Re: What should std::InputIterator<T>::pointer be? [n2193 / n2083]

From: (Niels Dekker - no return address)
Sun, 1 Apr 2007 19:52:33 GMT
Richard Smith wrote:

It's standard practice to implement operator-> on such iterators by
way of a proxy object in order to extend the lifetime of the
  class my_iterator {
    struct arrow_proxy { [...]
    arrow_proxy operator->() const { return **this; }
However, it is also current practice to make the 'pointer' typedef a
raw pointer (see the boost::iterator_facade and elsewhere in Boost for
  typedef value_type const* pointer;

Even the current (pre-concept) draft of the Standard explicitly
specifies 'pointer' to be the return type of operator->():
  shall be defined as the iterator's reference and pointer types,
  that is, for an iterator object a, the same type as the type of
  *a and a->, respectively.
(Iterator traits, section 24.3.1)

See also DR #445, by David Abrahams, 2003-12-09:

So I think that Boost's iterator_facade is slightly outdated, as the
return type of iterator_facade::operator->() might be different from

As arrow_type is convertible to pointer, my_iterator::operator->()
will satisfy this associated function requirement, but concept-based
code will now see an operator-> that returns a raw pointer.
  concept foo<typename T> { void T::bar(); };
  template < std::InputIterator Iter >
    requires std::SameType<Iter::value_type, foo>
  void fn Iter i ) { i->bar(); }
I'm not entirely sure I've got this correct syntacticaly

I guess you mean:
  template < std::InputIterator Iter >
    requires foo<Iter::value_type>
  void fn( Iter i ) { i->bar(); }

But assuming this is legal modulo any syntactic snafus,
does it cause undefined behaviour when used with my_iterator<foo>?
I.e. does it call foo::bar on a foo object after it has been
destroyed? So far as I can tell, the answer is 'yes'.

Yes, it does. So there's a serious problem if my_iterator::pointer is
not the return type of my_iterator::operator->().

Have I understood this correctly? If so, is there some way of around
this that doesn't require changing the 'pointer' typedef in all
existing code?

Yes, by writing your own concept map, that redefines both 'pointer' and
operator-> for my_iterator. For instance by using operator_arrow_proxy
from boost/iterator/iterator_facade.hpp, as follows:

namespace std {
  concept_map InputIterator<my_iterator> {
    typedef boost::detail::operator_arrow_proxy<value_type> pointer;
    pointer operator->(const my_iterator& X) {
      pointer result( X.operator->() );
      return result;
} } }

If it still does not compile on ConceptGCC, it's probably a compiler
bug! And indeed, ConceptGCC 4.1.1 alpha 5 says:
  error: result of 'operator->()' yields non-pointer result
Douglas Gregor told me that the current version of ConceptGCC does not
preserve the proxy as long as it should, he just didn't have the time to
fix it yet.

Niels Dekker
C++ programmer at LKEB, Leiden University Medical Center

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