NAME¶

std::basic_format_arg - std::basic_format_arg

Synopsis¶

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

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¶

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

Member functions¶

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

Non-member functions¶

visit_format_arg argument visitation interface for user-defined
(C++20) (deprecated in C++26) 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 > (1) (since C++26)
decltype(auto) visit( this basic_format_arg arg, Visitor&& vis );
template< class R, class Visitor > (2) (since C++26)
R visit( this basic_format_arg arg, Visitor&& vis );

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¶

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

Category:¶

* Todo no example