Why can I cast pointers of unrelated classes without error?

Question

I've been experimenting with pointers and written the following code:

#include <iostream>

using std::cout;

struct A
{
    int a;
    char b;
    A(){ a = 4; }
};

struct B
{
    int c;
    B(){ c = 5; }
};

A *a = new A;
B *b = new B;

int main()
{ 
    a = (A*)b; 
    cout << a -> a; //5
}

Why B* can be converted to A*? Can any pointer be converted to any other pointer?

Even if your compiler says nothing (because an explicit cast means "i´m aware of potential problems, don´t warn me"), it´s just plain wrong what you´re doing. That it gives the expected result is coincidence (because a and c have the same byte offsets and type in A and B) — deviantfan, Jul 14 '14 at 18:43
It is not quite true that a pointer of any type can be converted to any other type. But it can be converted to far more types than it should be. — Beta, Jul 14 '14 at 18:45
The language allows you to cast a pointer to anything to a pointer to (mostly) anything else. The result just can't be safely used except in a few limited cases. — T.C., Jul 14 '14 at 18:47
in c, you could cast types from void * to say char * quite safely iirc. — DTSCode, Jul 14 '14 at 18:47
c-style cast `(A*)` is a sledge hammer. Use a `static_cast` to let the compiler check if your types are related. @mark-b Maybe that was closed as dupe too quickly :-/ ... — πάντα ῥεῖ, Jul 14 '14 at 18:47
An example of what? A safe conversion, an unsafe conversion, or a conversion forbidden by the language? — Beta, Jul 14 '14 at 20:25
@DTSCode If the original pointer you stored in void * was in fact char *. Same in C++ — Neil Kirk, Jul 14 '14 at 21:12
i was talking about casting pointers. im aware. and it doesnt have to just be char * — DTSCode, Jul 14 '14 at 23:56
@_St_Antario Doesn't my answer solve your question. Is there something missing, you still don't understand? — πάντα ῥεῖ, Jul 15 '14 at 19:17

πάντα ῥεῖ · Answer 1 · 2014-07-14T21:20:53.940

"Can any pointer be converted to any other pointer?"

If you use a c-style cast like this, yes.

a = (A*)b;

The structure pointed to by b will be just (re-)interpreted, just as it would be an A. The correct c++ equivalent is

a = reinterpret_cast<A*>(b);

What you'll experience from such casts by means of consistency is unlikely to fit what's expected.
In other words: You'll experience all kinds of undefined behavior, accessing any members of a after doing such cast.

You should use a static_cast<> to let the compiler check, if those types are related, and can be casted somehow reasonably

a = static_cast<A*>(b);

Check these online samples to see the differences of static_cast<> and reinterpret_cast<>.

Why can I cast pointers of unrelated classes without error?

1 Answers1