Re: Confused by <bits/predefined_ops.h>

[Jonathan Wakely <jwakely at redhat dot com>]

On 19/01/17 08:32 +0000, Jonathan Wakely wrote:
> On 18/01/17 22:17 +0100, François Dumont wrote:
> > Hi
> >
> > Names of functions and struct in this file are surely not what I am 
> > most proud of in libstdc++. I even think that I confess it when I 
> > proposed the patch :-)
> >
> > As those elements are purely internal and we rarely need to be touch 
> > I don't consider it as a big issue but if you want to rename 
> > everything don't hesitate.
> >
> > Note that I also have this patch pending:
> >
> > https://gcc.gnu.org/ml/libstdc++/2015-10/msg00071.html
> >
> > But I can surely rework it on top of your changes unless you just 
> > want me to forget it.
>
> It looks good, let's return to it in stage 1 for gcc 8.
>
> Any thoughts on the questions repeated below?
>
> As for PR78346, I can fix it by making the relevant function object
> types take their constructor arguments by reference, so that the
> reference we store in _M_ref is bound to some iterator in the caller's
> scope, not the constructor argument that goes out of scope immediately
> (see patch below). But why do those objects store a reference anyway?
> Why don't they just store the iterator passed to the constructor and
> dereference it as needed? Is this to avoid dereferencing multiple
> times?  Because that might be slow?

This is the alternative I prefer, storing the iterator and
dereferencing as needed:

@@ -207,17 +207,17 @@ namespace __ops
  template<typename _Iterator1>
    struct _Iter_equals_iter
    {
-      typename std::iterator_traits<_Iterator1>::reference _M_ref;
+      _Iterator1 _M_it1;

      explicit
      _Iter_equals_iter(_Iterator1 __it1)
-       : _M_ref(*__it1)
+       : _M_it1(__it1)
      { }

      template<typename _Iterator2>
       bool
       operator()(_Iterator2 __it2)
-       { return *__it2 == _M_ref; }
+       { return *__it2 == *_M_it1; }
    };

  template<typename _Iterator>
@@ -273,16 +273,16 @@ namespace __ops
    struct _Iter_comp_to_iter
    {
      _Compare _M_comp;
-      typename std::iterator_traits<_Iterator1>::reference _M_ref;
+      _Iterator1 _M_it1;

      _Iter_comp_to_iter(_Compare __comp, _Iterator1 __it1)
-       : _M_comp(_GLIBCXX_MOVE(__comp)), _M_ref(*__it1)
+       : _M_comp(_GLIBCXX_MOVE(__comp)), _M_it1(__it1)
      { }

      template<typename _Iterator2>
       bool
       operator()(_Iterator2 __it2)
-       { return bool(_M_comp(*__it2, _M_ref)); }
+       { return bool(_M_comp(*__it2, *_M_it1)); }
    };

  template<typename _Compare, typename _Iterator>