are reference and pointer equal with regards to polymorphism

Question

I always think of having to use pointers for polymorphism. Using the canonical example:

DrawEngine:render(Shape *shape)
{
    shape->draw();
    shape->visible(true);
}

And passing in pointer to various Shape derived classes. Does it work the same with references

DrawEngine:render(Shape &shape)
{
     shape.draw();
     shape.visible(true);
}

is it even valid to do

engine.render(myTriangle); // myTriangle instance of class derived from Shape

If this works are there any differences between the two cases? I tried to find info in Stroustrup but found nothing.

I reopened this cos I wanted to explore just a tad more.

so at least one difference is dynamic_cast. For me polymorphism includes the use of dynamic_cast

can I go

Rhomboid & r = dynamic_cast<Rhomboid &>(shape);

what happens if the cast fails. Is this any different?

Rhomboid * r = dynamic_cast<Rhomboid*>(&shape);

Answer 1

With regard to polymorphism, references work just like pointers.

Answer 2

Pointers help in other ways as well. Like passing a string and accepting it as char* parameter in a function.

Consider the old example of reversing a string in place:

void reversestring(char* myString)
{
int firstPos=0;
int lastPos=myString.length - 1;
while (firstPos < lastPos)
{
  char temp=myString[firstPos];
  myString[firstPos]=myString[lastPos];
  myString[lastPos]=temp;
  firstPos++;
  lastPos--;
}
}

Writing code for string manipulation like these using references won't be that simple.

Answer 3

Regarding dynamic_cast, a failed cast produces a nullpointer with pointers, and results in a throw of a bad_cast (IIRC) exception with references.

One reason is that there's no such thing as a valid null-reference.

And possibly another reason (but it could just be an unintentionally useful emergent feature) is that sometimes one needs the exception and sometimes one needs the easy-to-check nullpointer, and no matter whether you have a reference or pointer at hand it takes at most a dereferencing or address operator to obtain the behavior you want.

Cheers & hth.,