C++头文件联合编译

main.cpp

#include <iostream>
#include "add.h" // this brings in the declaration for add()

int main()
{
   using namespace std;
   cout << "The sum of 3 and 4 is " << add(3,4) << endl;
   return 0;
}

add.cpp

int add(int x,int y)
{
   return x+y;
}

add.h

#ifndef ADD_H
#define ADD_H

int add(int x,int y); //function prototype for add.h

#endif

下Microsoft Visual C++ 2010
IDE下编译自动识别，编译不会出错
在Linux下
直接编译main.cpp是会报

 g++ -o main main.cpp
/tmp/ccVwA0IL.o: In function `main':
main.cpp:(.text+0x1a): undefined reference to `add(int, int)'
collect2: ld 返回 1

解决方法
g++ -o main add.cpp main.cpp
编译通过