Why can I compile successfully without implementing one of the functions?

Question

I would like to understand how do classes compile in C++.

Why does the code below compile successfully? Doesn't Foo() need an implementation for compilation to be successful?

class Test{
public:
    Test()  {}
    int Foo();
};

int main()
{
    Test obj;
    return 0;
}

"How does a compiler work?" - http://en.wikipedia.org/wiki/Compiler — Merlyn Morgan-Graham, Nov 06 '11 at 19:02
After editing this question it may actually be useful, maybe it should be un-closed? — Septagram, Nov 06 '11 at 20:35

score 7 · Answer 1 · answered Nov 06 '11 at 19:03

7

Nobody tries to call Foo, so the linker doesn't complain about the missing implementation, because it's not needed.

If you wrote virtual int Foo();, you would see a linker error.

answered Nov 06 '11 at 19:03

Karoly Horvath

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Why does _**virtual**_ cause the error ? What changes ? – Emadpres Nov 06 '11 at 19:11
1

@EmAdpres: See http://stackoverflow.com/q/3560786/14065 – Martin York Nov 06 '11 at 19:25
1

@EmAdpres: you need an entry in the vtable. – Karoly Horvath Nov 06 '11 at 19:30

score 2 · Answer 2 · answered Nov 06 '11 at 19:00

2

There is no Book() in your code. If you are talking about Foo(), you are never using that function within your code, so the linker never gets asked about it and fail with "undefined function" or some similar error.

answered Nov 06 '11 at 19:00

K-ballo

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Why can I compile successfully without implementing one of the functions?

2 Answers2