Standard library header <format> (C++20)

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros(C++20)

Language support library

Concepts library(C++20)

Diagnostics library

Memory management library

Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library(C++17)

Concurrency support library(C++11)

Execution control library(C++26)

Technical specifications

Symbols index

External libraries

 

Standard library headers

Language support <cfloat> <climits> <compare>(C++20) <contracts>(C++26) <coroutine>(C++20) <csetjmp> <csignal> <cstdarg> <cstddef> <cstdint>(C++11) <cstdlib> <exception> <initializer_list>(C++11)    <limits> <new> <source_location>(C++20) <stdfloat>(C++23) <typeinfo> <version>(C++20) Concepts <concepts>(C++20) Diagnostics <cassert> <cerrno> <debugging>(C++26) <stacktrace>(C++23) <stdexcept> <system_error>(C++11) Memory management <memory> <memory_resource>(C++17) <scoped_allocator>(C++11) Metaprogramming <type_traits>(C++11) <ratio>(C++11)

General utilities <any>(C++17) <bitset> <bit>(C++20) <charconv>(C++17) <expected>(C++23) <format>(C++20) <functional> <optional>(C++17) <stdbit.h>(C++26) <tuple>(C++11) <typeindex>(C++11) <utility> <variant>(C++17) Containers <array>(C++11) <deque> <flat_map>(C++23) <flat_set>(C++23) <forward_list>(C++11) <hive>(C++26) <inplace_vector>(C++26)    <list> <map> <mdspan>(C++23) <queue> <set> <span>(C++20) <stack> <unordered_map>(C++11) <unordered_set>(C++11) <vector> Iterators <iterator> Ranges <generator>(C++23) <ranges>(C++20)

Algorithms <algorithm> <numeric> Strings <cctype> <cstring> <cuchar>(C++11) <cwchar> <cwctype> <string_view>(C++17) <string> Text processing <clocale> <codecvt>(C++11/17/26*) <locale> <regex>(C++11) <text_encoding>(C++26)    Numerics <cfenv>(C++11) <cmath> <complex> <linalg>(C++26) <numbers>(C++20) <random>(C++11) <simd>(C++26) <stdckdint.h>(C++26) <valarray> Time <chrono>(C++11) <ctime> C compatibility <ccomplex>(C++11/17/20*) <ciso646>(until C++20) <cstdalign>(C++11/17/20*) <cstdbool>(C++11/17/20*) <ctgmath>(C++11/17/20*)

Input/output <cinttypes>(C++11) <cstdio> <filesystem>(C++17) <fstream> <iomanip> <iosfwd> <iostream> <ios> <istream> <ostream> <print>(C++23) <spanstream>(C++23) <sstream> <streambuf> <strstream>(C++98/26*) <syncstream>(C++20) Concurrency support <atomic>(C++11) <barrier>(C++20) <condition_variable>(C++11) <future>(C++11) <hazard_pointer>(C++26) <latch>(C++20) <mutex>(C++11) <rcu>(C++26) <semaphore>(C++20) <shared_mutex>(C++14) <stdatomic.h>(C++23) <stop_token>(C++20) <thread>(C++11) Execution support <execution>(C++17)

 

This header is part of the text processing library.

Concepts
formattable (C++23)	specifies that a type is formattable, that is, it specializes std::formatter and provides member functions parse and format (concept)
Enumerations
range_format (C++23)	specifies how a range should be formatted (enum)
Classes
formatter (C++20)	defines formatting rules for a given type (class template)
range_formatter (C++23)	class template that helps implementing std::formatter specializations for range types (class template)
basic_format_parse_contextformat_parse_contextwformat_parse_context (C++20)(C++20)(C++20)	formatting string parser state (class template)
basic_format_contextformat_contextwformat_context (C++20)(C++20)(C++20)	formatting state, including all formatting arguments and the output iterator (class template)
basic_format_arg (C++20)	class template that provides access to a formatting argument for user-defined formatters (class template)
basic_format_argsformat_argswformat_args (C++20)(C++20)(C++20)	class that provides access to all formatting arguments (class template)
basic_format_stringformat_stringwformat_string (C++20)(C++20)(C++20)	class template that performs compile-time format string checks at construction time (class template)
format_error (C++20)	exception type thrown on formatting errors (class)
Formatter specializations
std::formatter<pair-or-tuple> (C++23)	formatting support for pair and tuple (class template specialization)
std::formatter<range> (C++23)	formatting support for ranges (class template specialization)
Functions
format (C++20)	stores formatted representation of the arguments in a new string (function template)
format_to (C++20)	writes out formatted representation of its arguments through an output iterator (function template)
format_to_n (C++20)	writes out formatted representation of its arguments through an output iterator, not exceeding specified size (function template)
formatted_size (C++20)	determines the number of characters necessary to store the formatted representation of its arguments (function template)
runtime_format (C++26)	creates runtime format strings directly usable in user-oriented formatting functions (function)
vformat (C++20)	non-template variant of std::format using type-erased argument representation (function)
vformat_to (C++20)	non-template variant of std::format_to using type-erased argument representation (function template)
visit_format_arg (C++20)(deprecated in C++26)	argument visitation interface for user-defined formatters (function template)
make_format_argsmake_wformat_args (C++20)(C++20)	creates a type-erased object referencing all formatting arguments, convertible to format_args (function template)
Helpers
format_kind (C++23)	selects a suited std::range_format for a range (variable template)
enable_nonlocking_formatter_optimization (C++23)	indicates the argument type can be efficiently printed (variable template)

Synopsis

namespace std {// class template basic_format_contexttemplate<class Out, class CharT>class basic_format_context;using format_context = basic_format_context</* unspecified */, char>;using wformat_context = basic_format_context</* unspecified */, wchar_t>;   // class template basic_format_argstemplate<class Context>class basic_format_args;using format_args = basic_format_args<format_context>;using wformat_args = basic_format_args<wformat_context>;   // class template basic_format_stringtemplate<class CharT, class... Args>struct basic_format_string;   template<class... Args>using format_string = basic_format_string<char, type_identity_t<Args>...>;template<class... Args>using wformat_string = basic_format_string<wchar_t, type_identity_t<Args>...>;   // formatting functionstemplate<class... Args> string format(format_string<Args...> fmt, Args&&... args);template<class... Args> wstring format(wformat_string<Args...> fmt, Args&&... args);template<class... Args> string format(const locale& loc, format_string<Args...> fmt, Args&&... args);template<class... Args> wstring format(const locale& loc, wformat_string<Args...> fmt, Args&&... args);   string vformat(string_view fmt, format_args args); wstring vformat(wstring_view fmt, wformat_args args); string vformat(const locale& loc, string_view fmt, format_args args); wstring vformat(const locale& loc, wstring_view fmt, wformat_args args);   template<class Out, class... Args> Out format_to(Out out, format_string<Args...> fmt, Args&&... args);template<class Out, class... Args> Out format_to(Out out, wformat_string<Args...> fmt, Args&&... args);template<class Out, class... Args> Out format_to(Out out, const locale& loc, format_string<Args...> fmt, Args&&... args);template<class Out, class... Args> Out format_to(Out out, const locale& loc, wformat_string<Args...> fmt, Args&&... args);   template<class Out> Out vformat_to(Out out, string_view fmt, format_args args);template<class Out> Out vformat_to(Out out, wstring_view fmt, wformat_args args);template<class Out> Out vformat_to(Out out, const locale& loc, string_view fmt, format_args args);template<class Out> Out vformat_to(Out out, const locale& loc, wstring_view fmt, wformat_args args);   template<class Out>struct format_to_n_result { Out out; iter_difference_t<Out> size;};template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, format_string<Args...> fmt, Args&&... args);template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, wformat_string<Args...> fmt, Args&&... args);template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, const locale& loc, format_string<Args...> fmt, Args&&... args);template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, const locale& loc, wformat_string<Args...> fmt, Args&&... args);   template<class... Args> size_t formatted_size(format_string<Args...> fmt, Args&&... args);template<class... Args> size_t formatted_size(wformat_string<Args...> fmt, Args&&... args);template<class... Args> size_t formatted_size(const locale& loc, format_string<Args...> fmt, Args&&... args);template<class... Args> size_t formatted_size(const locale& loc, wformat_string<Args...> fmt, Args&&... args);   // formattertemplate<class T, class CharT =char>struct formatter;   // variable template enable_nonlocking_formatter_optimizationtemplate<class T>constexprbool enable_nonlocking_formatter_optimization =false;   // concept formattabletemplate<class T, class CharT> concept formattable =/* see description */;   template<class R, class CharT> concept __const_formattable_range =// exposition onlyranges::input_range<const R>&& formattable<ranges::range_reference_t<const R>, CharT>;   template<class R, class CharT>using __fmt_maybe_const =// exposition only conditional_t<__const_formattable_range<R, CharT>, const R, R>;   // class template basic_format_parse_contexttemplate<class CharT>class basic_format_parse_context;using format_parse_context = basic_format_parse_context<char>;using wformat_parse_context = basic_format_parse_context<wchar_t>;   // formatting of ranges// variable template format_kindenumclass range_format { disabled, map, set, sequence, string, debug_string };   template<class R>constexpr/* unspecified */ format_kind =/* unspecified */;   template<ranges::input_range R> requires same_as<R, remove_cvref_t<R>>constexpr range_format format_kind<R>=/* see description */;   // class template range_formattertemplate<class T, class CharT =char> requires same_as<remove_cvref_t<T>, T>&& formattable<T, CharT>class range_formatter;   // class template range-default-formattertemplate<range_format K, ranges::input_range R, class CharT>struct __range_default_formatter;// exposition only   // specializations for maps, sets, and stringstemplate<ranges::input_range R, class CharT> requires (format_kind<R>!= range_format::disabled)&& formattable<ranges::range_reference_t<R>, CharT>struct formatter<R, CharT>: __range_default_formatter<format_kind<R>, R, CharT>{};   // arguments// class template basic_format_argtemplate<class Context>class basic_format_arg;   template<class Visitor, class Context> decltype(auto) visit_format_arg(Visitor&& vis, basic_format_arg<Context> arg);   // class template format-arg-storetemplate<class Context, class... Args>class __format_arg_store;// exposition only   template<class Context = format_context, class... Args> __format_arg_store<Context, Args...> make_format_args(Args&&... fmt_args);template<class... Args> __format_arg_store<wformat_context, Args...> make_wformat_args(Args&&... args);   // class format_errorclass format_error;}

Class template std::basic_format_string

namespace std {template<class CharT, class... Args>struct basic_format_string {private: basic_string_view<CharT> str;// exposition only   public:template<class T> consteval basic_format_string(const T& s);   constexpr basic_string_view<CharT> get()constnoexcept{return str;}};}

Concept std::formattable

template<class T, class CharT> concept formattable = semiregular<formatter<remove_cvref_t<T>, CharT>>&& requires(formatter<remove_cvref_t<T>, CharT> f, const formatter<remove_cvref_t<T>, CharT> cf, T t, basic_format_context<__fmt_iter_for<CharT>, CharT> fc, basic_format_parse_context<CharT> pc){{ f.parse(pc)}-> same_as<basic_format_parse_context<CharT>::iterator>;{ cf.format(t, fc)}-> same_as<__fmt_iter_for<CharT>>;};

Class template std::basic_format_parse_context

namespace std {template<class CharT>class basic_format_parse_context {public:using char_type = CharT;using const_iterator =typename basic_string_view<CharT>::const_iterator;using iterator = const_iterator;   private: iterator begin_;// exposition only iterator end_;// exposition onlyenum indexing { unknown, manual, automatic };// exposition only indexing indexing_;// exposition only size_t next_arg_id_;// exposition only size_t num_args_;// exposition only   public:constexprexplicit basic_format_parse_context(basic_string_view<CharT> fmt, size_t num_args =0)noexcept; basic_format_parse_context(const basic_format_parse_context&)= delete; basic_format_parse_context& operator=(const basic_format_parse_context&)= delete;   constexpr const_iterator begin()constnoexcept;constexpr const_iterator end()constnoexcept;constexprvoid advance_to(const_iterator it);   constexpr size_t next_arg_id();constexprvoid check_arg_id(size_t id);   template<class... Ts>constexprvoid check_dynamic_spec(size_t id)noexcept;constexprvoid check_dynamic_spec_integral(size_t id)noexcept;constexprvoid check_dynamic_spec_string(size_t id)noexcept;};}

Class template std::basic_format_context

namespace std {template<class Out, class CharT>class basic_format_context { basic_format_args<basic_format_context> args_;// exposition only Out out_;// exposition only   basic_format_context(const basic_format_context&)= delete; basic_format_context& operator=(const basic_format_context&)= delete;   public:using iterator = Out;using char_type = CharT;template<class T>using formatter_type = formatter<T, CharT>;   basic_format_arg<basic_format_context> arg(size_t id)constnoexcept;std::locale locale();   iterator out();void advance_to(iterator it);};}

Variable template std::format_kind

template<ranges::input_range R> requires same_as<R, remove_cvref_t<R>>constexpr range_format format_kind<R>=/* see description */;

Class template std::range_formatter

namespace std {template<class T, class CharT =char> requires same_as<remove_cvref_t<T>, T>&& formattable<T, CharT>class range_formatter { formatter<T, CharT> underlying_;// exposition only basic_string_view<CharT> separator_ =// exposition only __STATICALLY_WIDEN<CharT>(", "); basic_string_view<CharT> __opening_bracket_ =// exposition only __STATICALLY_WIDEN<CharT>("["); basic_string_view<CharT> __closing_bracket_ =// exposition only __STATICALLY_WIDEN<CharT>("]");   public:constexprvoid set_separator(basic_string_view<CharT> sep);constexprvoid set_brackets(basic_string_view<CharT> opening, basic_string_view<CharT> closing);constexpr formatter<T, CharT>& underlying(){return underlying_;}constexprconst formatter<T, CharT>& underlying()const{return underlying_;}   template<class ParseContext>constexprtypename ParseContext::iterator parse(ParseContext& ctx);   template<ranges::input_range R, class FormatContext> requires formattable<ranges::range_reference_t<R>, CharT>&& same_as<remove_cvref_t<ranges::range_reference_t<R>>, T>typename FormatContext::iterator format(R&& r, FormatContext& ctx)const;};}

Class template __range_default_formatter

namespace std {template<ranges::input_range R, class CharT>struct __range_default_formatter<range_format::sequence, R, CharT>{private:using __maybe_const_r = __fmt_maybe_const<R, CharT>; range_formatter<remove_cvref_t<ranges::range_reference_t<__maybe_const_r>>, CharT> underlying_;// exposition only   public:constexprvoid set_separator(basic_string_view<CharT> sep);constexprvoid set_brackets(basic_string_view<CharT> opening, basic_string_view<CharT> closing);   template<class ParseContext>constexprtypename ParseContext::iterator parse(ParseContext& ctx);   template<class FormatContext>typename FormatContext::iterator format(__maybe_const_r& elems, FormatContext& ctx)const;};}

Specialization of __range_default_formatter for maps

namespace std {template<ranges::input_range R, class CharT>struct __range_default_formatter<range_format::map, R, CharT>{private:using __maybe_const_map = __fmt_maybe_const<R, CharT>;// exposition onlyusing __element_type =// exposition only remove_cvref_t<ranges::range_reference_t<__maybe_const_map>>; range_formatter<__element_type, CharT> underlying_;// exposition onlypublic:constexpr __range_default_formatter();   template<class ParseContext>constexprtypename ParseContext::iterator parse(ParseContext& ctx);   template<class FormatContext>typename FormatContext::iterator format(__maybe_const_map& r, FormatContext& ctx)const;};}

Specialization of __range_default_formatter for sets

namespace std {template<ranges::input_range R, class CharT>struct __range_default_formatter<range_format::set, R, CharT>{private:using __maybe_const_set = __fmt_maybe_const<R, CharT>;// exposition only range_formatter<remove_cvref_t<ranges::range_reference_t<__maybe_const_set>>, CharT> underlying_;// exposition onlypublic:constexpr __range_default_formatter();   template<class ParseContext>constexprtypename ParseContext::iterator parse(ParseContext& ctx);   template<class FormatContext>typename FormatContext::iterator format(__maybe_const_set& r, FormatContext& ctx)const;};}

Specialization of __range_default_formatter for strings

namespace std {template<range_format K, ranges::input_range R, class CharT> requires (K == range_format::string|| K == range_format::debug_string)struct __range_default_formatter<K, R, CharT>{private: formatter<basic_string<CharT>, CharT> underlying_;// exposition only   public:template<class ParseContext>constexprtypename ParseContext::iterator parse(ParseContext& ctx);   template<class FormatContext>typename FormatContext::iterator format(/* see description */& str, FormatContext& ctx)const;};}

Class template std::basic_format_arg

namespace std {template<class Context>class basic_format_arg {public:class handle;   private:using char_type =typename Context::char_type;// exposition only   variant<monostate, bool, char_type, int, unsignedint, longlongint, unsignedlonglongint, float, double, longdouble, const char_type*, basic_string_view<char_type>, constvoid*, handle> value;// exposition only   template<class T>explicit basic_format_arg(T&& v)noexcept;// exposition onlyexplicit basic_format_arg(float n)noexcept;// exposition onlyexplicit basic_format_arg(double n)noexcept;// exposition onlyexplicit basic_format_arg(longdouble n)noexcept;// exposition onlyexplicit basic_format_arg(const char_type* s);// exposition only   template<class traits>explicit basic_format_arg( basic_string_view<char_type, traits> s)noexcept;// exposition only   template<class traits, class Allocator>explicit basic_format_arg(const basic_string<char_type, traits, Allocator>& s)noexcept;// exposition only   explicit basic_format_arg(nullptr_t)noexcept;// exposition only   template<class T>explicit basic_format_arg(T* p)noexcept;// exposition only   public: basic_format_arg()noexcept;   explicit operator bool()constnoexcept;   template<class Visitor> decltype(auto) visit(this basic_format_arg arg, Visitor&& vis);template<class R, class Visitor> R visit(this basic_format_arg arg, Visitor&& vis);};}

Class std::basic_format_arg::handle

namespace std {template<class Context>class basic_format_arg<Context>::handle{constvoid* ptr_;// exposition onlyvoid(*format_)(basic_format_parse_context<char_type>&, Context&, constvoid*);// exposition only   template<class T>explicit handle(T&& val)noexcept;// exposition only   friendclass basic_format_arg<Context>;// exposition only   public:void format(basic_format_parse_context<char_type>&, Context& ctx)const;};}

Class template __format_arg_store

namespace std {template<class Context, class... Args>class __format_arg_store {// exposition only array<basic_format_arg<Context>, sizeof...(Args)> args;// exposition only};}

Class template std::basic_format_args

namespace std {template<class Context>class basic_format_args { size_t size_;// exposition onlyconst basic_format_arg<Context>* data_;// exposition only   public:template<class... Args> basic_format_args(const __format_arg_store<Context, Args...>& store)noexcept;   basic_format_arg<Context> get(size_t i)constnoexcept;};}

Tuple formatter

namespace std {template<class CharT, formattable<CharT>... Ts>struct formatter<__pair_or_tuple<Ts...>, CharT>{private: tuple<formatter<remove_cvref_t<Ts>, CharT>...> underlying_;// exposition only basic_string_view<CharT> separator_ =// exposition only __STATICALLY_WIDEN<CharT>(", "); basic_string_view<CharT> opening_bracket_ =// exposition only __STATICALLY_WIDEN<CharT>("("); basic_string_view<CharT> closing_bracket_ =// exposition only __STATICALLY_WIDEN<CharT>(")");   public:constexprvoid set_separator(basic_string_view<CharT> sep);constexprvoid set_brackets(basic_string_view<CharT> opening, basic_string_view<CharT> closing);   template<class ParseContext>constexprtypename ParseContext::iterator parse(ParseContext& ctx);   template<class FormatContext>typename FormatContext::iterator format(/* see description */& elems, FormatContext& ctx)const;};}

Class std::format_error

namespace std {class format_error :public runtime_error {public:explicit format_error(const string& what_arg);explicit format_error(constchar* what_arg);};}