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      std::format_to_n, std::format_to_n_result

      From cppreference.com
      < cpp‎ | utility‎ | format
       
      C++
       
      Text processing library
       
      Formatting library
      Standard format specification
      Formatting functions
      (C++20)
      (C++20)
      format_to_n
      (C++20)
      (C++20)
      (C++20)
      (C++20)
      Format strings
      (C++20)(C++20)(C++20)
      (C++26)
      Formatting concepts
      (C++23)
      Formatter
      (C++20)
      (C++23)
      (C++23)
      (C++23)
      (C++23)
      (C++20)(C++20)(C++20)
      (C++20)(C++20)(C++20)
      (C++23)
      (C++23)
      Formatting arguments
      (C++20)
      (C++20)
      (C++20)(C++20)(C++20)
      (C++20)(deprecated in C++26)
      (C++20)(C++20)
      Format error
      (C++20)
       
      Defined in header <format>
      template<class OutputIt, class... Args>

      std::format_to_n_result<OutputIt>
          format_to_n( OutputIt out, std::iter_difference_t<OutputIt> n,

                       std::format_string<Args...> fmt, Args&&... args);
      		 (1) (since C++20)
      template<class OutputIt, class... Args>

      std::format_to_n_result<OutputIt>
          format_to_n( OutputIt out, std::iter_difference_t<OutputIt> n,

                       std::wformat_string<Args...> fmt, Args&&... args);
      		 (2) (since C++20)
      template<class OutputIt, class... Args>

      std::format_to_n_result<OutputIt>
          format_to_n( OutputIt out, std::iter_difference_t<OutputIt> n,
                       conststd::locale& loc,

                       std::format_string<Args...> fmt, Args&&... args);
      		 (3) (since C++20)
      template<class OutputIt, class... Args>

      std::format_to_n_result<OutputIt>
          format_to_n( OutputIt out, std::iter_difference_t<OutputIt> n,
                       conststd::locale& loc,

                       std::wformat_string<Args...> fmt, Args&&... args);
      		 (4) (since C++20)
      Helper types
      template<class OutputIt >

      struct format_to_n_result {
          OutputIt out;
          std::iter_difference_t<OutputIt> size;

      };
      		 (5) (since C++20)

      Format args according to the format string fmt, and write the result to the output iterator out. At most n characters are written. If present, loc is used for locale-specific formatting.

      Let CharT be char for overloads (1,3), wchar_t for overloads (2,4).

      These overloads participate in overload resolution only if OutputIt satisfies the concept std::output_iterator<const CharT&>.

      The behavior is undefined if OutputIt does not model (meet the semantic requirements of) the concept std::output_iterator<const CharT&>, or if std::formatter<std::remove_cvref_t<Ti>, CharT> does not meet the BasicFormatter requirements for any Ti in Args.

      5)std::format_to_n_result has no base classes, or members other than out, size and implicitly declared special member functions.

      Contents

      [edit]Parameters

      out - iterator to the output buffer
      n - maximum number of characters to be written to the buffer
      fmt -
      [edit]
      an object that represents the format string. The format string consists of
      • ordinary characters (except { and }), which are copied unchanged to the output,
      • escape sequences {{ and }}, which are replaced with { and } respectively in the output, and
      • replacement fields.

      Each replacement field has the following format:

      {arg-id(optional)} (1)
      {arg-id(optional):format-spec} (2)
      1) replacement field without a format specification
      2) replacement field with a format specification
      arg-id - specifies the index of the argument in args whose value is to be used for formatting; if it is omitted, the arguments are used in order.

      The arg-id s in a format string must all be present or all be omitted. Mixing manual and automatic indexing is an error.

      format-spec - the format specification defined by the std::formatter specialization for the corresponding argument. Cannot start with }.

      (since C++23)
      (since C++26)
      • For other formattable types, the format specification is determined by user-defined formatter specializations.
      args... - arguments to be formatted
      loc - std::locale used for locale-specific formatting

      [edit]Return value

      A format_to_n_result such that the out member is an iterator past the end of the output range, and the size member is the total (not truncated) output size.

      [edit]Exceptions

      Propagates any exception thrown by formatter or iterator operations.

      [edit]Notes

      The libstdc++ implementation prior to GCC-13.3 had a bug in reporting the correct format_to_n_result::out value.

      [edit]Example

      At Godbolt's Compiler Explorer: clang (trunk) + libc++, GCC (trunk) + libstdc++.

      Run this code
      #include <format>#include <initializer_list>#include <iomanip>#include <iostream>#include <string_view>   int main(){char buffer[64];   for(std::size_t max_chars_to_write :{std::size(buffer)-1, 23uz, 21uz}){const std::format_to_n_result result = std::format_to_n( buffer, max_chars_to_write, "Hubble's H{2} {3} {0}{4}{1} km/sec/Mpc.", // 24 bytes w/o formatters71, // {0}, occupies 2 bytes8, // {1}, occupies 1 byte"\u2080", // {2}, occupies 3 bytes, '₀' (SUBSCRIPT ZERO)"\u2245", // {3}, occupies 3 bytes, '≅' (APPROXIMATELY EQUAL TO)"\u00B1"// {4}, occupies 2 bytes, '±' (PLUS-MINUS SIGN));// 24 + 2 + 1 + 3 + 3 + 2 == 35, no trailing '\0'   *result.out='\0';// adds terminator to buffer   conststd::string_view str(buffer, result.out);   std::cout<<"Buffer until '\\0': "<<std::quoted(str)<<'\n'<<"Max chars to write: "<< max_chars_to_write <<'\n'<<"result.out offset: "<< result.out- buffer <<'\n'<<"Untruncated output size: "<< result.size<<"\n\n";}}

      Output: 

      Buffer until '\0': "Hubble's H₀ ≅ 71±8 km/sec/Mpc." Max chars to write: 63 result.out offset: 35 Untruncated output size: 35   Buffer until '\0': "Hubble's H₀ ≅ 71±8" Max chars to write: 23 result.out offset: 23 Untruncated output size: 35   Buffer until '\0': "Hubble's H₀ ≅ 71�" Max chars to write: 21 result.out offset: 21 Untruncated output size: 35

      [edit]Defect reports

      The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

      DR Applied to Behavior as published Correct behavior
      P2216R3C++20 throws std::format_error for invalid format string invalid format string results in compile-time error
      P2418R2C++20 objects that are neither const-usable nor copyable
      (such as generator-like objects) are not formattable       		allow formatting these objects
      P2508R1C++20 there's no user-visible name for this facility the name basic_format_string is exposed

      [edit]See also

      (C++20)
      		 stores formatted representation of the arguments in a new string
      (function template)[edit]
      (C++20)
      		 writes out formatted representation of its arguments through an output iterator
      (function template)[edit]
      (C++20)
      		 determines the number of characters necessary to store the formatted representation of its arguments
      (function template)[edit]
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