boostboost-iteratorsiterator-facade

boost iterator facade and dereference() function


I'm trying to create an iterator which only can dereference to real value types, not to references. Is this possible using boost::iterator_facade, or does it require me to have values that can be returned by adress\reference.

To be more specfic, my iterator returns a std::pair of references, which means my iterators value_type is not stored anywhere, but created on the fly on dereferencing (like std::map::iterator).


Solution

  • Yes, thing you want is possible. Please, take a look at boost/iterator_facade.hpp (example is for Boost lib of version 1.49.0 but it is ok for its new distributions also):

      template <
        class Derived
      , class Value
      , class CategoryOrTraversal
      , class Reference   = Value&
      , class Difference  = std::ptrdiff_t
    >
    class iterator_facade
    

    Template argument Reference is the key. You should just specify Reference when deriving from boost::iterator_facade. For example, your code can look like as the following:

    template<typename value_type>
    class custom_iterator
        :    public boost::iterator_facade<
                 custom_iterator<value_type>,
                 value_type,
                 boost::forward_traversal_tag,
                 value_type
             >
    {
        ...
        value_type dereference() const{ return value_type(...); }
        ...
    };