std:: basic_format_arg

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C++
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Relational operators (deprecated in C++20)
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Standard format specification
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Formatting arguments
basic_format_arg
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Format error
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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

This section is incomplete
Reason: no example

See also

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