Class and structure data placement C/C++

Question

As far as I know, class and struct data is placed one variable after another, for example:

class Foo
{
    int A;
    char B;
    float* C;
    double* D;
};

Foo Object;
char* ptr = &Object;

&(Object.A) == ptr;    // all these are true
&(Object.B) == ptr+sizeof(int);
&(Object.C) == ptr+sizeof(int)+sizeof(char);
&(Object.D) == ptr+sizeof(int)+sizeof(char)+sizeof(float*);

Does it always work that way? Would it also work if I made all these data static in the class?

Thanks very much. That's what I wanted to know if this is some kind of standard or if it just sometimes happen. — Avert, Sep 15 '15 at 17:46
Please remove the C tag since `class` is not a keyword in C language. — Thomas Matthews, Sep 15 '15 at 17:54
There is very well a standard: The ABI and/or PCS of your platform/architecture. — too honest for this site, Sep 15 '15 at 18:21
@DDPWNAGE, what does C# have to do with this question? We're talking the difference between C and C++. Common misconception is that they are the same language. — Thomas Matthews, Sep 15 '15 at 20:25
@ThomasMatthews it has nothing to do with the question. I saw your comment, but I didn't read the title. I had a brain fart, I'm sorry. — DDPWNAGE, Sep 15 '15 at 20:26

score 1 · Accepted Answer · edited May 23 '17 at 12:14

Well, the comments are a bit misleading, although all are correct. (Perhaps because this is in fact a duplicate of an old question)

Your original assumption is "basically" correct. Well, basically.

The only thing you can (at least most of the time) rely on is the order of the elements.

The actual size of the elements varies greatly across platforms and compilers. This is an issue of itself, but an int is most of the time 32bit but sometimes 16bit or even 64bit.

On top of that the compilers may (or may not) add some padding in between the elements, sometimes depending on the element type, sometimes not.

In conclusion: if you write code that depends on pointer arithmetic like the one you proposed, your code will be platform dependent and implementation/compiler dependent.

Class and structure data placement C/C++

1 Answers1