template<typename T> Array<T>::Array(int size) : _size(size), _data(new T(size)) // should have been new T[size] {} That's not the only thing wron

template<typename T> Array<T>::Array(int size) : _size(size), _data(new T(size)) // should have been new T[size] {} That's not the only thing wrong here. private: T* _data; int _size; }; The order of initialization is the order of declaration, NOT the order you use in your constructor. _data depends on _size being initialized first. Therefore _size must be declared above _data. Note: I understand the next example goes over this issue. I just think this example should've been declared properly as to avoid distracting the reader from the main problem, "() vs []".