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The C and C++ Include Header Files

cat -n /usr/include/c++/15/bits/charconv.h

     1	// Numeric conversions (to_string, to_chars) -*- C++ -*-
     2	
     3	// Copyright (C) 2017-2025 Free Software Foundation, Inc.
     4	//
     5	// This file is part of the GNU ISO C++ Library.  This library is free
     6	// software; you can redistribute it and/or modify it under the
     7	// terms of the GNU General Public License as published by the
     8	// Free Software Foundation; either version 3, or (at your option)
     9	// any later version.
    10	
    11	// This library is distributed in the hope that it will be useful,
    12	// but WITHOUT ANY WARRANTY; without even the implied warranty of
    13	// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    14	// GNU General Public License for more details.
    15	
    16	// Under Section 7 of GPL version 3, you are granted additional
    17	// permissions described in the GCC Runtime Library Exception, version
    18	// 3.1, as published by the Free Software Foundation.
    19	
    20	// You should have received a copy of the GNU General Public License and
    21	// a copy of the GCC Runtime Library Exception along with this program;
    22	// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
    23	// <http://www.gnu.org/licenses/>.
    24	
    25	/** @file bits/charconv.h
    26	 *  This is an internal header file, included by other library headers.
    27	 *  Do not attempt to use it directly. @headername{charconv}
    28	 */
    29	
    30	#ifndef _GLIBCXX_CHARCONV_H
    31	#define _GLIBCXX_CHARCONV_H 1
    32	
    33	#ifdef _GLIBCXX_SYSHDR
    34	#pragma GCC system_header
    35	#endif
    36	
    37	#if __cplusplus >= 201103L
    38	
    39	#include <type_traits>
    40	#include <ext/numeric_traits.h>
    41	
    42	namespace std _GLIBCXX_VISIBILITY(default)
    43	{
    44	_GLIBCXX_BEGIN_NAMESPACE_VERSION
    45	namespace __detail
    46	{
    47	#if __cpp_variable_templates
    48	  // This accepts 128-bit integers even in strict mode.
    49	  template<typename _Tp>
    50	    constexpr bool __integer_to_chars_is_unsigned
    51	      = ! __gnu_cxx::__int_traits<_Tp>::__is_signed;
    52	#endif
    53	
    54	  // Generic implementation for arbitrary bases.
    55	  template<typename _Tp>
    56	    _GLIBCXX14_CONSTEXPR unsigned
    57	    __to_chars_len(_Tp __value, int __base = 10) noexcept
    58	    {
    59	#if __cpp_variable_templates
    60	      static_assert(__integer_to_chars_is_unsigned<_Tp>, "implementation bug");
    61	#endif
    62	
    63	      unsigned __n = 1;
    64	      const unsigned __b2 = __base  * __base;
    65	      const unsigned __b3 = __b2 * __base;
    66	      const unsigned long __b4 = __b3 * __base;
    67	      for (;;)
    68		{
    69		  if (__value < (unsigned)__base) return __n;
    70		  if (__value < __b2) return __n + 1;
    71		  if (__value < __b3) return __n + 2;
    72		  if (__value < __b4) return __n + 3;
    73		  __value /= __b4;
    74		  __n += 4;
    75		}
    76	    }
    77	
    78	  // Write an unsigned integer value to the range [first,first+len).
    79	  // The caller is required to provide a buffer of exactly the right size
    80	  // (which can be determined by the __to_chars_len function).
    81	  template<typename _Tp>
    82	    _GLIBCXX23_CONSTEXPR void
    83	    __to_chars_10_impl(char* __first, unsigned __len, _Tp __val) noexcept
    84	    {
    85	#if __cpp_variable_templates
    86	      static_assert(__integer_to_chars_is_unsigned<_Tp>, "implementation bug");
    87	#endif
    88	
    89	      constexpr char __digits[201] =
    90		"0001020304050607080910111213141516171819"
    91		"2021222324252627282930313233343536373839"
    92		"4041424344454647484950515253545556575859"
    93		"6061626364656667686970717273747576777879"
    94		"8081828384858687888990919293949596979899";
    95	      unsigned __pos = __len - 1;
    96	      while (__val >= 100)
    97		{
    98		  auto const __num = (__val % 100) * 2;
    99		  __val /= 100;
   100		  __first[__pos] = __digits[__num + 1];
   101		  __first[__pos - 1] = __digits[__num];
   102		  __pos -= 2;
   103		}
   104	      if (__val >= 10)
   105		{
   106		  auto const __num = __val * 2;
   107		  __first[1] = __digits[__num + 1];
   108		  __first[0] = __digits[__num];
   109		}
   110	      else
   111		__first[0] = '0' + __val;
   112	    }
   113	
   114	} // namespace __detail
   115	_GLIBCXX_END_NAMESPACE_VERSION
   116	} // namespace std
   117	#endif // C++11
   118	#endif // _GLIBCXX_CHARCONV_H