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.

A basic_format_arg object behaves as if it stores a std::variant of the following types:

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

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

(C++20)

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.

format

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

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 const T& that refers 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));

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.

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.