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
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