std::basic_format_arg - cppreference.com (original) (raw)

| | | | | ---------------------------------------------------- | | ------------- | | 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 contains 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_viewContext::char\_type\
• const void*
• basic_format_arg::handle

Contents

• 1 Member classes
• 2 Member functions
• 3 Non-member functions
• 4 std::basic_format_arg::basic_format_arg
• 5 std::basic_format_arg::operator bool
• 6 std::basic_format_arg::visit
  • 6.1 Notes
  • 6.2 Example
  • 6.3 See also

[edit] Member classes

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

[edit] 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 contained formatting argument (public member function) |

[edit] Non-member functions

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 contained 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 contained object does not have type std::monostate), false otherwise.

std::basic_format_arg::visit

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.

[edit] Notes

[edit] Example

[edit] See also