Question

I am a little confused on whether I should prefer to initialize my main widgets on the stack or on the heap. In “C++ GUI Programming with QT 4,” main widgets are initialized on the stack. Before I say more, I will explain what I mean:

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    MainWindow mainWin;
    mainWin.show();
    return app.exec();
}

Now, perhaps this is simply because it is safer, perhaps it is because they don’t want to confuse readers about memory allocation in QT. Leaving out any deletes on objects inheriting from QObject certainly does allow readers to “forget” memory-management with QT objects. But, my question is, should we prefer that method, or the following:

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    MainWindow* mainWin = new MainWindow;
    mainWin->show();
    int execReturn = app.exec();
    delete mainWin;
    return execReturn;
}

The main reason I bring this question is I normally prefer to follow a pattern with choosing heap vs stack:

• If the item is large – Heap
• If the item is long-term – Heap
• Otherwise – Stack

Now, I guess my question boils down to two questions:

• Is QWidget large to the point where I should fear stack-overflows?
• How large is the stack for the average application? When should I fear for overflowing the stack (Other than obviously recursive functions)?

I realize it is unlikely that QWidget itself causes a stack-overflow, but of course this will be on top of any other stack usage my application may have.

Answer 1

Your pattern for choosing heap vs stack sounds reasonable but I wouldn’t worry so much about the size of the object. Any large object should use the heap internally. std::vector is usually the size of three pointers but can be very large.

I don’t think you should fear that any object is large enough to overflow the stack by itself. While possible it is certainly very rare (I haven’t seen one).

I would recommend thinking about simplicity also, you could allocate any local variable on the heap and then free it before the function returns, but this would be unnecessarily complicated and generally considered bad practice.

Stack size is usually configured through linker settings. On Windows it’s 1MB by default.