template <class T>class Array { protected : T* data; int size; }; template<class T>class Stack : protected Array<T> { int top; public: Stack(){}; public: void Push(T x) {data[++top] = x;} }; Why it say '"data" was not declared in this scope' in Push? How can I fix this? When I delete each template<T>, it works normally. Is something wrong in my template?
You need:
template <class T>class Array { protected : T* data; int size; }; template<class T>class Stack : protected Array<T> { int top; public: Stack(){}; public: // void Push(T x) {data[++top] = x;} <-- Won't compile void Push(T x) {this->data[++top] = x;} // <-- Will compile // void Push(T x) {Array<T>::data[++top] = x;} <-- Will also compile }; Because within the implementation of a class template derived from a class template, members of the base template must be referred to via the this pointer or with base-class qualification.
As an addition to @Mike's answer you might consider using using declaration:
template <class T>class Array { protected : T* data; int size; }; template<class T>class Stack : protected Array<T> { using Array<T>::data; // <--- HERE int top; public: Stack(){}; public: void Push(T x) {data[++top] = x;} }; Now data becomes available in Stack class.