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std::basic_format_arg<Context>::handle - cppreference.com

template< class Context >
class basic_format_arg<Context>::handle;

A type-erased wrapper that allows formatting an object of a user-defined type.

handle objects are typically created by std::make_format_args and accessed through std::visit_format_arg or the visit member functions of std::basic_format_arg(since C++26).

A typical implementation of handle is TriviallyCopyable and only stores two non-static data members:

• a const void* pointer to the object to be formatted, and
• a void (*)(std::basic_format_parse_context<Context::char_type>&, Context&, const void*) function pointer to the function performing needed operations in the format member function (see below).

Let

• T be the type of the formatting argument,
• TD be std::remove_const_t<T>,
• TQ be const TD if const TD satisfies __formattable_with<Context> or TD otherwise, and
• ref be a reference to the formatting argument.

Equivalent to: typename Context::template formatter_type<TD> f;
parse_ctx.advance_to(f.parse(parse_ctx));
format_ctx.advance_to(f.format(const_cast<TQ&>(static_cast<const TD&>(ref)), format_ctx));

A handle has reference semantics for the formatted argument and does not extend its lifetime. It is the programmer's responsibility to ensure that the argument outlives the handle. Usually, a handle is only used within formatting functions.

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