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The C and C++ Include Header Files

cat -n /usr/include/c++/15/ciso646

     1	// -*- C++ -*- forwarding header.
     2	
     3	// Copyright (C) 2001-2025 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 ciso646
    26	 *  This is a Standard C++ Library file.  You should @c \#include this file
    27	 *  in your programs, rather than any of the @a *.h implementation files.
    28	 *
    29	 *  This is the C++ version of the Standard C Library header @c iso646.h,
    30	 *  which is empty in C++.
    31	 *
    32	 *  @since C++11 (removed in C++20)
    33	 */
    34	#ifndef _GLIBCXX_CISO646
    35	#define _GLIBCXX_CISO646
    36	
    37	#ifdef _GLIBCXX_SYSHDR
    38	#pragma GCC system_header
    39	#endif
    40	
    41	#include <bits/c++config.h>
    42	
    43	#if __cplusplus >= 202002L
    44	#  if ! _GLIBCXX_USE_DEPRECATED
    45	#    error "<ciso646> is not a standard header since C++20, use <version> to detect implementation-specific macros"
    46	#  elif defined __DEPRECATED
    47	#    pragma GCC diagnostic push
    48	#    pragma GCC diagnostic ignored "-Wc++23-extensions"
    49	#    warning "<ciso646> is not a standard header since C++20, use <version> to detect implementation-specific macros"
    50	#    pragma GCC diagnostic pop
    51	#  endif
    52	#endif
    53	
    54	#endif