static_cast and reference to pointers

Question

Can anyone tell me why this doesn't compile:

struct A { };
struct B : public A { };

int main()
{
  B b;
  A* a = &b;
  B* &b1 = static_cast<B*&>(a);
  return 0;
}

Now, if you replace the static cast with:

B* b1 = static_cast<B*>(a);

then it does compile.

Edit: It is obvious that the compiler treats A* and B* as independent types, otherwise this would work. The question is more about why is that desirable?

@user814628 Only if you do not know (because of program logic) that the cast is valid, and using `dynamic_cast` forces the compiler to embed runtime type information into that class hierarchy. Combine this with the relatively slow operation of `dynamic_cast` and it's a solution which you don't want to use unless it's necessary. — Agentlien, Jan 17 '13 at 16:03

score 7 · Accepted Answer · answered Jan 17 '13 at 16:03

B is derived from A, but B* isn't derived from A*. A pointer to a B is not a pointer to an A, it can only be converted to one. But the types remain distinct (and the conversion can, and often will, change the value of the pointer). A B*& can only refer to a B*, not to any other pointer type.

score 3 · Answer 2 · 2013-01-17T15:55:17.240

3

non-constant lvalue reference (B*&) cannot bind to a unrelated type (A*).

edited Jan 17 '13 at 15:55

answered Jan 17 '13 at 15:47

score 0 · Answer 3 · answered Jan 17 '13 at 16:04

0

Handling of references is something the compiler does for you, there should be no need to cast to reference.

If we refactor the code to:

B b;
A* a = &b;
B* b_ptr = static_cast<B*>(a);
B*& p1 = b_ptr;

It will compile.

answered Jan 17 '13 at 16:04

Periodic Maintenance

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I agree (see my post), but this doesn't do the same. For example, changing the pointee in ``p1`` doesn't change the pointee in ``a``. – PierreBdR Jan 17 '13 at 16:40

score -1 · Answer 4 · answered Jan 17 '13 at 15:55

-1

You are trying to cast an A* to a B*. This is the wrong way around and not very useful. You probably want to store a pointer to derived in a pointer to base, which is useful and doesn't even need a cast.

I suppose a dynamic_cast might work here, but the result is implementation defined if I'm not mistaken.

answered Jan 17 '13 at 15:55

rubenvb

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You cannot use `dynamic_cast` to convert a `A*` to a `B*&`. It's not legal, and the code won't compile. – James Kanze Jan 17 '13 at 16:05
Casting from ``A*`` to ``B*`` is the only direction in which ``static_cast`` make sense. In the other direction, there is nothing to be done. – PierreBdR Jan 17 '13 at 16:39

static_cast and reference to pointers

4 Answers4

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