"undefined reference to `vtable.." - How to know which method isn't defined?

Question

When linking e.g. with GCC on linux I get:

undefined reference to `vtable for MyClass'

Problem is, the MyClass ancestors have about 100 pure virtual methods, and I don't know which of them isn't defined. Do I really have to go through all of them to find out?

Yes and no. You could try (from above link): `nm -C NameOfObjectFile.o | grep MyClass::` — Jonas, Nov 24 '16 at 09:50
@Jonas: I'm afraid that doesn't help me. I need to know which methods are **not** defined. — Yaron Cohen-Tal, Nov 24 '16 at 09:53
I suppose you should made a list of all methods, and the subtract those the previous command returns. — Jonas, Nov 24 '16 at 09:54

score 3 · Answer 1 · edited May 23 '17 at 12:13

As mentioned down on the link from @Jonas comment, the message "undefined reference to vtable" only happens if the first declared virtual method is not defined: https://stackoverflow.com/a/28458010/1274747

And as I tried just now, this is really the case: If the first declared overridden virtual method is not defined, I get the vtable error. If some other function down the line is not defined, I get undefined reference error of that particular method.

Example:

struct base
{
    virtual ~base() {}
    virtual int func1() = 0;
    virtual int func2() = 0;
};

struct derived: base
{
    virtual int func1();
    virtual int func2();
};

// switch the comment to define one or the other
int derived::func1() // undefined reference to `derived::func2()'
// int derived::func2() // "undefined reference to `vtable for derived'"
{
    return 5;
}

int main()
{
    derived d;
    return 0;
}

That means, check the first virtual function declared in the derived class (it might also be a virtual destructor).

Thanx! It would have been nice though if the compiler/linker were smart enough to tell me that :) — Yaron Cohen-Tal, Nov 24 '16 at 22:03

"undefined reference to `vtable.." - How to know which method isn't defined?

1 Answers1