Forward Declaration of a Base Class

class B;

class A : public B
{

    // ...
}

namespace std
{
    class string;
}

class A
{
    string aStringToTest;
}

typedef basic_string<char> string; // aproximate

template <typename T, typename U> class Test; // correct
//template <typename T> class Test; // incorrect even if U has a default type

template <typename T, typename U = int> class Test {
   // ...
};