std::basic_format_arg - cppreference.com

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`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

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

Member functions

	constructs a `std::basic_format_arg` (public member function)
	checks if the current object holds a formatting argument (public member function)
	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) [edit]

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`