Docs4dev
Docs
cpp / latest / utility / format / basic_format_arg.html

std::basic_format_arg

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

Provides access to a formatting argument.

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

A basic_format_arg object behaves as if it stores a std::variant of the following types:

  • std::monostate (only if the object was default-constructed)
  • bool
  • Context::char_type
  • int
  • unsigned int
  • long long int
  • unsigned long long int
  • float
  • double
  • long double
  • const Context::char_type*
  • std::basic_string_view<Context::char_type>
  • const void*
  • basic_format_arg::handle

Member classes

(C++20)
type-erased wrapper that allows formatting an object of user-defined type
(public member class)

Member functions

(constructor)
(C++20)
constructs a std::basic_format_arg
(public member function)
operator bool
(C++20)
checks if the current object holds a formatting argument
(public member function)
visit
(C++26)
visit the stored formatting argument
(public member function)

Non-member functions

(C++20) (deprecated in C++26)
argument visitation interface for user-defined formatters
(function template)

std::basic_format_arg::basic_format_arg

basic_format_arg() noexcept;
 (since C++20)

Default constructor. Constructs a basic_format_arg that does not hold a formatting argument. The stored object has type std::monostate.

To create a basic_format_arg that holds a formatting argument, std::make_format_args has to be used.

std::basic_format_arg::operator bool

explicit operator bool() const noexcept;
 (since C++20)

Checks whether *this holds a formatting argument.

Returns true if *this holds a formatting argument (i.e. the stored object does not have type std::monostate), false otherwise.

std::basic_format_arg::visit

template< class Visitor >
decltype(auto) visit( this basic_format_arg arg, Visitor&& vis );
 (1) (since C++26) 
template< class R, class Visitor >
R visit( this basic_format_arg arg, Visitor&& vis );
 (2) (since C++26)

Applies the visitor vis to the object contained in arg.

The visit functions do not modify the basic_format_arg object on which it is called because a copy of the object is used when calling vis.

1) Equivalent to return std::visit(std::forward<Visitor>(vis), v);, where v is the std::variant stored in arg.
2) Equivalent to return std::visit<R>(std::forward<Visitor>(vis), v);, where v is the std::variant stored in arg.

Notes

Feature-test macro Value Std Feature
__cpp_lib_format 202306L (C++26) Member visit

Example

See also

(C++20)(C++20)(C++20)
class that provides access to all formatting arguments
(class template)

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/utility/format/basic_format_arg