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Boost.Intrusive implements C++14 Null Forward Iterators, a feature that was introduced with C++14. This means that equality and inequality comparison are defined over all iterators for the same underlying sequence and the value initialized-iterators.
Value initialized iterators behave as if they refer past the end of the same empty sequence:
list<MyType> l = { ... }; auto ni = list<MyType>::iterator(); auto nd = list<MyType2>::iterator(); ni == ni; // True. nd != nd; // False. ni == nd; // Won't compile.
        The paper N2913, titled SCARY
        Iterator Assignment and Initialization, proposed a requirement that
        a standard container's iterator types have no dependency on any type argument
        apart from the container's value_type,
        difference_type, pointer type,
        and const_pointer type. In
        particular, according to the proposal, the types of a standard container's
        iterators should not depend on the container's key_compare,
        hasher, key_equal,
        or allocator types.
      
That paper demonstrated that SCARY operations were crucial to the performant implementation of common design patterns using STL components. It showed that implementations that support SCARY operations reduce object code bloat by eliminating redundant specializations of iterator and algorithm templates.
        Boost.Intrusive containers are a bit different
        from standard containers. In particular, they have no allocator parameter
        and they can be configured with additional options not present in STL-like
        containers. Thus Boost.Intrusive offers
        its own SCARY iterator
        implementation, where iterator types don't change when the container is configured
        with an option that does not alter the value <-> node transformation.
        More concretely, the following options and conditions guarantee that iterator
        types are unchanged:
      
size_type<>, constant_time_size<>,
          slist:
            cache_last<>,
            linear<>,
          unordered_[multi]set:
            hash<>,
            equal<>,
            power_2_buckets<>,
            cache_begin<>.
          [multi]set,
            avl_[multi]set, sg_[multi]set,
            bs_[multi]set, splay_[multi]set,
            treap_[multi]set): compare<>.
          treap_[multi]set:
            priority<>
          bs_[multi]set,
            sg_[multi]set, treap_[multi]set,
            splay_[multi]set: They share the same
            iterator type when configured with the same options.