Calculate offsets of structure members without instantiation

Question

What about this code, is it standard ? I've seen that kind of code in a Visual C++ header, it can calculate the offsets of some structure members but how does it works exactly ?

#include <iostream>

struct Foo
{
    int a;
    int b;
    int c;
};


int main(int argc, char** argv)      
{
    int* i1 = (&((Foo*)0)->a);
    int* i2 = (&((Foo*)0)->b);
    int* i3 = (&((Foo*)0)->c);

    std::cout << "i1 = " << i1 << "\ni2 = " << i2 << "\ni3 = " << i3 << "\n";
    return 0;
}

Results : i1 = 0, i2 = 4, i3 = 8

Edit : Just remembered where I saw this code before. It's in WinNT.h

#define FIELD_OFFSET(type, field)    ((LONG)(LONG_PTR)&(((type *)0)->field))

There is a standard macro `offsetof(structure,field)` defined in ``, which is doing just that. So yes, the code is valid, but using the standard macro would make it more standard. — DYZ, Jan 02 '17 at 00:36
TL;DR: The implementation can do stuff like that, you can't. — Baum mit Augen, Jan 02 '17 at 00:42
@nikau6 You "can" also run red lights, that works too (most of the time). Does not mean it's legal or a good idea though. — Baum mit Augen, Jan 02 '17 at 00:45
That was my question, is it standard ? Previous answer said it's valid. It's valid or it's not, it can't be both. — nikau6, Jan 02 '17 at 00:51
As the answers in the dupe explain, it's valid when the implementation does it as long as it works, it's not valid when you do it. — Baum mit Augen, Jan 02 '17 at 00:54

score 2 · Accepted Answer · answered Jan 02 '17 at 00:38

2

That is pretty common, as macro offsetof. Take a look at http://en.cppreference.com/w/cpp/types/offsetof.

It works as the distance of each member's memory location and struct's memory location. In case of your example, you set struct's memory location as 0, thus no need to calculate distance.

answered Jan 02 '17 at 00:38

xosp7tom

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If you set your memory location as 0 ... wouldn't the operator "->" be segmentation fault? – ChaoSXDemon Jan 02 '17 at 00:39
3

segfault happens when you are actually trying to access the member variable or call the member function. With &, you are just calculating memory address. – xosp7tom Jan 02 '17 at 00:40
1

Even starting at location 0 it can still be useful to calculate the offsets. – nikau6 Jan 02 '17 at 00:48

Calculate offsets of structure members without instantiation

1 Answers1