template<typename T> class A
{
    public :
        T t;
};

template<typename T> class B : public A<T>
{
    public :
        T f() { return t; }
};

In member function 'T B<T>::f()':
error: 't' was not declared in this scope
         T f() { return t; }
                        ^

template<typename T> class B : public A<T>
{
    public :
        T f() { return A<T>::t; }
};

template<typename T> class B : public A<T>
{
    public :
        T f() { return this->t; }
};