>Can anyone tell me why?

You need to initialize the base class, not the base class’s members. So:

class Usedchild:public Used{
public:
    Usedchild(Matrix inputmatrix, Vec inputvec):Used(inputmatrix,inputvec){};

};

should work. It may also be worth your while to look at a text on C++ syntax before diving too deep into the complicated parts of Boost.