Stack vs Heap C++

Question

I just had a quick question about how stack variables versus heap variables work. As I understand it, stack variables are variables that after functions return will be deleted, and heap variables are persistent. But what I'm really confused about is how to allocate heap variables inside functions:

int MyObject::addObject(const char* a){
    MyObject newObject(a);
    return 0;
}

Say that I have a constructor for MyObject that is newObject(const char * a). Then in this function when it is called, after the return does the newly constructed newObject get deleted? If yes, how can you allocate to the heap within a function then? If not, how do you cleanup your memory later?

Furthermore, what exactly is the role of a destructor and when is it called?

Answer 1

A constructor for the class MyObject is MyObject(), not newObject(). In your example, newObject is the name of your variable, not the constructor.

To allocate on the heap inside a function, you need to call the new operator:

int MyObject::addObject(const char* a){
    MyObject* newObject = new MyObject(a);
    //newObject is allocated on the heap

    //... Some more code...

    delete newObject;
    //you have to explicitly delete heap-allocated variables
    //if not, you get memory leaks
    return 0;
}

Your code:

MyObject newObject(a);

creates an automatic storage (stack) MyObject called newObject that lives until the scope it was declared in ends - i.e. closing }.

Furthermore, what exactly is the role of a destructor and when is it called?

To clean up memory the class allocated with new or new[] (or malloc) and it owns. It is called either when the object goes out of scope for automatic objects, or when you explicitly call delete for dynamic objects.