# std::basic_format_arg

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< cpp‎ | utility‎ | format

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.

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 | Definition
handle | type-erased wrapper that allows formatting an object of user-defined type (class)

### Member functions

(constructor) | constructs a `std::basic_format_arg` (public member function)
operator bool | checks if the current object holds a 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::handle

template< class Context > class basic_format_arg<Context>::handle; |  |

Allows formatting an object of a user-defined type.

### Member functions

format | formats the referenced object with the given contexts (public member function)

## std::basic_format_arg::handle::format

void format( std::basic_format_parse_context<Context::char_type>& parse_ctx, Context& format_ctx ) const; |  |

Let `T` be the type of the associated formatting argument, `ref` be a reference to the formatting argument. Equivalent to:

typename Context::template formatter_type<T> f;
parse_ctx.advance_to(f.parse(parse_ctx));
format_ctx.advance_to(f.format(ref, format_ctx));

## std::basic_format_arg::basic_format_arg

basic_format_arg() noexcept; |  |

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; |  |

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.

### Example

| This section is incomplete Reason: no example

### See also

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

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