Force template parameter class to inherit from another templated class with partially fulfilled parameters

Question

So I have the following two classes:

template < class Cost >
class Transformation {
  public:
    virtual Cost getCost() = 0;
};

template < class Trans         


        

Answer 1

Does this suit your need?

template < class Cost >
class Transformation {
public:
    typedef Cost TransformationCost;
    virtual Cost getCost() = 0;
};

template < class TransfCl, class Cost = typename TransfCl::TransformationCost>
class State {
    static_assert(
                std::is_base_of< Transformation< Cost >, TransfCl >::value,
                "TransfCl class in State must be derived from Transformation< Cost >"
            );
protected:
    State(){}
public:
    virtual void apply( const TransfCl& ) = 0;
};

class TransfImpl : public Transformation< int >{
public:
    int getCost(){ return 0; }
};

class StateImpl : public State< TransfImpl >{
public:
   StateImpl(){}
   void apply( const TransfImpl & ){}
};

ADDED: Live Demo