there could be two type of problem here i can see !

First

template <class D>SLList<D> SLList<D>&::operator+(SLList<D> ob2) {
return this+obj2;
}

Return type should be Reference to...

template <class D>SLList<D> SLList<D>::operator+(SLList<D> ob2) { return *this;}

for this funtion u have to traverse all the lists and add them up !

this return status just returning the one...