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

From cppreference.com

This header is part of the text processing library.

Concepts

(C++23)

specifies that a type is formattable, that is, it specializes std::formatter and provides member functions parse and format
(concept) [edit]

Enumerations

range_format

(C++23)

specifies how a range should be formatted
(enum) [edit]

Classes

formatter

(C++20)

defines formatting rules for a given type
(class template) [edit]

range_formatter

(C++23)

class template that helps implementing std::formatter specializations for range types
(class template) [edit]

basic_format_parse_contextformat_parse_contextwformat_parse_context

(C++20)(C++20)(C++20)

formatting string parser state
(class template) [edit]

basic_format_contextformat_contextwformat_context

(C++20)(C++20)(C++20)

formatting state, including all formatting arguments and the output iterator
(class template) [edit]

basic_format_arg

(C++20)

class template that provides access to a formatting argument for user-defined formatters
(class template) [edit]

basic_format_argsformat_argswformat_args

(C++20)(C++20)(C++20)

class that provides access to all formatting arguments
(class template) [edit]

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) [edit]

format_error

(C++20)

exception type thrown on formatting errors
(class) [edit]

Formatter specializations

std::formatter<pair-or-tuple>

(C++23)

formatting support for pair and tuple
(class template specialization) [edit]

std::formatter<range>

(C++23)

formatting support for ranges
(class template specialization) [edit]

Functions

format

(C++20)

stores formatted representation of the arguments in a new string
(function template) [edit]

format_to

(C++20)

writes out formatted representation of its arguments through an output iterator
(function template) [edit]

format_to_n

(C++20)

writes out formatted representation of its arguments through an output iterator, not exceeding specified size
(function template) [edit]

formatted_size

(C++20)

determines the number of characters necessary to store the formatted representation of its arguments
(function template) [edit]

runtime_format

(C++26)

creates runtime format strings directly usable in user-oriented formatting functions
(function) [edit]

vformat

(C++20)

non-template variant of std::format using type-erased argument representation
(function) [edit]

vformat_to

(C++20)

non-template variant of std::format_to using type-erased argument representation
(function template) [edit]

visit_format_arg

(C++20) (deprecated in C++26)

argument visitation interface for user-defined formatters
(function template) [edit]

make_format_argsmake_wformat_args

(C++20)(C++20)

creates a type-erased object referencing all formatting arguments, convertible to format_args
(function template) [edit]

Helpers

format_kind

(C++23)

selects a suited std::range_format for a range
(variable template)[edit]

enable_nonlocking_formatter_optimization

(C++23)

indicates the argument type can be efficiently printed
(variable template)[edit]

[edit] 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 CharT> struct /*runtime-format-string*/
  {                                   // exposition-only
  private:
    basic_string_view<CharT> /*str*/; // exposition-only
  public:
    /*runtime-format-string*/(basic_string_view<CharT> s) noexcept
      : /*str*/(s)
    {
    }
    /*runtime-format-string*/(const /*runtime-format-string*/&) = delete;
    /*runtime-format-string*/& operator=(const /*runtime-format-string*/&) = delete;
  };
  /*runtime-format-string*/<char> runtime_format(string_view fmt) noexcept { return fmt; }
  /*runtime-format-string*/<wchar_t> runtime_format(wstring_view fmt) noexcept
  {
    return fmt;
  }
 
  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;
 
  // formatter locking
  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>
  {};
 
  template<ranges::input_range R>
    requires(format_kind<R> != range_format::disabled)
  constexpr bool enable_nonlocking_formatter_optimization<R> = false;
 
  // arguments
  // class template basic_format_arg
  template<class Context> class basic_format_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;
}

[edit] 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);
    basic_format_string(/*runtime-format-string*/<CharT> s) noexcept
      : str(s./*str*/)
    {
    }
 
    constexpr basic_string_view<CharT> get() const noexcept { return /*str*/; }
  };
}

This section is incomplete
Reason: create page: concept formattable

[edit] 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>>;
    };

[edit] 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) 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;
  };
}

[edit] 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);
  };
}

[edit] 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 */;

[edit] 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_*/ =
      /*STATICALLY-WIDEN*/<CharT>(", "); // exposition-only
    basic_string_view<CharT> /*opening-bracket_*/ =
      /*STATICALLY-WIDEN*/<CharT>("[");  // exposition-only
    basic_string_view<CharT> /*closing-bracket_*/ =
      /*STATICALLY-WIDEN*/<CharT>("]");  // exposition-only
 
  public:
    constexpr void set_separator(basic_string_view<CharT> sep) noexcept;
    constexpr void set_brackets(basic_string_view<CharT> opening,
                                basic_string_view<CharT> closing) noexcept;
    constexpr formatter<T, CharT>& underlying() noexcept { return /*underlying_*/; }
    constexpr const formatter<T, CharT>& underlying() const noexcept
    {
      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;
  };
}

[edit] Class template __range_default_formatter

namespace std {
  template<ranges::input_range R, class CharT>
  struct /*range-default-formatter*/<range_format::sequence, R, CharT>
  {                                                          // exposition-only
  private:
    using /*maybe-const-r*/ = /*fmt-maybe-const*/<R, CharT>; // exposition-only
    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) noexcept;
    constexpr void set_brackets(basic_string_view<CharT> opening,
                                basic_string_view<CharT> closing) noexcept;
 
    template<class ParseContext>
    constexpr typename ParseContext::iterator parse(ParseContext& ctx);
 
    template<class FormatContext>
    typename FormatContext::iterator format(/*maybe-const-r*/& elems,
                                            FormatContext& ctx) const;
  };
}

[edit] 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;
  };
}

[edit] 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;
  };
}

[edit] 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;
  };
}

[edit] 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
 
  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);
  };
}

[edit] 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
 
  public:
    void format(basic_format_parse_context<char_type>&, Context& ctx) const;
  };
}

[edit] 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
  };
}

[edit] 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;
  };
 
  template<class Context, class... Args>
  basic_format_args(/*format-arg-store*/<Context, Args...>) -> basic_format_args<Context>;
}

[edit] 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_*/ =
      /*STATICALLY-WIDEN*/<CharT>(", ");                            // exposition-only
    basic_string_view<CharT> /*opening-bracket_*/ =
      /*STATICALLY-WIDEN*/<CharT>("(");                             // exposition-only
    basic_string_view<CharT> /*closing-bracket_*/ =
      /*STATICALLY-WIDEN*/<CharT>(")");                             // exposition-only
 
  public:
    constexpr void set_separator(basic_string_view<CharT> sep) noexcept;
    constexpr void set_brackets(basic_string_view<CharT> opening,
                                basic_string_view<CharT> closing) noexcept;
 
    template<class ParseContext>
    constexpr typename ParseContext::iterator parse(ParseContext& ctx);
 
    template<class FormatContext>
    typename FormatContext::iterator format(/* see description */ &elems,
                                            FormatContext& ctx) const;
  };
 
  template<class... Ts>
  constexpr bool enable_nonlocking_formatter_optimization</*pair-or-tuple*/<Ts...>> =
    (enable_nonlocking_formatter_optimization<Ts> && ...);
}

[edit] Class std::format_error

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

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

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[edit] Synopsis

[edit] Class template std::basic_format_string

[edit] Concept `std::formattable`

[edit] Class template std::basic_format_parse_context

[edit] Class template std::basic_format_context

[edit] Variable template std::format_kind

[edit] Class template std::range_formatter

[edit] Class template __range_default_formatter

[edit] Specialization of __range_default_formatter for maps

[edit] Specialization of __range_default_formatter for sets

[edit] Specialization of __range_default_formatter for strings

[edit] Class template std::basic_format_arg

[edit] Class std::basic_format_arg::handle

[edit] Class template __format_arg_store

[edit] Class template std::basic_format_args

[edit] Tuple formatter

[edit] Class std::format_error

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

Contents

Concepts

Enumerations

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Formatter specializations

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[edit] Synopsis

[edit] Class template std::basic_format_string

[edit] Concept std::formattable

[edit] Class template std::basic_format_parse_context

[edit] Class template std::basic_format_context

[edit] Variable template std::format_kind

[edit] Class template std::range_formatter

[edit] Class template __range_default_formatter

[edit] Specialization of __range_default_formatter for maps

[edit] Specialization of __range_default_formatter for sets

[edit] Specialization of __range_default_formatter for strings

[edit] Class template std::basic_format_arg

[edit] Class std::basic_format_arg::handle

[edit] Class template __format_arg_store

[edit] Class template std::basic_format_args

[edit] Tuple formatter

[edit] Class std::format_error

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[edit] Concept `std::formattable`