inheriting from a template class causes errors

Question

I have a template class MyTemplate. It works fine. But as soon as I create another class that derives from it, I get errors.

//main.cpp
template <typename T> class MyTemplate {
    public:
        T* test() {
            return new T(this); //error here.
        }
};

template <typename T> class MyTemplate2 : public MyTemplate<T> { 
};

class MyClass {
    public:
        MyClass(MyTemplate2<MyClass>* source) {
        }
};

int main() {
    MyTemplate2<MyClass>().test();
    return 0;
}

The error I get is: main.cpp|4|error: invalid conversion from 'MyTemplate<MyClass>* const' to 'MyTemplate2<MyClass>*'

As I understand the error, "this" in MyTemplate is of type MyTemplate. But I want it to be MyTemplate2. I can do an explicit cast, but this requires passing a second argument to the template class, and it seems like there should be a better solution to this. Is there?

The other solution would be to make MyClass's constructor take a MyTemplate<MyClass>* argument. — UncleBens
– UncleBens, Commented Aug 13, 2010 at 22:21
@UncleBens In my real code, MyTemplate2 has some additional properties/methods which are used in MyClass's constructor. — Ponkadoodle
– Ponkadoodle, Commented Aug 13, 2010 at 22:24
then do the cast inside the constructor of MyClass, if you don't want to do the cast in the MyTemplate constructor (which would be a bit disturbing, because you would introduce knowledge about existence of derived classes into that base). — Johannes Schaub - litb
– Johannes Schaub - litb, Commented Aug 13, 2010 at 22:36

Johannes Schaub - litb · Accepted Answer · 2010-08-13 22:28:37Z

3

What you try is simply to pass a Base* (which is this) to a Derived*, which is the wrong way around. You need to explicitly cast to perform this downward conversion.

answered Aug 13, 2010 at 22:28

Johannes Schaub - litb

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Comments

Chubsdad · Accepted Answer · 2010-08-17 09:03:11Z

0

How about using the "clone" approach?

template <typename T> class MyTemplate { 
    public: 
        T* test() { 
            return clone(); //error here. 
        } 
        virtual T *clone() = 0;
}; 

template <typename T> class MyTemplate2 : public MyTemplate<T> {  
    public:
        T *clone(){return new T(this);}
}; 

class MyClass { 
    public: 
        MyClass(MyTemplate2<MyClass>* source) { 
        } 
}; 

int main() { 
    MyTemplate2<MyClass>().test(); 
    return 0; 
}

answered Aug 17, 2010 at 9:03

Chubsdad

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