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

This header is part of the format 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_context
  template<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_args
  template<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_string
  template<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 functions
  template<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);
 
  // formatter
  template<class T, class CharT = char>
    struct formatter;
 
  // variable template enable_nonlocking_formatter_optimization
  template<class T>
    constexpr bool enable_nonlocking_formatter_optimization = false;
 
  // concept formattable
  template<class T, class CharT>
    concept formattable = /* see description */;
 
  template<class R, class CharT>
    concept __const_formattable_range =                               // exposition only
      ranges::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_context
  template<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_kind
  enum class 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_formatter
  template<class T, class CharT = char>
    requires same_as<remove_cvref_t<T>, T> && formattable<T, CharT>
  class range_formatter;
 
  // class template range-default-formatter
  template<range_format K, ranges::input_range R, class CharT>
    struct __range_default_formatter;                                 // exposition only
 
  // specializations for maps, sets, and strings
  template<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_arg
  template<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-store
  template<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_error
  class 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() const noexcept { 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 only
    enum indexing { unknown, manual, automatic };       // exposition only
    indexing indexing_;                                 // exposition only
    size_t next_arg_id_;                                // exposition only
    size_t num_args_;                                   // exposition only
 
  public:
    constexpr explicit 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() const noexcept;
    constexpr const_iterator end() const noexcept;
    constexpr void advance_to(const_iterator it);
 
    constexpr size_t next_arg_id();
    constexpr void check_arg_id(size_t id);
 
    template<class... Ts>
    constexpr void check_dynamic_spec(size_t id) noexcept;
    constexpr void check_dynamic_spec_integral(size_t id) noexcept;
    constexpr void 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) const noexcept;
    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:
    constexpr void set_separator(basic_string_view<CharT> sep);
    constexpr void set_brackets(basic_string_view<CharT> opening,
                                basic_string_view<CharT> closing);
    constexpr formatter<T, CharT>& underlying() { return underlying_; }
    constexpr const formatter<T, CharT>& underlying() const { return underlying_; }
 
    template<class ParseContext>
      constexpr typename 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:
    constexpr void set_separator(basic_string_view<CharT> sep);
    constexpr void set_brackets(basic_string_view<CharT> opening,
                                basic_string_view<CharT> closing);
 
    template<class ParseContext>
      constexpr typename 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 only
    using __element_type =                                          // exposition only
      remove_cvref_t<ranges::range_reference_t<__maybe_const_map>>;
    range_formatter<__element_type, CharT> underlying_;             // exposition only
  public:
    constexpr __range_default_formatter();
 
    template<class ParseContext>
      constexpr typename 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 only
  public:
    constexpr __range_default_formatter();
 
    template<class ParseContext>
      constexpr typename 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>
      constexpr typename 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, unsigned int, long long int, unsigned long long int,
            float, double, long double,
            const char_type*, basic_string_view<char_type>,
            const void*, handle> value;                                // exposition only
 
    template<class T> explicit basic_format_arg(T&& v) noexcept;       // exposition only
    explicit basic_format_arg(float n) noexcept;                       // exposition only
    explicit basic_format_arg(double n) noexcept;                      // exposition only
    explicit basic_format_arg(long double n) noexcept;                 // exposition only
    explicit 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() const noexcept;
 
    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 {
    const void* ptr_;                                           // exposition only
    void (*format_)(basic_format_parse_context<char_type>&,
                    Context&, const void*);                     // exposition only
 
    template<class T> explicit handle(T&& val) noexcept;        // exposition only
 
    friend class 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 only
    const 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) const noexcept;
  };
}

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:
    constexpr void set_separator(basic_string_view<CharT> sep);
    constexpr void set_brackets(basic_string_view<CharT> opening,
                                basic_string_view<CharT> closing);
 
    template<class ParseContext>
      constexpr typename 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(const char* what_arg);
  };
}

Compiler support
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Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

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

Concepts

Enumerations

Classes

Formatter specializations

Functions

Helpers

Synopsis

Class template std::basic_format_string

Concept std::formattable

Class template std::basic_format_parse_context

Class template std::basic_format_context

Variable template std::format_kind

Class template std::range_formatter

Class template __range_default_formatter

Specialization of __range_default_formatter for maps

Specialization of __range_default_formatter for sets

Specialization of __range_default_formatter for strings

Class template std::basic_format_arg

Class std::basic_format_arg::handle

Class template __format_arg_store

Class template std::basic_format_args

Tuple formatter

Class std::format_error

Navigation

Concept `std::formattable`