Making a class abstract without any pure virtual methods

Question

I have a class which is to listen to mouse events. However, I do not want to force the user to implement any specific one, but I do want to make it clear that they must inherit it.

Is there a way to do this?

Thanks

Answer 1

You can declare a pure virtual destructor, but give it a definition. The class will be abstract, but any inheriting classes will not by default be abstract.

struct Abstract {      virtual ~Abstract() = 0; };  Abstract::~Abstract() {}  struct Valid: public Abstract {         // Notice you don't need to actually overide the base         // classes pure virtual method as it has a default };   int main() {     // Abstract        a;  // This line fails to compile as Abstract is abstract     Valid           v;  // This compiles fine. } 

Answer 2

Specify the constructor of the base as protected. This does mean that you cannot construct it directly, but forces inheritance. There is nothing that makes a developer inherit from that class aside from good documentation though!

Example:

struct Abstract { protected:     Abstract() {} };  struct Valid: public Abstract {     // No need to override anything. };   int main() {     // Abstract a;  // This line fails constructor is protected     Valid v;  // This compiles fine. }