template<typename T>
class A
{
public:
    typedef std::vector<T> Vec_t;
};


template<typename T>
class B : public A<T>
{
private:
    Vec_t v;  // fails - Vec_t is not recognized
};

typename A<T>::Vec_t v;

typename A<T>::Vec_t v;  ← good

template <class T>
class X
{
    std::string s;
}

std::vector<int>::iterator foo_iterator;

A<T>::Vec_t v;
OR
A<int>::Vec_t int_type;