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From: Sean Parent (sparent_at_[hidden])
Date: 2006-07-11 17:19:18

> Sean Parent wrote:
> > From item 6 - I'd remove the operator < from shared_ptr. If we were
> > going to keep operator < for shared ptr, then I would have to argue
> > to change all of the examples from 6 for consistency (or provide a
> > semantic difference between shared_ptr and these cases.
> There are two problems with using std::less instead of operator< to
> supply a
> set/map order. First, it doesn't propagate to composite types; if you
> provide a specialization of less<K>, less< pair<K, int> > doesn't
> work.
Well the same is true for operator < and a simple struct. I'm
proposing that the standard be extended to require std::pair<> to
supply a less<> specialization.
> Second, less<K> is defined as always using K::operator< unless K is
> a raw
> pointer; it is basically a different name for operator<, one that
> can be
> used as a predicate. This gives rise to the interpretation that
> wherever
> less<K> is encountered, the implementation is entitled to use
> K::operator<
> directly.
Not true. gives a fair amount of leeway in the case where
operator < does not provide a total ordering (a raw pointer is the
only instance given in the standard). We are allowed to specialize
classes in the standard with our own types. So boost::tuple could
specialize std::less<> without raising any conflicts.
> This could have been avoided by defining a separate relation for
> set/map
> order, either as a function reachable via ADL, or as a function
> object, then
> making sure that it is defined for all standard value types.
Why is this necessary? std::less<> serves us well and avoids the
whole ADL mess.
> With suitable changes to the standard, less<K> can be made that
> relation,
> and if this happens, shared_ptr ought to also be changed to reflect
> that.
There isn't any reason that I can find in the standard not to use
less<K> for shared_ptr. It won't work with std::pair<> but I consider
that a minor inconvenience.

(should we move the discussion to the std reflector?)


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