CODEWITHSUNDEEP
c++classtemplatescasting
Alberto
Alberto

Reputation: 12899

Can't cast Template subclass to a different Template instantiation

suppose having a Template stack<class T, int maxElements which have a subclass Entry, which will build the Linked List for the stack, so something like this:

template <class T, int maxElements>
class Stack{
private: 
     class Entry{
        friend class Stack;
        T info;
        Entry* next;
     };
     Entry* first;
public:
    template<class U, int n>
    void append(const Stack<U, n>& s){
          if(isEmpty()) first = s.first; /* here is the problem*/
          ....
    }

};

So, the problem is in the line marked, and it's assigning to 'Stack<char, 100>::Entry *' from incompatible type 'Stack<char, 10>::Entry *const', and that because it's building a "class" Entry for each template instantiation, but the point is that Entry doesn't depends from the maxElement parameter, so I would like to know if there is a way to tell this to the compiler.
So far, the only way i can think that can do this, is to take out the class from the template, and making itself a template based only on T

PS: i know i'm sharing memory in the line where i'm having the error, one thing at time

Upvotes: 0

Views: 61

Answers (1)

Ulrich Eckhardt
Ulrich Eckhardt

Reputation: 17415

The problem is that different template instantiations are different types. With that, nested types (Entry in this case) are different types as well.

The solution is just as simple: Move parts to a (private) baseclass that only depends on the type:

template<typename T>
class StackBase {
protected:
    struct Entry {
        T info;
        Entry* next;
    };
};

Then you derive from this baseclass:

template<typename T, int maxElements>
class Stack: private StackBase<T> {
    ...
};

Upvotes: 2

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