Is the allocation of C++ non pointer data members in pointer class instance on the heap or on the stack

Question

I have a the following class:

class CObj {
private:
    MyOtherClass _member;
};

and the following code that creates an instance of CObj class:

CObj* obj = new Cobj;

obj is allocated on the heap, but: Are CObj::_member allocated on the heap too? or on the stack?

It's a *member* of `CObj`, so what do you think? :) – jrok Oct 22 '13 at 11:49 — jrok, Oct 22 '13 at 11:49
Most importantly - why do you care? – Kerrek SB Oct 22 '13 at 12:00 — Kerrek SB, Oct 22 '13 at 12:00

Matteo Italia · Accepted Answer · 2013-10-22T12:19:37.430

4

obj is a pointer allocated "on the stack"; the object obj points to is "on the heap", and, being obj->_member a member (=a part) of such an object, it's on the heap too.

In general, members, being part of the parent object, are allocated wherever their parent is stored.

edited Oct 22 '13 at 12:19

answered Oct 22 '13 at 11:49

Matteo Italia

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@IvanIshchenko Not enough code to know where it is, but it looks like automatic storage, or "the stack". You are thinking of the thing created by `new`, not `obj` itself. – juanchopanza Oct 22 '13 at 11:52
As I understood, by obj OP ment an instance of Cobj, not a pointer. – Ivan Ishchenko Oct 22 '13 at 11:54
1

@IvanIshchenko OP is confused. This is a good opportunity to fix that :) – juanchopanza Oct 22 '13 at 11:59
Wow, thanks for so much replies I thank all of you for taking your time to my question. – adrianoviana87 Oct 22 '13 at 14:24

score 3 · Answer 2 · edited May 23 '17 at 12:23

3

_member has automatic storage duration - it is allocated where it's owning object is allocated. So, if you create an instance of CObj with dynamic storage duration, like in your example, it will also be allocated in dynamic storage (the heap). If you create an object with automatic storage duration, it will be on the stack.

The problem with such questions is that C++ does not have any concept of stack and heap - it's just storage durations.

edited May 23 '17 at 12:23

Community

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answered Oct 22 '13 at 11:55

Björn Pollex

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`_member` does _not_ have automatic storage duration; it has the storage duration of the object which contains it. – James Kanze Oct 22 '13 at 12:45
And of course, it's not just storage durations; there's also object lifetime (which is determined when the constructors and the destructors run). – James Kanze Oct 22 '13 at 12:46

score 0 · Answer 3 · answered Oct 22 '13 at 11:54

0

When you call new OS allocate as much memory as size of class on heap and call class contructor.

From this you can see that since MyOtherClass is in CObj and CObj holds MyOtherClass , MyOtherClass is also on heap in same space as CObj .

answered Oct 22 '13 at 11:54

Luka Rahne

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Is the allocation of C++ non pointer data members in pointer class instance on the heap or on the stack

3 Answers3