Question

This occured in the line of thought following Template specialization or conditional expressions?.

I am using template specialisation in a project of mine and came across this example from Stroustrup: Matrix.h, where he declares a MatrixBase template class

template<class T> class Matrix_base

for common elements and a Matrix template class

template<class T = double, int D = 1> class Matrix

as a “prop” (whatever that is) for specialisations. He declares the constructor as private so that only specialisations can be instanciated. These are declared:

template<class T> class Matrix<T,1> : public Matrix_base<T> {...};
template<class T> class Matrix<T,2> : public Matrix_base<T> {...};
template<class T> class Matrix<T,3> : public Matrix_base<T> {...};

My question is: In this case, what is the advantage of specialisation? Obviously there is no code that the three specialisations have in common, so why not cut out the general template and declare:

template<class T> class Matrix_1<T> : public Matrix_base<T> {...};
template<class T> class Matrix_2<T> : public Matrix_base<T> {...};
template<class T> class Matrix_3<T> : public Matrix_base<T> {...};

?

Answer 1

Because by having the second template parameter, one allows for specializations as well as a general, non-specialized implementation. So

Matrix<float, 1000> m;

might do something reasonable but non specialized, whereas you would have to define a Matrix_1000<T>.
Edit: the first point applies in general, but not to this particular case, where the general case has a private constructor.

Furthermore, it allows you to do stuff like

Matrix<double, SOME_CONSTANT> m;

which you cannot do with your _N solution.