C++ Pointer returning NULL

Question

Trying to update to a pointer from a function return. Just for background this is a template that acts like the stl vector. This is the returning function.

    ////////////////////////////////////////////////////////////////////////////////
//removes an item from the array
const T& remove(int pos)
{
    if(pos > cnt)
        pos = cnt;
    if(pos < 0)
        pos = 0;
    static T v;
    for(int i,k = 0; i < cnt; i++,k++)
    {
        if(i == pos)
        {
            v = element[i];
            i++;
        }
        else
            element[k] = element[i];
    }
    cnt--;
    return v;
}

/////////////////////////////////////////////////////////////////////////

The pointer variable I am trying to update:

TVector<Member*> members;
Member* backmember;

backmember = members.remove(members.size()-1);

but backmember always returns null. I am sure I am missing something simple, just not sure what. Any ideas? Let me know if you have any questions and thanks in advance.

Just curious, why can't you use en.wikipedia.org/wiki/Erase-remove_idiom instead of writing your own method. — vine'th
– vine'th, Commented Nov 23, 2011 at 4:05
remove returns a const T&. Your backmember takes a pointer, which is not the same thing as a reference. It also takes a non-const pointer. So that's two ways that this code is wrong. Are you sure this is the code in question? How does this compile, let alone return NULL? — Nicol Bolas
– Nicol Bolas, Commented Nov 23, 2011 at 4:07
Making a static copy of the object to return is a bad idea - it means two threads can't access different arrays at the same time. If you want to return the removed value, you should do it by value. — Mike Seymour
– Mike Seymour, Commented Nov 23, 2011 at 4:09
Yes I understand guys this is not my normal programming style. I had to fill in the functions and the return types have to match. I know the general rule is not to post assignment work to stackoverflow, but I knew it was something simple I was overlooking. This is what happens when you have a java instructor teaching C. :) — Austin Cherry
– Austin Cherry, Commented Nov 23, 2011 at 4:17

Blastfurnace · Accepted Answer · 2011-11-23 04:04:20Z

6

I don't think this initializes i:

for(int i,k = 0; i < cnt; i++,k++)

Try changing it to this and testing:

for(int i = 0,k = 0; i < cnt; i++,k++)

answered Nov 23, 2011 at 4:04

Blastfurnace

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1 Comment

Chris Parton Over a year ago

It's possible that the OP is under the impression that int i,k = 0 initialises both i and k, whereas it only initialises k to 0. Of course, this is exactly what you've just said, I just wanted to state it more verbosely :) Also, I personally find for loops containing multiple variables to be something to avoid wherever possible. They're too hard to read and understand at a glance.

zaharpopov · Accepted Answer · 2011-11-23 04:13:16Z

2

for(int i,k = 0; i < cnt; i++,k++)

i,k uses comma operator, which evaluate arguments in order, returning result of last one. So this means "declare i, then declare k and set it to 0"

i is not set. So most likely it get some large random value from memory and your loop never run

answered Nov 23, 2011 at 4:13

zaharpopov

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