Question

In this question the OP asked for a solution to template typedef which is not possible in C++. The OP also presented a solution themselves but did not like it:

template<size_t N, size_t M>
class Matrix {
    // ....
};

// Problem - will not compile
typedef Matrix<N,1> Vector<N>;

// Solution
template <int N>
class Vector: public Matrix<N,1>
{ };

My question is, what advantage does the Helper::type solution give us over the OP’s solution (assuming these classes are never meant to be used by a base pointer or new‘d as such)? An empty class should carry no overhead in release (or does it?). The only disadvantage I can see is that in debug builds you will have to expand the base class when debugging.

EDIT: In addition to the selected answer, see @Dani’s answer who suggested that the inherited version would require constructors to be defined, which is an added inconvenience.

Answer 1

The point of typedef is to define a type alias. A subclass is not a type alias – it is a new type.

For example, imagine some library function

template<size_t N, size_t M>
Matrix<N, M> * createMatrix();

Now with helper type

Vector<3>::type * var = createMatrix<3, 1>();

is legal.
With inheritance

Vector<3> * var = createMatrix<3, 1>();

is not.