static abstract methods in c++

Question

I have an abstract base class

class IThingy
{
  virtual void method1() = 0;
  virtual void method2() = 0;
};

I want to say - \"all classes

Answer 1

There is no sure way to prescribe such a contract in C++, as there is also no way to use this kind of polymorphism, since the line

Concrete::Factory()

is always a static compile-time thing, that is, you cannot write this line where Concrete would be a yet unknown client-provided class.

You can make clients implement this kind of "contract" by making it more convenient than not providing it. For example, you could use CRTP:

class IThingy {...};

template 
class AThingy : public IThingy
{
public:
  AThingy() { &Derived::Factory; } // this will fail if there is no Derived::Factory
};

and tell the clients to derived from AThingy (you could enforce this with constructor visibility tweaking, but you cannot ensure the clients don't lie about their_class_name).

Or you could use the classic solution, create a separate hierarchy of factory classes and ask the clients to provide their ConcreteFactory object to your API.