Lodle Lodle - 7 months ago 29
C++ Question

When to use virtual destructors?

I have a solid understanding of most OO theory but the one thing that confuses me a lot is virtual destructors.

I thought that the destructor always gets called no matter what and for every object in the chain.

When are you meant to make them virtual and why?


Virtual destructors are useful when you can delete an instance of a derived class through a pointer to base class:

class Base 
    // some virtual methods

class Derived : public Base
        // Do some important cleanup

Here, you'll notice that I didn't declare Base's destructor to be virtual. Now, let's have a look at the following snippet:

Base *b = new Derived();
// use b
delete b; // Here's the problem!

Since Base's destructor is not virtual and b is a Base* pointing to a Derived object, delete b has undefined behaviour. In most implementations, the call to the destructor will be resolved like any non-virtual code, meaning that the destructor of the base class will be called but not the one of the derived class, resulting in a resources leak.

To sum up, always make base classes' destructors virtual when they're meant to be manipulated polymorphically.

If you want to prevent the deletion of an instance through a base class pointer, you can make the base class destructor protected and nonvirtual; by doing so, the compiler won't let you call delete on a base class pointer.

You can learn more about virtuality and virtual base class destructor in this article from Herb Sutter.