std::basic_format_arg

 

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Formatting library

Formatting functions

format (C++20)

format_to (C++20)

format_to_n (C++20)

formatted_size (C++20)

vformat (C++20)

vformat_to (C++20)

Formatter

formatter (C++20)

basic_format_parse_contextformat_parse_contextwformat_parse_context (C++20)(C++20)(C++20)

basic_format_contextformat_contextwformat_context (C++20)(C++20)(C++20)

Formatting arguments

basic_format_arg (C++20)

basic_format_argsformat_argswformat_args (C++20)(C++20)(C++20)

visit_format_arg (C++20)

make_format_argsmake_wformat_args (C++20)(C++20)

Format error

format_error (C++20)

 

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)

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 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));

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.