Question

I’m using a bit of legacy type code that runs on a framework, so I can’t really explain whats going on at a lower level as I don’t know.

However my code creates an array of objectives.

int maxSize = 20;
    myObjects = new Object*[maxSize+1];

    myObjects[0] = new item1(this);
    myObjects[1] = new item2(this);

    for(int i=2; i != maxSize+1; i++){
          myObjects[i] = new item3(this);
        }

    myObjects[maxSize+1] = NULL;

If maxSize is larger than 30 I get a whole load of errors I’ve never seen. Visual Studio draws up an error in xutility highlighting:

const _Container_base12 *_Getcont() const
    {   // get owning container
    return (_Myproxy == 0 ? 0 : _Myproxy->_Mycont);
    }

I’ve never used Malloc before, but is this where the problem lies. Should I be assigning using it to avoid this problem?

Answer 1

The absolute value of maxSize is probably not a culprit: allocating 30 pointers should go without trouble on any computer, including most micro-controllers. Using malloc is not going to change anything: you are doing your allocation the way you’re supposed to do it in C++.

Here is the likely source of your error:

myObjects[maxSize+1] = NULL;

You have allocated storage for maxSize+1 items, so the valid indexes are between 0 and maxSize. Writing one past the last element is undefined behavior, meaning that a crash could happen. You got lucky with 20 elements, but 30 smoked out this bug for you. Using valgrind utility is a good way to catch memory errors that could cause crashes, even if they currently don’t cause them.

int maxSize = 20;
myObjects = new Object*[maxSize+1];

myObjects[0] = new item1(this);
myObjects[1] = new item2(this);

// if maxsize is 1, this loop could be trouble
for(int i=2; i != maxSize; i++){
    myObjects[i] = new item3(this);
}

myObjects[maxSize] = NULL;