Defining a member function of a template class

[Flurite]

I am working with this code:

#include <iostream>
using namespace std;
 
template<typename T, int size=0>
class ArrayClass
{
private:
        T array[size];
public:
        T& operator [](int index);
};
 
template<typename T>
T& ArrayClass<T>::operator [](int index)
{
        return array[index];
}
 
int main()
{
        ArrayClass<int> arrayVariable;
        arrayVariable[3] = 5;
        cout << arrayVariable[3] << "\n";
 
        return 0;
}

and getting this error:

invalid use of incomplete type 'class ArrayClass<T, 0>'
declaration of 'class ArrayClass<T, 0>'

Does anyone know what's causing this problem? If I define the function within the class itself, it works fine, but defining it outside gives me these errors.

[ne555]

template<typename T, int size=0> class ArrayClass; Two (count them) template parameters

template<typename T> //Just one
T& ArrayClass<T>::operator [](int index)

Edit: ¿zero-size arrays?

[Flurite]

Still gives me the error if I do:

template<typename T, int size>
T& ArrayClass<T>::operator [](int index)
{
        return array[index];
}

Edit: I changed the default parameter to 5.

[ne555]

template<typename T, int size> //two
T& 
ArrayClass<T>:: //one
operator [](int index)
{
        return array[index];
}

[Flurite]

Thanks.