Question

Calling Virtual methods during construction and destruction causes a compiler error? I heard that it is a dangerous thing to do.

I know that if I have a Class Base that defines a virtual method foo(), the resolution of foo() is dynamic in all subclasses of Base. So if the subclass Derived overrides foo() then Derived::foo() is called. So why calling virtual methods during construction/destruction confuses the compiler?

What is the difference between calling them from the constructor, and outside it?

Answer 1

During construction of Base(), the derived class hasn’t been initialized yet, and doesn’t really exist yet. When you call a virtual method, your call will go to Base::foo(). This isn’t necessarily wrong, but is often unexpected, hence the general prohibition against it.

Similarly during destruction, Derived is destroyed before ~Base() is called, so again virtual method calls will go to Base::foo(), which is again often unexpected.