Getting value of C++ list iterator problem

Question

I have code like this:

struct sWindowInfo {
    WNDPROC pPrevWndProc;
};
typedef std::list<sWindowInfo> windowsList;

And function, which returns pointer to iterator of first window-info struct in list:

windowsList::const_iterator* windowGet();

Code like this works fine:

if (windowsList::const_iterator* itr = windowGet())
{
    WNDPROC wndProc = (*itr)->pPrevWndProc;        
    return CallWindowProc(wndProc, hWnd, msg, wParam, lParam);
}

But if i try:

if (windowsList::const_iterator* itr = windowGet())
    return CallWindowProc((*itr)->pPrevWndProc, hWnd, msg, wParam, lParam);

Runtime error occurs and i see strange values in debugger. I don't understand, why? In my opinion this is identical code.

Answer 1

Both implementations are wrong. You should not return pointers to iterator since it will be invalid after windowGet() call. It is the same thing as doing this:

int* getInt()
{
    int a = 10;
    return &a;
}

int* a = getInt();


int v = *a ; // v may be 10 or may be not 

You may ask why first code is working? It is working just by luck: it happened that for that code compiler generates code that is not using the stack memory which was used by iterator. In second example compiler may generate a different code in which the iterator memory was used and changed.