Edit

Editing: stringfwd.h

// <string> Forward declarations -*- C++ -*-

// Copyright (C) 2001-2018 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file bits/stringfwd.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{string}
 */

//
// ISO C++ 14882: 21 Strings library
//

#ifndef _STRINGFWD_H
#define _STRINGFWD_H 1

#pragma GCC system_header

#include <bits/c++config.h>
#include <bits/memoryfwd.h>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

/**
   *  @defgroup strings Strings
   *
   *  @{
  */

template<class _CharT>
    struct char_traits;

template<> struct char_traits<char>;

#ifdef _GLIBCXX_USE_WCHAR_T
  template<> struct char_traits<wchar_t>;
#endif

#if ((__cplusplus >= 201103L) \
     && defined(_GLIBCXX_USE_C99_STDINT_TR1))
  template<> struct char_traits<char16_t>;
  template<> struct char_traits<char32_t>;
#endif

_GLIBCXX_BEGIN_NAMESPACE_CXX11

template<typename _CharT, typename _Traits = char_traits<_CharT>,
           typename _Alloc = allocator<_CharT> >
    class basic_string;

/// A string of @c char
  typedef basic_string<char>    string;

#ifdef _GLIBCXX_USE_WCHAR_T
  /// A string of @c wchar_t
  typedef basic_string<wchar_t> wstring;   
#endif

#if ((__cplusplus >= 201103L) \
     && defined(_GLIBCXX_USE_C99_STDINT_TR1))
  /// A string of @c char16_t
  typedef basic_string<char16_t> u16string;

/// A string of @c char32_t
  typedef basic_string<char32_t> u32string; 
#endif

_GLIBCXX_END_NAMESPACE_CXX11

/** @}  */

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif	// _STRINGFWD_H

Back