加C头文件是C++为了兼容纯C语法而对.h文件进行了处理，例如cstdio

// cstdio standard header

#pragma once

#ifndef _CSTDIO_

#define _CSTDIO_

#include

#ifdef _STD_USING

#undef _STD_USING

#include

#define _STD_USING

#else /* _STD_USING */

#include

#endif /* _STD_USING */

#define _HAS_CONVENTIONAL_CLIB 1

#define _IOBASE _base

#define _IOPTR _ptr

#define _IOCNT _cnt

#ifndef RC_INVOKED

#if _GLOBAL_USING

_STD_BEGIN

using ::size_t; using ::fpos_t; using ::FILE;

using ::clearerr; using ::fclose; using ::feof;

using ::ferror; using ::fflush; using ::fgetc;

using ::fgetpos; using ::fgets; using ::fopen;

using ::fprintf; using ::fputc; using ::fputs;

using ::fread; using ::freopen; using ::fscanf;

using ::fseek; using ::fsetpos; using ::ftell;

using ::fwrite; using ::getc; using ::getchar;

using ::gets; using ::perror;

using ::putc; using ::putchar;

using ::printf; using ::puts; using ::remove;

using ::rename; using ::rewind; using ::scanf;

using ::setbuf; using ::setvbuf; using ::sprintf;

using ::sscanf; using ::tmpfile; using ::tmpnam;

using ::ungetc; using ::vfprintf; using ::vprintf;

using ::vsprintf;

_STD_END

#endif /* _GLOBAL_USING */

#endif /* RC_INVOKED */

#ifndef _Filet

#define _Filet FILE

#endif /* _Filet */

#ifndef _FPOSOFF

#define _FPOSOFF(fp) ((long)(fp))

#endif /* _FPOSOFF */

#endif /* _CSTDIO_ */

/*

* Copyright (c) 1992-2005 by P.J. Plauger. ALL RIGHTS RESERVED.

* Consult your license regarding permissions and restrictions.

V4.05:0009 */

从上面的代码可以看出C++将所有原来.h中定义的东西都加入到了std命名空间中。。。。

所以C++代码中请尽量用加C头文件。