Are references and pointers equal with regards to polymorphism?

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 po开发者_如何学Gointer 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 information in Stroustrup, but I found nothing.

I reopened this because 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);

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With regard to polymorphism, references work just like pointers.

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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.,

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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.