Question

is there a difference, from prospective of meta-programming for example, between the two
declarations?

template<typename T>
struct matrix {
    typedef matrix self_type;    // or
    typedef matrix<T> self_type;
};

thank you

Answer 1

In this particular situation (inside a class template), matrix is a shorthand for matrix<T>. When you write lots of hairy templates all day long while trying to fit everything in 80 columns, the shorthand is welcome.

Note that you can also abbreviate method arguments:

template <typename T>
struct matrix
{
    typedef matrix my_type;
    matrix(); // constructor is abbreviated too
    matrix& operator=(matrix);
};

// Method argument types can be abbreviated too
// but not result types.
template <typename T>
matrix<T>& matrix<T>::operator=(matrix m)
{
    // ...
}