Pointer doesn't get modified in class member function

Question

I'm making a Dynamic Array with templates and classes.

This is the code I'm having issues with:

template<typename GType>
class GArray
{
   GType* array_type = nullptr;
   int size = 0;

public:
GArray(GType Size)
{
    size = Size;
    array_type = new GType[size];

    for (int i = 0; i < size; i++)
        array_type[i] = NULL;
}

void Push(GType Item)
{
    size++;

    GType* temp = new GType[size];

    for (int i = 0; i < size-1; i++)
        temp[i] = array_type[i];

    temp[size] = Item;

    delete[] array_type;
    array_type = temp;
    temp = nullptr;
}

GType& operator[] (int Index)
{
    if (Index >= 0 && Index < size)
        return array_type[Index];
}
};

int main()
{
GArray<int> arr(2);
arr[0] = 10;
arr[1] = 20;
arr.Push(30);


// print array
for (int i = 0; i < arr.Size(); i++)
    cout << arr[i] << endl;

return 0;
}

In the main(), when I print the whole array values, the last one (which should be 30) is an undefined value (like -842150451).

With several tests, I can say that INSIDE the Push() function, the array_type pointer changes. When I come back to the main(), it's like array_type didn't change, it's the same as before.

Your operator [] does not always return a value! Should not compile — Ed Heal
– Ed Heal, Commented Jan 4, 2016 at 11:53
Using case to determine the variable is not a good idea - typos hard to sport (line size = Size;) — Ed Heal
– Ed Heal, Commented Jan 4, 2016 at 11:54
Should learn about const and reference i.e line ` void Push(GType Item)` should be ` void Push(const GType& Item)` — Ed Heal
– Ed Heal, Commented Jan 4, 2016 at 11:55
Yes, the standard defines it as UB: "Flowing off the end of a function is equivalent to a return with no value; this results in undefined behavior in a value-returning function." — In clang and gcc -Werror=return-type can be used to emit an error. — flogram_dev
– flogram_dev, Commented Jan 4, 2016 at 12:15
@Michele - You set the warning level for the compiler. You have to google it as I have not used VS for a while — Ed Heal
– Ed Heal, Commented Jan 4, 2016 at 12:16

Adi Levin · Accepted Answer · 2016-01-04 12:02:32Z

6

The reason for the bug is that

temp[size] = Item

Is wrong. It should be replaced by

temp[size-1] = Item

answered Jan 4, 2016 at 12:02

Adi Levin

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