Question

I am trying to implement a dynamic array:

template <typename Item>
class Array {
private:
    Item *_array;
    int _size;
public:
    Array();
    Array(int size);
    Item& operator[](int index);
};

template <typename Item>
Array<Item>::Array() {
    Array(5);
}

template <typename Item>
Array<Item>::Array(int size) {
    _size = size;
    _array = new Item [size];

    for (int i = 0; i < size; i++)
        cout << i << " " << _array[i] << " " << &_array[i] << endl;
}

template <class Item>
Item& Array<Item>::operator[](int index) {
    if (index < 0 || index > _size-1)
        cout << "this: " << this << ". Index out of range" << endl;

    return _array[index];
}

When used like this, it works as expected, i.e. prints 5:

Array< int > testArray(5);
testArray[0] = 5;
cout << testArray[0] << endl;

However, I would like to use the class for a two-dimensional dynamic array. I thought that the following would just magically work and print 5…

Array< Array<int> > testArray(5);
testArray[0][0] = 5;
cout << testArray[0][0] << endl;

…but it does not work. It crashes when I try to set the value at [0][0]. The debugger shows me that this has _size set to 0 and _array to NULL. this at that point points to the first element of the _array of the last created Array instance.

One of the things I don’t get is when the “inner” array calls its constructor. Stepping through the code, I see that Array(int size) is called once and Array() five times. I would like to create the inner array with a specific size, but using Array< Array<int>(10) > testArray(5) does not compile.

Could you provide me with some insight on this? It seems I could not quite wrap my head around templates yet…

Answer 1

You cannot chain constructor calls in C++. Your first constructor implementation does nothing, so the 5 instances contained in the parent Array end up being uninitialized, resulting in undefined behavior.

To fix, you can either add a default value to the size parameter of the other constructor, or factor out the initialization logic in a separate (private) function, and call it from both constructors.

EDIT: The reason why the first constructor does nothing is that the line

Array(5)

does not call the constructor of the current instance, but instead allocates a new (unnamed) temporary Array instance, which is immediately destructed at the end of the line.