使用Visual studio code 配置好C++开发环境后想在main函数里引用其他文件的函数，但是在Mac下总是报Undefined symbols for architecture x86_64:错误

代码如下：
a.h文件

int func();

a.cpp文件

#include <iostream>
#include "a.h"
using namespace std;
int func(){
    return 0;
}

main.cpp文件

#include <iostream>

#include "a.h"

using namespace std;
int main()
{
    int b = func();
    cout << b << endl;
}

按F5 启动调试 输出：

> Executing task: g++ /Users/stanhu/Desktop/Git/Foundation/CPP_Learn/main.cpp -o /Users/stanhu/Desktop/Git/Foundation/CPP_Learn/main.out -g -Wall -fcolor-diagnostics -std=c++11 <

Undefined symbols for architecture x86_64:
  "func()", referenced from:
      _main in main-d53c96.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
The terminal process terminated with exit code: 1

怎么解决这个问题呢。另外，如果怎么正常引用第三方的h文件？