polymorphic C++ references

Question

I was wondering how you can do polymorphism with references, as opposed to pointers.

To clarify, see the following minimal example:

cl         


        

Answer 1

There's nothing odd. Polymorphisms works both for pointers and references:

struct Base { };
struct Derived : Base;

void foo(Base &);

int main() {
  Derived x;
  foo(x);    // fine
}

You're conflating this with another issue, namely creating a reference to a dynamic object:

T * pt = new T;
T & rt = *pt;

T & x = *new T;  // same effect

Note that it's generally very bad style to track a dynamic object only by reference, because the only way to delete it is via delete &x;, and it's very hard to see that x needs cleaning up.

There are two immediate alternatives for your design: 1) make a a member object in B, or 2) make a a shared_ptr or unique_ptr and change the initalizer to a(new A1). It all depends on whether you actually need the polymorphic behaviour, i.e. if you have other constructors for B which assign a different derived class to a other than A1.