我是C ++新手，感到困惑

Question

我是C ++的新手，请键入以下代码来练习指针和取消引用运算符。但是，当我打印出来时，它给出了一条错误消息。我检查了错误消息，它们是来自链接器的。有人可以解释我做错了什么吗？

我正在使用Microsoft Visual Studio 2019社区。

using namespace std;
/*
Errors: 
LNK1120
LNK2019
*/

int main() {
    int num1;
    int *pNum;
    int pointed;
    num1 = 3;
    pNum = &pointed;
    pointed = 5;
    passByValue(num1);
    std::cout << "In main, num1 is " << num1 << endl;
    return 0;
}

---

using namespace std;
void passByValue(int num)
 {
cout << "You are using passByValue()." << endl;
cout << "num used to be " << num << "." << endl;
num = num*num;
cout << "num is now " << num << "." << endl;
}

我收到此错误：

Errors: LNK1120: 1 unresolved externals
LNK2019: unresolved external symbol "void__cdeclpassByValue(int)"?
passByValue@@YAXH@Z referenced in function_main