How can I create a polymorphic object on the stack?

Question

How do I allocate a polymorphic object on the stack? I\'m trying to do something similar to (trying to avoid heap allocation with new)?:

A* a = NULL;

switch         


        

Answer 1

If B is your base types D1, D2, and D3 are your derived types:

void foo()
{
    D1  derived_object1;
    D2  derived_object2;
    D3  derived_object3;
    B *base_pointer;

    switch (some_var)
    {
        case 1:  base_pointer = &derived_object1;  break;
        ....
    }
}

If you want to avoid wasting the space of the three derived objects, you could break up your method into two parts; the part that chooses which type you need, and the part of the method that works on it. Having decided which type you need, you call a method that allocates that object, creates a pointer to it, and calls the second half of the method to complete the work on the stack-allocated object.