Misunderstanding in use of class variables from a header file - c++

Question

struct TodoItem
{
std::string todo;
};
const int MAX_STACK_SIZE = 5;
class TodoStackArray
{
public:
TodoStackArray(); //confusion here<---
bool isEmpty();
bool isFull();
void push(std::string todoItem);
void pop();
TodoItem* peek();
/* for grading purposes we need these following methods */
int getStackTop() { return stackTop; }
TodoItem** getStack() { return stack; }
private:
int stackTop;
TodoItem* stack[MAX_STACK_SIZE];
};
Above, is some declarations and a class from a header file for a current assignment. Our duty was to implement a stack program using this header file. However, I am a little confused as to the purpose of the TodoStackArray() in the Header File. Is this supposed to be a base constructor? Am I supposed to use it for anything??
I understand that this is somewhat situation specific but given that stack Arrays are STL I figured you might all be able to provide some insight. Thanks!
In case you want to see what I did with this header file...
TodoStackArray::TodoStackArray() //This, unsurprisingly, produces an error.
{
stackTop = -1;
stack[stackTop];
}
bool TodoStackArray::isEmpty()
{
return (stackTop == -1);
}
TodoItem* TodoStackArray::peek()
{
if(stackTop ==-1)
{
cout<< "Stack empty, cannot peak."<<endl;
}
else
{
return(stack[stackTop]);
}
}
bool TodoStackArray::isFull()
{
return(stackTop == 4);
}
void TodoStackArray::push(std::string todoItem)
{
if(stackTop >= 5)
{
cout<<"Stack full, cannot add new todo item."<<endl;
}
else
{
stack[stackTop++];
stack[stackTop]->todo = todoItem;
}
}
void TodoStackArray::pop()
{
if(stackTop == -1)
{
cout<<"Stack empty, cannot pop an item."<<endl;
}
else
{
stackTop--;
}
}
Also, to be clear, we were not provided driver software. They will be assessing from their own stuff so we have to write our own driver software to test our functions. Hence the lack of any Main implementation.

Answer 1

Yes, TodoStackArray::TodoStackArray() really is your class' default constructor.
What you are supposed to do with it just as with any default constructor:
How are the class members supposed to be initialized if I create a new TodoStackArray?
The answer depends on what your class is supposed to look like in its initial state.
In you situation, "by default" a stack is empty. Setting the stackTop to -1 was not a bad idea and the rest of your implementation is consistent with that (stackTop == -1 clearly means "empty stack").
The odd thing that happens in your constructor is this:
stack[stackTop];
What is, according to you, the purpose of this line? You are reading the value in your array at a negative index, and we all know this kind of things never end well.
Constructors are made to initialize your data. Here you are reading something you didn't initialize at an index that doesn't exist. Doesn't make much sense, does it? :)
Just get rid of this line and you should be able to move on. Here is an equivalent implementation using member initializer list (which are considered nicer):
TodoStackArray::TodoStackArray() : stackTop(-1)
{
// notice: no body required here
}
Have fun!