Provides declarations of apply_visitor for three or more variant parameters. Controls maximum amount of variant parameters for multi visistors. 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 MultiVisitor & Variant1 & Variant2 & Variant3 & ... typename MultiVisitor::result_type const MultiVisitor & Variant1 & Variant2 & Variant3 & ...