std::basic_format_arg<Context>:: handle

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basic_format_arg::handle
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Defined in header <format>
template < class Context >
class basic_format_arg < Context > :: handle ;
(since C++20)

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) .

Data members

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).

Member functions

format
(C++20)
formats the referenced object with the given contexts
(public member function)

std::basic_format_arg<Context>::handle:: format

void format ( std:: basic_format_parse_context < Context :: char_type > & parse_ctx,
Context & format_ctx ) const ;
(since C++20)

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 ) ) ;

Notes

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.

See also

(C++20)
class template that provides access to a formatting argument for user-defined formatters
(class template)
(C++20) (C++20)
creates a type-erased object referencing all formatting arguments, convertible to format_args
(function template)
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