1. #include <string.h>

C语言的头文件，包含比如strcpy之类的字符串处理函数。注意C语言里没有string类的概念，不要弄混。

 

2. #include <string>

C++的头文件，包含std::string的定义，属于STL范畴

 

3. #include <cstring>

C++版本的头文件，和C语言版本的<string.h>对应，将C语言的函数定义在std命名空间内。

定义：

/** @file cstring
 *  This is a Standard C++ Library file.  You should @c \#include this file
 *  in your programs, rather than any of the @a *.h implementation files.
 *
 *  This is the C++ version of the Standard C Library header @c string.h,
 *  and its contents are (mostly) the same as that header, but are all
 *  contained in the namespace @c std (except for names which are defined
 *  as macros in C).
 */

//
// ISO C++ 14882: 20.4.6  C library
//

#pragma GCC system_header

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

#ifndef _GLIBCXX_CSTRING
#define _GLIBCXX_CSTRING 1

// Get rid of those macros defined in <string.h> in lieu of real functions.
#undef memchr
#undef memcmp
#undef memcpy
#undef memmove
#undef memset
#undef strcat
#undef strchr
#undef strcmp
#undef strcoll
#undef strcpy
#undef strcspn
#undef strerror
#undef strlen
#undef strncat
#undef strncmp
#undef strncpy
#undef strpbrk
#undef strrchr
#undef strspn
#undef strstr
#undef strtok
#undef strxfrm

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

  using ::memchr;
  using ::memcmp;
  using ::memcpy;
  using ::memmove;
  using ::memset;
  using ::strcat;
  using ::strcmp;
  using ::strcoll;
  using ::strcpy;
  using ::strcspn;
  using ::strerror;
  using ::strlen;
  using ::strncat;
  using ::strncmp;
  using ::strncpy;
  using ::strspn;
  using ::strtok;
  using ::strxfrm;
  using ::strchr;
  using ::strpbrk;
  using ::strrchr;
  using ::strstr;

#ifndef __CORRECT_ISO_CPP_STRING_H_PROTO
  inline void*
  memchr(void* __s, int __c, size_t __n)
  { return __builtin_memchr(__s, __c, __n); }

  inline char*
  strchr(char* __s, int __n)
  { return __builtin_strchr(__s, __n); }

  inline char*
  strpbrk(char* __s1, const char* __s2)
  { return __builtin_strpbrk(__s1, __s2); }

  inline char*
  strrchr(char* __s, int __n)
  { return __builtin_strrchr(__s, __n); }

  inline char*
  strstr(char* __s1, const char* __s2)
  { return __builtin_strstr(__s1, __s2); }
#endif

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace