Nested class as a template parameter of parent class in C++

Question

I want to implement an algorithm as a class deriving from a pure virtual class representing the kind of problem the particular algorithm solves.

The general interface would look like this:

template<typename A, typename B>
class ISolutionToProblem
{
public:
    virtual void Init(const A & input, const B & param) = 0;
    virtual const B & ComputeSolution() = 0;

    virtual ~ISolutionToProblem() {}
};

And the implementation would be for example:

template<typename T>
class MyAlgorithm:
    public ISolutionToProblem<typename MyAlgorithm<T>::WorkData, T>
{
public:
    struct WorkData { /* Stuff using T... */ };
    virtual void Init(const WorkData & input, const T & param);
    virtual const T & ComputeSolution();

virtual ~MyAlgorithm();
};

(to be more specific, the problem is actually path finding, but I don't think it is relevant)

My problem is the inheritance part: I am using a nested struct as a template parameter, and no matter how nicely I try to talk to the compiler, it keeps refusing to compile my code.

I could go lazy and just put the inner structure outside of the class, but if possible I'd prefer it to stay neatly placed in the class.

1. So is what I am trying to do actually possible (in C++98)?
2. If so, how should I write it ? (bonus points if you get me to understand why the syntax doesn't accept the form above)
3. Otherwise, what am I doing wrong? (is my design flawed to begin with?)

---

Here is how the compiler error looks like.

• g++ (4.8):

  error: no type named ‘WorkData’ in ‘class MyAlgorithm<int>’

• clang (3.1):

  error: no type named 'WorkData' in 'MyAlgorithm<T>'

• VS2012:

  error C2146: syntax error : missing ',' before identifier 'WorkData'
  see reference to class template instantiation 'MyAlgorithm<T>' being compiled
  error C2065: 'WorkData' : undeclared identifier
  error C2955: 'ISolutionToProblem' :
               use of class template requires template argument list
  see declaration of 'ISolutionToProblem'

Answer 1

I think your problem is the the compiler doesn't know what the inner class looks like until it has defined the outer class and the outer class is defined with the inner class as a template parameter. I'm not 100% sure this can't be made to work. The CRTP is an example similar to this that is known to work.

Templates can be used to create inheritance hierarchies, but should not be use the in the definition of a hierarchy. If that sounds confusing, it's because it is. Inheritance and template classes don't mix well. Notice that even though CRTP uses inheritance and templates it does not use virtual functions.