C++: Pure virtual type

Question

I'm exploring template shenanigans in C++ (C++11), and one thing I'd like to have is a pure virtual type in an abstract class. This would be like Scala's abstract types . In C++ I'd want to do something like the following:

struct Base {
  // Says any concrete subclass must define Type, but doesn't
  // require that it be anything in particular.
  virtual typedef MyType; 
};

struct Derived : Base {
  // Won't compile unless this typedef exists.
  typedef int MyType;
};

Any idea how to do this?

Answer 1

I am not sure there is a real need for this in C++.

Trying to put myself in the position of a designer who is looking for such a solution, I would say this kind of constraint would be needed to enforce some types to adhere to some syntactic convention.

Most likely, this is needed because a generic algorithm requires that syntactic convention: it cannot work with types that do not define such a type association.

For instance, the algorithm below requires the type of its argument to have an associated bar_type :

template<typename T>
bool foo(T t)
{
    typedef typename T::bar_type FT;
    FT ft;
    ...
}

But if this is the case, there is no need for enforcing a typedef to effectively constraint the input of foo<>() : simply omitting a type definition for bar_type won't make it possible to use that type with foo<>() .

Of course, you would discover this only as soon as you actually try to do so, and not before. And being able to define a concept such as HasBarType , and then to enforce some types to realize that concept would be nice; on the other hand, concepts are not yet part of C++ and, as much as they are desirable, it is possible to live without them.

Answer 2

edit

This doesn't work, but I think the curiously recurring template pattern might be the way to go.

/edit

template<typename Dependent>
class Base : public Dependent {
    typedef typename Dependent::MyType MyType;
};

Then use the curiously recurring template pattern :

struct Derived : Base<Derived> {
  // Won't compile unless this typedef exists.
  typedef int MyType;
};