Access to class member by increment operator

Question

I have a class, for example:

class Vector
{
    float x, y, z
};
Vector v;

And a pointer:

float *c = &v.x;

Will it be works correctly, when I'll use increment operator for access to y and z members?

P.S. Bad style to do by this way, but it's sport interest.

you can try it and you'll have the answer :) – Reda Jun 13 '13 at 21:11 — Reda, Jun 13 '13 at 21:11
@RedaBalkouch that's not at all how C++ works. – Luchian Grigore Jun 13 '13 at 21:12 — Luchian Grigore, Jun 13 '13 at 21:12
the code is working in my laptop – Reda Jun 13 '13 at 21:18 — Reda, Jun 13 '13 at 21:18

Pubby · Accepted Answer · 2013-06-13T21:29:47.377

1

Will it be works correctly, when I'll use increment operator for access to y and z members?

No. Undefined behavior. You can't perform pointer arithmetic across objects and you can't guarantee struct padding.

You can do stuff like this though:

class Vector
{
    float v[3];
    int& x() { return v[0]; }
    int  x() const { return v[0]; }
    // and so on ...
};

edited Jun 13 '13 at 21:29

answered Jun 13 '13 at 21:11

Pubby

Is it? Can you provide a reference? (consider his exact example, where `Vector` is a POD.) – Luchian Grigore Jun 13 '13 at 21:13
Why is "undefined behavior"? More detailed answer please, or link. Are the members sometimes separated in memory? Thanks for code, it is obviously – fpohtmeh Jun 13 '13 at 21:15
1

@LuchianGrigore Two reasons: struct padding and pointer arithmetic rules. I'll look for a reference. – Pubby Jun 13 '13 at 21:24
Maybe you should add that to the answer. – Luchian Grigore Jun 13 '13 at 21:28
I've found the [answer](http://stackoverflow.com/questions/5397447/struct-padding-in-c). Thanks for all. – fpohtmeh Jun 13 '13 at 21:36

1 Answers1