c++头文件_#include

头文件是c++自己定义库函数的地方，虽然它定义的函数的方式我们看不懂，但是像int,char,memset等函数都是他在头文件里给你编好的，下边给大家看一下我们熟悉的iostream头文件里面是什么样的：

// Standard iostream objects -*- C++ -*-
 
// Copyright (C) 1997-2014 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 include/iostream
 *  This is a Standard C++ Library header.
 */
 
//
// ISO C++ 14882: 27.3  Standard iostream objects
//
 
#ifndef _GLIBCXX_IOSTREAM
#define _GLIBCXX_IOSTREAM 1
 
#pragma GCC system_header
 
#include <bits/c++config.h>
#include <ostream>
#include <istream>
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
  /**
   *  @name Standard Stream Objects
   *
   *  The &lt;iostream&gt; header declares the eight <em>standard stream
   *  objects</em>.  For other declarations, see
   *  http://gcc.gnu.org/onlinedocs/libstdc++/manual/io.html
   *  and the @link iosfwd I/O forward declarations @endlink
   *
   *  They are required by default to cooperate with the global C
   *  library's @c FILE streams, and to be available during program
   *  startup and termination. For more information, see the section of the
   *  manual linked to above.
  */
  //@{
  extern istream cin;		/// Linked to standard input
  extern ostream cout;		/// Linked to standard output
  extern ostream cerr;		/// Linked to standard error (unbuffered)
  extern ostream clog;		/// Linked to standard error (buffered)
 
#ifdef _GLIBCXX_USE_WCHAR_T
  extern wistream wcin;		/// Linked to standard input
  extern wostream wcout;	/// Linked to standard output
  extern wostream wcerr;	/// Linked to standard error (unbuffered)
  extern wostream wclog;	/// Linked to standard error (buffered)
#endif
  //@}
 
  // For construction of filebuffers for cout, cin, cerr, clog et. al.
  static ios_base::Init __ioinit;
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
 
#endif /* _GLIBCXX_IOSTREAM */

iostream库里面就定义了我们所熟知的cin和cout函数

 其他的还有cerr，clog，wcin，wcout，wcerr，wclog等等函数。而有些人就不太明白了，那头文件还能导入头文件吗？

答案是肯定的，因为它里面的一些函数如static，extern，namespace等函数也需要引入头文件，因为里面定义函数的过程过于深奥，所以不在这里过多讲述。

下面再给大家啃一个不同的头文件内部：

// Components for manipulating sequences of characters -*- C++ -*-
 
// Copyright (C) 1997-2014 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 include/string
 *  This is a Standard C++ Library header.
 */
 
//
// ISO C++ 14882: 21  Strings library
//
 
#ifndef _GLIBCXX_STRING
#define _GLIBCXX_STRING	1
 
#pragma GCC system_header
 
#include <bits/c++config.h>
#include <bits/stringfwd.h>
#include <bits/char_traits.h>  // NB: In turn includes stl_algobase.h
#include <bits/allocator.h>
#include <bits/cpp_type_traits.h>
#include <bits/localefwd.h>    // For operators >>, <<, and getline.
#include <bits/ostream_insert.h>
#include <bits/stl_iterator_base_types.h>
#include <bits/stl_iterator_base_funcs.h>
#include <bits/stl_iterator.h>
#include <bits/stl_function.h> // For less
#include <ext/numeric_traits.h> 
#include <bits/stl_algobase.h> 
#include <bits/range_access.h>
#include <bits/basic_string.h>
#include <bits/basic_string.tcc> 
 
#endif /* _GLIBCXX_STRING */

这个头文件就是属于很多人也都知道的<string>头文件，看到这个头文件，你们有没有发现什么？对了，就是这个头文件string基本都是由其他的头文件组成的，什么<bits/range_access.h>等等的头文件。引入头文件就相当于把这个头文件拥有了其他头文件的能力，这个能力指的就是其他头文件定义的函数，数据类型等。我们的程序要导入头文件也是同一个原因。

那头文件有没有基本上都是程序的呢？也有。再来看下一个：

// Helpers for ostream inserters -*- C++ -*-
 
// Copyright (C) 2007-2014 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/ostream_insert.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{ostream}
 */
 
#ifndef _OSTREAM_INSERT_H
#define _OSTREAM_INSERT_H 1
 
#pragma GCC system_header
 
#include <iosfwd>
#include <bits/cxxabi_forced.h>
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
  template<typename _CharT, typename _Traits>
    inline void
    __ostream_write(basic_ostream<_CharT, _Traits>& __out,
		    const _CharT* __s, streamsize __n)
    {
      typedef basic_ostream<_CharT, _Traits>       __ostream_type;      
      typedef typename __ostream_type::ios_base    __ios_base;
 
      const streamsize __put = __out.rdbuf()->sputn(__s, __n);
      if (__put != __n)
	__out.setstate(__ios_base::badbit);
    }
 
  template<typename _CharT, typename _Traits>
    inline void
    __ostream_fill(basic_ostream<_CharT, _Traits>& __out, streamsize __n)
    {
      typedef basic_ostream<_CharT, _Traits>       __ostream_type;      
      typedef typename __ostream_type::ios_base    __ios_base;
 
      const _CharT __c = __out.fill();
      for (; __n > 0; --__n)
	{
	  const typename _Traits::int_type __put = __out.rdbuf()->sputc(__c);
	  if (_Traits::eq_int_type(__put, _Traits::eof()))
	    {
	      __out.setstate(__ios_base::badbit);
	      break;
	    }
	}
    }
 
  template<typename _CharT, typename _Traits>
    basic_ostream<_CharT, _Traits>&
    __ostream_insert(basic_ostream<_CharT, _Traits>& __out,
		     const _CharT* __s, streamsize __n)
    {
      typedef basic_ostream<_CharT, _Traits>       __ostream_type;
      typedef typename __ostream_type::ios_base    __ios_base;
 
      typename __ostream_type::sentry __cerb(__out);
      if (__cerb)
	{
	  __try
	    {
	      const streamsize __w = __out.width();
	      if (__w > __n)
		{
		  const bool __left = ((__out.flags()
					& __ios_base::adjustfield)
				       == __ios_base::left);
		  if (!__left)
		    __ostream_fill(__out, __w - __n);
		  if (__out.good())
		    __ostream_write(__out, __s, __n);
		  if (__left && __out.good())
		    __ostream_fill(__out, __w - __n);
		}
	      else
		__ostream_write(__out, __s, __n);
	      __out.width(0);
	    }
	  __catch(__cxxabiv1::__forced_unwind&)
	    {
	      __out._M_setstate(__ios_base::badbit);
	      __throw_exception_again;
	    }
	  __catch(...)
	    { __out._M_setstate(__ios_base::badbit); }
	}
      return __out;
    }
 
  // Inhibit implicit instantiations for required instantiations,
  // which are defined via explicit instantiations elsewhere.
#if _GLIBCXX_EXTERN_TEMPLATE
  extern template ostream& __ostream_insert(ostream&, const char*, streamsize);
 
#ifdef _GLIBCXX_USE_WCHAR_T
  extern template wostream& __ostream_insert(wostream&, const wchar_t*,
					     streamsize);
#endif
#endif
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
 
#endif /* _OSTREAM_INSERT_H */

这个是一个不太知名的头文件<ostream_insert.h>头文件，他基本全是由代码组成，也就是定义的函数和数据类型较多，导入的头文件较少。

接下来就到了一个重头戏：如何导入头文件？

这个问题可能有些人看完前面的就已经有答案了，就是#include+<导入头文件名称>就行了。

十分简单。