Why does it make sense to give definition for a pure virtual function?

Question

Scott said on Effective C++, 3rd Edition, pg. 43 that to make an abstract class, we just need to give it a pure virtual destructor:

class AWOV {                  // AWOV = "Abstract w/o Virtuals" public:   virtual ~AWOV() = 0;        // declare pure virtual destructor }; 

Then, he went on said that there is one twist: we must provide a definition for the pure virtual destructor:

AWOV::~AWOW() {}              // definition of pure virtual dtor 

My question is, by specifiying = 0, for pure virtual functions, we are saying that the function cannot have any definition for the class where this pure virtual function is declared.

Why is it OK to provide a definition (even it is empty) for the pure virtual destructor here?

Answer 1

"we are saying that the function cannot have any definition for the class where this pure virtual function is declared."

That's not what pure virtual means. Pure virtual only means that the containing class cannot be instantiated (is abstract), so it has to be subclassed, and subclasses must override the method. E.g.,

struct A {     virtual ~A() = 0; };  A::~A() {}  struct B : A {};  int main() {     A a;  // error     B b;  // ok } 

Here, the B destructor is implicitly defined. If it was another method that is pure virtual, you'd have to explicitly override it:

struct A {     virtual void foo() = 0; };  void A::foo() {}  struct B : A {};  int main() {     B b;  // error } 

Providing a definition for a pure virtual method is desirable when the base class must be abstract but still provide some default behavior.

In the specific case of a destructor, it has to be provided because it will be called automatically when subclass instances are destroyed. A program that tries to instantiate a subclass of a class with a pure virtual destructor without a definition will not pass the linker.

Answer 2

Making it pure forces derived (non-abstract) classes to implement their own.

Providing an implementation allows derived classes to invoke base class behavior (which destructors do by default).