I'm trying to use the iterator_facade and am running into trouble. It's not really caused by the iterator_facade, but I figured I'd ask anyway. This takes some explanation, so if anyone reads this, know that I *do* appreciate it. :-) TLDR version: I'm getting a compiler error about converting between pointers of iterator_facade<> instantiations with different const qualifications on the value_type. The code's attached, in addition to mostly appearing (with a few minor formatting changes) inline below. Please help me figure out what to do to fix it. What I'm doing is a bit roundabout, but here goes. One fault with C++ iterators (at least from some point of view) is that if you want to operate on different kinds of iterators you have to care about too much -- you need to know more than just the type that op* returns (which is all you care about most of the time, iterator_traits notwithstanding), but what the underlying iterator type is. This means that if you want to write code that operates on multiple kinds of iterators, it must be a template. Templates don't suit my needs at this time, so I'm trying to write a couple wrapper classes that will do dynamic dispatch to different iterator types. Think of this as writing a C++ version of Java's Iterator interface. Thus I define a base class for my iterators: template<typename ValueType, typename ReferenceType> class iterator_base : public iterator_facade<iterator_base<ValueType, ReferenceType>, ValueType, boost::forward_traversal_tag, ReferenceType> { friend class boost::iterator_core_access; protected: virtual void increment() = 0; virtual ReferenceType dereference() const = 0; virtual bool equal(iterator_base const & other) const = 0; public: virtual iterator_base* clone() const = 0; }; (I'm only worrying about forward iterators at this time. I'm also ignoring ptrdiff_t) The first three functions are straight from the iterator_facade class, and the fourth is your typical clone() method. Now, I define a template that wraps any iterator that doesn't implement my iterator_base; it just stores an instance of the backing iterator type and does the corresponding thing (I'm removing some 'typenames' in there to avoid line wraps): template<typename BackingIter> class cplusplus_iterator_adaptor : public iterator_base<iterator_traits<BackingIter>::value_type, iterator_traits<BackingIter>::reference> { private: BackingIter backing; typedef typename iterator_traits<BackingIter>::value_type ValueType; typedef typename iterator_traits<BackingIter>::reference Reference; public: cplusplus_iterator_adaptor() {} cplusplus_iterator_adaptor(cplusplus_iterator_adaptor const & other) : backing(other.backing) {} explicit cplusplus_iterator_adaptor(BackingIter iter) : backing(iter) {} cplusplus_iterator_adaptor* clone() const { return new cplusplus_iterator_adaptor(*this); } // Actual iterator functions void increment() { ++backing; } bool equal(iterator_base<ValueType, Reference> const& other) const { const cplusplus_iterator_adaptor * p = dynamic_cast<cplusplus_iterator_adaptor const*>(&other); return p && backing == p->backing; } Reference dereference() const { return *backing; } }; Okay, now we have something that we can use like cplusplus_iterator_adaptor<vector<int>::iterator> my_itr(vec.begin()); *my_itr; ++my_itr; and it will work the same as vector<int>::iterator, except that you can treat it polymorphically from an iterator_base<int> reference or pointer, thus freeing you from caring that it wraps a vector iterator instead of just caring that it gives you an int&. There's one problem with this however, which is that if I <i>actually</i> want to deal with it polymorphically, I have to use a reference or pointer; but (1) this kinda sucks and (2) lots of code is written that operates on iterators by value (like, say, the STL). To solve this problem, I implemented a wrapper class that gives value semantics to a iterator_base<> pointer. This wrapper class also implements the elements required of the iterator concept, and additionally behaves like a cloning smart pointer. template<typename ValueType, typename ReferenceType = ValueType &> class iterator_base_wrapper : public iterator_facade<iterator_base_wrapper<ValueType, ReferenceType>, ValueType, boost::forward_traversal_tag, ReferenceType> { private: iterator_base<ValueType, ReferenceType> * backing; public: iterator_base_wrapper() : backing(0) {} iterator_base_wrapper(iterator_base_wrapper const & other) : backing(other.backing->clone()) {} explicit iterator_base_wrapper(iterator_base const & base) : backing(base.clone()) {} private: void swap(iterator_base_wrapper & other) const { std::swap(backing, other.backing); } public: iterator_base_wrapper& operator=(iterator_base_wrapper const&other) { iterator_base_wrapper copy(other); swap(copy); return *this; } ~iterator_base_wrapper() { delete backing; } private: friend class boost::iterator_core_access; virtual void increment() { ++(*backing); } virtual bool equal(iterator_base_wrapper const & other) const { return backing->equal(*other.backing); } ReferenceType dereference() const { return **backing; } }; OK, goal achieved! Somewhat. It's now possible to use the iterator_base_wrapper like you would a normal iterator: int run() { const int arr[] = {0,1,2,3,4,5,6,7,8,9,10}; const int size_arr = sizeof(arr)/sizeof(arr[0]); vector<int> v(arr, arr+size_arr); cplusplus_iterator_adaptor<vector<int>::iterator> a_beg(v.begin()); cplusplus_iterator_adaptor<vector<int>::iterator> a_end(v.end()); iterator_base_wrapper<int> ii(a_beg); iterator_base_wrapper<int> ii_end(a_end); vector<int>::const_iterator vi = v.begin(); while(ii != ii_end) { assert( *vi == *ii ); ++vi; ++ii; } assert( vi == v.end() ); return 0; } (BY the way, I totally intend on writing a couple make_* functions, a la make_pair.) OK, but now, what happens if we change things to be const_iterators? I can still make the cplusplus_iterator_adaptor: cplusplus_iterator_adaptor<vector<int>::const_iterator> a_beg(v.begin()); But I *can't* make my wrapper, using either iterator_base_wrapper<const int, const int &> ii(a_beg); or iterator_base_wrapper<int, const int &> ii(a_begin); because of this error: cannot convert from 'iterator_base<int, const int&>*' to 'iterator_base<const int, const int&>*'. The reason for *this* is that the cplusplus_iterator_adaptor uses the iterator_traits<>::value_type of the underlying iterator -- vector's const_iterator -- which isn't const-qualified when it subclasses iterator_facade. This means that the iterator_facade base class in both cases are different types. What's the best way to fix this? The best I can come up with is to add like a clone_const() function that returns a const-qualified version of the value type. But that seems ugly. Is there something elegant I'm missing? Evan Driscoll