C++ Interface Throwing Unresolved External Symbol Error

Question

Alright, I've got what should be an easy question but there are literally no examples of what I am looking for online, only copies of the same example which does everything in one file. I'm trying to make an interface and a method to use it like the example below, but I am getting unresolved external symbol errors from visual studio when I compile. Any ideas about what I am doing wrong?

IFoo.h

class IFoo {
public:
    virutal int Bar();
}

Foo.h

class Foo : public IFoo {
    virtual int Bar();
}

Foo.cpp

int Foo::Bar() {
    return 1;
}

Answer 1

In IFoo.h, the virtual method must be pure virtual:

class IFoo {
public:
    virtual int Bar() = 0;
};

Alternately, provide an default implementation, either inline or in an IFoo.cpp. However, that would make the I prefix misleading, as it wouldn't act like an interface, then.

Answer 2

Assuming that the unresolved symbol is for int IFoo::Bar(), please note that this function is declared to be a callable function and it can, indeed, be called, e.g. using

p->IFoo::Bar()

That is, you either need to define this function or you need to declare it as not existing:

virtual int Bar() = 0;

(which still allows you to define it, though).