From cppreference.com
template<classCharT>classbasic_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
when parsing the format specification.
A program that declares an explicit or partial specialization of std::basic_format_parse_context is ill-formed, no diagnostic required.
Several typedefs for common character types are provided:
Type Definition std::format_parse_contextstd::basic_format_parse_context<char>std::wformat_parse_contextstd::basic_format_parse_context<wchar_t>Member types
Type Definition char_typeCharTiteratorstd::basic_string_view<CharT>::const_iteratorconst_iteratorstd::basic_string_view<CharT>::const_iteratorMember 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
constexprexplicitbasic_format_parse_context(std::basic_string_view<CharT>fmt,std::size_tnum_args=0)noexcept; (1)(until C++26)constexprexplicitbasic_format_parse_context(std::basic_string_view<CharT>fmt)noexcept;(since C++26)basic_format_parse_context(constbasic_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
constexprconst_iteratorbegin()constnoexcept;Returns an iterator to the beginning of the format string range.
std::basic_format_parse_context::end
constexprconst_iteratorend()constnoexcept;Returns an iterator to the end of the format string range.
std::basic_format_parse_context::advance_to
constexprvoidadvance_to(const_iteratorit);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
from it.
std::basic_format_parse_context::next_arg_id
constexprstd::size_tnext_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
.
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
constexprvoidcheck_arg_id(std::size_tid);Enters manual argument indexing mode.
If *this has already entered automatic argument indexing mode, throws
.
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>constexprvoidcheck_dynamic_spec(std::size_tid)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
) 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.
The program is ill-formed unless sizeof...(Ts)>=1, the types in Ts... are unique, and each type is one of bool, char_type, int, unsignedint, longlongint, unsignedlonglongint, float, double, longdouble, constchar_type*, std::basic_string_view<char_type>, or constvoid*.
std::basic_format_parse_context::check_dynamic_spec_integral
constexprvoidcheck_dynamic_spec_integral(std::size_tid)noexcept;(since C++26)Equivalent to call check_dynamic_spec<int,unsignedint,longlongint,unsignedlonglongint>(id). A call to check_dynamic_spec_integral has no effect at runtime.
std::basic_format_parse_context::check_dynamic_spec_string
constexprvoidcheck_dynamic_spec_string(std::size_tid)noexcept;(since C++26)Equivalent to call check_dynamic_spec<constchar_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
C++20 check_arg_id has a compile-time argument
id check, but next_arg_id did not have added
C++20 user specialization of basic_format_parse_context was allowed disallowed