std::basic_format_arg
| 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
| (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
| (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.
v is the std::variant stored in arg.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
| (C++20)(C++20)(C++20) |
class that provides access to all formatting arguments (class template) |