variant1 &
Variant2 &
Variant3 &
...