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

To strictly answer your question - what you have now does just that - i.e. a = A(); and a = B() and a = C(), but these objects are sliced.

To achieve polymorphic behavior with the code you have, I', afraid that's not possible. The compiler needs to know the size beforehand of the object. Unless you have references or pointers.

If you use a pointer, you need to make sure it doesn't end up dangling:

A* a = NULL;

switch (some_var)
{
case 1:
    A obj;
    a = &obj;
    break;
}

won't work because obj goes out of scope. So you're left with:

A* a = NULL;
A obj1;
B obj2;
C obj3;
switch (some_var)
{
case 1:
    a = &obj1;
    break;
case 2:
    a = &obj2;
    break;
case 3:
    a = &obj3;
    break;
}

This of course is wasteful.

For references it's a bit trickier because they have to be assigned on creation, and you can't use temporaries (unless it's a const reference). So you'll probably need a factory that returns a persistent reference.