Thread: user-defined iterator

/usr/include/c++/3.2/bits/stl_iterator.h: In constructor
`__gnu_cxx::__normal_iterator<_Iterator,
_Container>::__normal_iterator(const __gnu_cxx::__normal_iterator<_Iter,
_Container>&) [with _Iter = Expression* const*, _Iterator = Expression**,
_Container = std::vector<Expression*, std::allocator<Expression*> >]':
set.hpp:184: instantiated from `Set<T>::iterator Set<T>::begin() const [with T = Expression]'
expression.cpp:99: instantiated from here
/usr/include/c++/3.2/bits/stl_iterator.h:589: invalid conversion from `
Expression* const* const' to `Expression**'

Line 99 is j = n.begin()
expression.cpp:

Code:

void Expression::substitute(const Set<Expression>& exp, const Set<Variable_Plain>& var) {
  Set<Variable_Plain>::iterator i;
  Set<Expression>::iterator j;
  Set<Expression> n(exp);
  Set<Variable_Plain> v(var);
  for (i = v.begin(),
	 j = n.begin();
       (i != v.end())
	 && (j != n.end());
       ++i,++j) {
    //    std::cerr << "beg"<<std::endl;
    Factor_Expression f_e(*j);
    Variable vv(*i);
    algo::for_each(terms.begin(),terms.end(),&Expression::sub_substitute,this,f_e,vv);
    //    std::cerr << "end"<<std::endl;
  }
  //  std::cerr << "end substitute" << std::endl;
}

set.hpp
line 184: is "i.it = (members.begin());"

Code:

template<typename T>
class Set {
//...
  class iterator {
    public:
      typename std::vector<T*>::iterator it;
    public:
      //iterator(const typename std::vector<T*>::iterator& i) : it(i) {}
      T& operator*() const {
        return **it;
      }
      T operator++() {
        ++it;
        return **this;
      }
      T& operator++(int)  {
        it++;
        return **this;
      }
      T operator--() {
        --it;
        return **this;
      }
      T& operator--(int) {
        it--;
        return **this;
      }
      iterator operator+(int x) const {
        iterator i(it);
        for (int n=0;n<x;++n)
	  ++i;
        return i;
      }
      const bool operator==(const iterator& i) const {
        return (it == i.it);
      }
      const bool operator!=(const iterator& i) const {
        return !(*this == i);
      }
      iterator operator-(int x) const {
        iterator i(it);
        for (int n=0;n<x;++n)
	  --i;
        return i;
      }
    };
//...
  iterator begin() const {
    iterator i;
    i.it = (members.begin());
    return i;
  }
  iterator end() const {
    iterator i;
    i.it = (members.end());
    return i;
  }
};

My question is... why is it giving me that error?
invalid conversion from Expression* const* const to Expression**
Wouldn't it cast itself implicitly?

Thanks, I'll try it out tomorrow

I'm suprised someone actually understood my problem...