Using pointer member in C++ class

Question

I am trying to do some work with my class and std::vector. I encountered with folloing problem,=

Here is my code:

class A
{
    private:
        ........
    public:
        vector<int>* ret_vec();
};

vector<int>* A::ret_vec()
{
    vector<int> vec;         // vec is declared as a local variable
    ......
    return &vec;
}

int main()
{
    .....
    vector<int>* vec = A.ret_vec();       // size of vec is 0 !!!
}

As you can see in the code, vec in vector<int>* A::ret_vec() is declared as a local variable, and when I assgin it to a pointer in main, the size of vec is 0.

Thus, I used following method instead:

class A
{
    private:
        vector<int> vec;         // vec is declared as a member variable
    public:
        vector<int>* ret_vec();
};

vector<int>* A::ret_vec()
{
    ......
    return &vec;
}

int main()
{
    .....
    vector<int>* vec = A.ret_vec();       // size of vec is ok
}

Now, as I declare vec as a member variable, when I assign it to a local pointer in main, its size is exactly as I expected.

May I know why is this? Many thanks.

========================================================== EDIT:

I'm sorry i haven't explained myself clear at the first place. I am dealing with a large data set vec, and the time performance is my current priority. Thus I'm afraid that return vector by values is not an option.

Return the vector by value. Your compiler will probably use the move assignement operator. — Jiwan, Aug 27 '13 at 09:37
You're lucky that it returned `0` in the first case, worse could've happened. — Uchia Itachi, Aug 27 '13 at 09:39
Hence you expose the member (2nd example) please avoid pointers, use a reference (which is always valid, not null), instead. If a member would be a pointer and that pointer is never zero return *member (a reference) — , Aug 27 '13 at 09:52
For your reference check this http://stackoverflow.com/questions/367633/what-are-all-the-common-undefined-behaviour-that-a-c-programmer-should-know-ab — Uchia Itachi, Aug 27 '13 at 09:54
Returning by value is not necessarily slower (see [Want Speed? Pass By Value](http://cpp-next.com/archive/2009/08/want-speed-pass-by-value/)) and is definitely safer than your alternative. — Jonathan Wakely, Aug 27 '13 at 10:00

score 7 · Answer 1 · answered Aug 27 '13 at 09:34

7

You return a dangling pointer and run into undefined behavior. It's not legal to return a pointer to a local automatic variable.

answered Aug 27 '13 at 09:34

Luchian Grigore

253,575
64
457
625

score 0 · Answer 2 · answered Aug 27 '13 at 09:54

0

You return a pointer to a local variable,which will be deleted after the function call. So returning pointer is not a good practice. Just return the object.

answered Aug 27 '13 at 09:54

Ani

42
4

I will be destroyed, not deleted. – Jonathan Wakely Aug 27 '13 at 09:58

Using pointer member in C++ class

2 Answers2