HELP! will this code cause memory fault after C++ object is out of scope? (STL problem)

Question

I just came across a piece of code written by my ex-colleague few years ago. Honestly, I'm not an C++ expert, so I am seeking help.

The code looks like this:

std::vector<OBJ> objects;

void initobjs()
{
    for (int i=0; i<10; i++)
    {
        OBJ obj;
        obj.type=i;
        obj.len=16;

        objects.push_back(obj);
    }
}

My question is: after function initobjs() returns, aren't all obj instances already out of scope and automatically freed by the C++ runtime system? If they are, will any reference to contents of any of the objects added to the vector cause memory fault or unexpected result?

Guys, the formatting is fixed, stop mixing edits. – GManNickG Aug 09 '10 at 20:33 — GManNickG, Aug 09 '10 at 20:33
What is the definition of OBJ? – quamrana Aug 09 '10 at 20:48 — quamrana, Aug 09 '10 at 20:48

score 13 · Accepted Answer · answered Aug 09 '10 at 20:27

13

Your concern can be made even more local: obj ends at the bottom of the for-loop.

That said, a container makes a copy of its argument*, and does not store any reference or pointer to the "original" value.

_{*Hence all containers require their elements be copy-constructible and copy-assignable.}

answered Aug 09 '10 at 20:27

GManNickG

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+1 ; to be more explicit, your container stores `OBJ` objects, not `OBJ&` or `OBJ*` – Tomaka17 Aug 09 '10 at 20:30
How `OBJ`'s copy constructor behaves could dramatically alter the postconditions of this for loop. – fbrereto Aug 09 '10 at 20:32

score 7 · Answer 2 · answered Aug 09 '10 at 20:30

The original objects will be out of scope, but the push_back() method actually creates copies of them, because they are passed by value. So when the initobjs() function exits, the copies are still in the vector; they will be deallocated when you remove them from the vector, or when the vector itself goes out of scope.

If the vector were a vector<OBJ*>, then that would be a different matter altogether: it would mean that you'd have to manually delete every obj the moment you remove it from the vector (unless you store it elsewhere).

Note that C++ does not have any memory management built in at all, except stack-allocated variables, which are deallocated when their scope ends. RAII is the buzz-word you're looking for in case you want to enlighten yourself a bit more.

score 1 · Answer 3 · answered Aug 09 '10 at 20:32

1

The objects vector will contain "copies" of the OBJ in the for loop. Depending on what the copy constructor of OBJ does this might yield the appropriate results or not.

I would go and check the copy constructor of OBJ for suspicious things.

answered Aug 09 '10 at 20:32

jdehaan

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HELP! will this code cause memory fault after C++ object is out of scope? (STL problem)

3 Answers3

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