std::basic_format_arg - cppreference.com
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    std::basic_format_arg

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    C++
     
    Text processing library
     
    Formatting library
    Standard format specification
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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

    (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) [edit]

    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

    Example

    This section is incomplete
    Reason: no example

    See also