只引用头文件不能调用函数

Question

```c
//源.cpp
#include "add.h"
//#include "add.c"

int main()
{
    int a = 10;
    int b = 20;
    //函数声明
    //int add(int x, int y);

    printf("%d\n", add(a, b));
    return 0;
}

//add.h
#pragma once

int add(int x, int y);

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

```

Answer 1

add.c中也需要#include "add.h"