C++ Templates type casting with derivates

Question

I\'m trying to cast from one generic to another, say:

myClass anItem = myclass anotherObject;
         


        

Answer 1

You can't static cast, as they are incompatible types. You can sometimes create an operator to coerce the type instead

#include 

class A { };

class B : public A { };


template
struct holder {
    T* value;

    holder ( T*value ) : value ( value ) { }

    template < typename U > // class T : public U
    operator holder () const
    {
        return holder( value );
    }
};


int main ()
{
    using namespace std;

    B   b;

    holder   hb ( &b );
    holder   ha  = hb;

    cout << boolalpha;

    cout << ( hb.value == ha.value ) << endl;

    return 0;
}

Whether this is a meaningful operation rather depends on the semantic of the template class - if the aFunction can put anything into the handler, you don't want the more specific object being mutated. Hence you copy somehow, either with a coercion operator or with a template copy constructor and assignment. ( the coercion is less code but might result in more objects being created if you don't use reference parameters )