std::basic_format_arg

 

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basic_format_arg (C++20)

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visit_format_arg (C++20) (deprecated in C++26)

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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

handle (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

visit_format_arg (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

See also

basic_format_argsformat_argswformat_args (C++20)(C++20)(C++20)

class that provides access to all formatting arguments (class template)