std::visit_format_arg - cppreference.com

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`template< class Visitor, class Context > /* see below */ visit_format_arg( Visitor&& vis, std::basic_format_arg<Context> arg );`		(since C++20) (deprecated in C++26)

Applies the visitor vis to the object contained in arg.

Equivalent to std::visit(std::forward<Visitor>(vis), value), where value is the std::variant stored in arg.

vis	-	a Callable that accepts every possible alternative from `arg`
arg	-	a std::basic_format_arg to be visited

The value returned by the selected invocation of the visitor.

As of C++26, std::visit_format_arg is deprecated in favor of the visit member functions of std::basic_format_arg.