c/c++可以有多个函数声明，但实现只能有一个
例子：

    //file t_defs.h
    #ifndef _T_DEFS_H_
    #define _T_DEFS_H_

    void say(void);

    #endif

    //file t_include.cpp

    #include <iostream>
    #include "t_defs.h"

    using namespace std;

    void say(void);
    int main(void){

            say();

    }

    void say(void){

            cout << " hello" << endl;
    }

    //file t_include.cpp

    #include <iostream>
    #include "t_defs.h"

    using namespace std;

    void say(void);

    /* error： multiple definition of `say()'
    void say(void){

            cout << " hello" << endl;
    }
    ×/

#g++ t_include.cpp t_include_1.cpp
#./a.out

    hello