Legally invoking a pure virtual function

Question

I'm sure we've all seen code that crashes due to a bug that results in a pure virtual function being called. One simple example is like this: ``` struct Base { Base() { method(); } virtual void method() = 0; }; struct Derived : Base { void method() {}; }; int main() { Derived d; } ``` In this case, the call to `method()` in the `Base` constructor is specifically cited as undefined behaviour by section 10.4/6 of the C++ standard, so it's no surprise that we end up crashing. (Both g++ and Clang warn about this, and in fact linking fails with g++ with this example, though Clang succeeds.) But, just for fun, can anybody come up with a way to invoke a pure virtual function which does not rely on undefined behaviour? (I suppose you could argue that if such a method exists then there's a defect in the C++ standard, but I'm just curious...) EDIT: Several answers guys and thank you, but I should have made it clear that I realise it's legal to make a non-virtual call to a pure virtual function…

Accepted Answer

It's perfectly legal to call a pure virtual function non-virtually:

```
Derived d;
d.Base::method();
```

Of course, this requires the function to be defined, which isn't the case in your example.

Legally invoking a pure virtual function

Solution

Problem