Virtual destructors are useful when you might potentially delete an instance of a derived class through a pointer to base class:
class Base
{
// some virtual methods
};
class Derived : public Base
{
~Derived()
{
// 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
delete b
], if the static type of the
object to be deleted is different from its dynamic type, the static
type shall be a base class of the dynamic type of the object to be
deleted and the static type shall have a virtual destructor or the
behavior is undefined.
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.