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std::basic_format_parse_context

Defined in header `<format>`
template< class CharT > class basic_format_parse_context;		(since C++20)

Provides access to the format string parsing state consisting of the format string range being parsed and the argument counter for automatic indexing.

A std::basic_format_parse_context instance is passed to Formatter when parsing the format specification.

Several typedefs for common character types are provided:

Defined in header `<format>`
Type	Definition
`std::format_parse_context`	std::basic_format_parse_context<char>
`std::wformat_parse_context`	std::basic_format_parse_context<wchar_t>

Member types

Type	Definition
`char_type`	`CharT`
`iterator`	std::basic_string_view<CharT>::const_iterator
`const_iterator`	std::basic_string_view<CharT>::const_iterator

Member functions

(constructor)	constructs a `std::basic_format_parse_context` instance from format string and argument count (public member function)
operator= [deleted]	`std::basic_format_parse_context` is not copyable (public member function)
begin	returns an iterator to the beginning of the format string range (public member function)
end	returns an iterator to the end of the format string range (public member function)
advance_to	advances the begin iterator to the given position (public member function)
next_arg_id	enters automatic indexing mode, and returns the next argument index (public member function)
check_arg_id	enters manual indexing mode, checks if the given argument index is in range (public member function)

std::basic_format_parse_context::basic_format_parse_context

constexpr explicit basic_format_parse_context( std::basic_string_view<CharT> fmt, std::size_t num_args = 0 ) noexcept;	(1)
basic_format_parse_context( const basic_format_parse_context& ) = delete;	(2)

1) Constructs a std::basic_format_parse_context instance. Initializes the format string range to [fmt.begin(), fmt.end()), and the argument count to num_args.

2) The copy constructor is deleted. std::basic_format_parse_context is not copyable.

std::basic_format_parse_context::begin

constexpr const_iterator begin() const noexcept;

Returns an iterator to the beginning of the format string range.

std::basic_format_parse_context::end

constexpr const_iterator end() const noexcept;

Returns an iterator to the end of the format string range.

std::basic_format_parse_context::advance_to

constexpr void advance_to( const_iterator it );

Sets the beginning of the format string range to it. After a call to advance_to(), subsequent calls to begin() will return a copy of it.

The behavior is undefined if end() is not reachable from it.

std::basic_format_parse_context::next_arg_id

constexpr std::size_t next_arg_id();

Enters automatic argument indexing mode, and returns the next argument index, starting from 0.

If *this has already entered manual argument indexing mode, throws std::format_error.

If the next argument index is larger than or equal to the num_args provided in the constructor, the call is not a core constant expression.

std::basic_format_parse_context::check_arg_id

constexpr void check_arg_id( std::size_t id );

Enters manual argument indexing mode.

If *this has already entered automatic argument indexing mode, throws std::format_error.

If id is larger than or equal to the num_args provided in the constructor, the call is not a core constant expression.

Example

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3825	C++20	`check_arg_id` has a compile-time argument `id` check, but `next_arg_id` did not have	added

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