Re: [boost] Please don't define a fake"operator<"just for orderedcontainers

From: Peter Dimov (pdimov_at_[hidden])
Date: 2006-07-11 14:15:46

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.

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.

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.

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.