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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)
check_dynamic_spec
(C++26)
checks if the type of the corresponding format argument with the given argument index is in the given type template arguments
(public member function)
check_dynamic_spec_integral
(C++26)
checks if the type of the corresponding format argument with the given argument index is an integral type
(public member function)
check_dynamic_spec_string
(C++26)
checks if the type of the corresponding format argument with the given argument index is a string type
(public member function)

std::basic_format_parse_context::basic_format_parse_context

(1) 
constexpr explicit 
basic_format_parse_context( std::basic_string_view<CharT> fmt,
                            std::size_t num_args = 0 ) noexcept;
 (until C++26) 
constexpr explicit 
basic_format_parse_context( std::basic_string_view<CharT> fmt ) noexcept;
 (since C++26) 
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(until C++26)​0​(since C++26).

Any call to next_arg_id, check_arg_id, or check_dynamic_spec on an instance of std::basic_format_parse_context initialized using this constructor is not a core constant expression.

 (since C++26)
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.

std::basic_format_parse_context::check_dynamic_spec

template< class... Ts >
constexpr void check_dynamic_spec( std::size_t id ) noexcept;
 (since C++26)

If id is larger than or equal to the num_args provided in the constructor or the type of the corresponding format argument (after conversion to std::basic_format_arg) is not one of the types in Ts..., the call is not a core constant expression. A call to check_dynamic_spec has no effect at runtime.

  • This overload participates in overload resolution only if the types in Ts... are unique and each type is one of bool, char_type, int, unsigned int, long long int, unsigned long long int, float, double, long double, const char_type*, std::basic_string_view<char_type>, or const void*.

std::basic_format_parse_context::check_dynamic_spec_integral

constexpr void check_dynamic_spec_integral( std::size_t id ) noexcept;
 (since C++26)

Equivalent to call check_dynamic_spec<int, unsigned int, long long int, unsigned long long int>(id). A call to check_dynamic_spec_integral has no effect at runtime.

std::basic_format_parse_context::check_dynamic_spec_string

constexpr void check_dynamic_spec_string( std::size_t id ) noexcept;
 (since C++26)

Equivalent to call check_dynamic_spec<const char_type*, std::basic_string_view<char_type>>(id). A call to check_dynamic_spec_string has no effect at runtime.

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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