[boost] Re: shared_ptr: operator<

From: Kevlin Henney (kevlin_at_[hidden])
Date: 2000-02-28 02:39:18

In message <Pine.LNX.4.10.10002272359490.17343-100000_at_boneone.endoframe.
com>, Braden N. McDaniel <braden_at_[hidden]> writes
>Kevlin: I think I see your point. But while it's true that the particulars
>the ordering implied by operator< are almost certainly uninteresting, this
>really isn't the point. I think it may be useful for a containter to be
>keyed on a shared_ptr: for some uses of std::set and std::map, the order
>of the elements may be uninteresting to the user; rather, the use of these
>containers may be motivated by other features.
>
>It is certainly true that one can create one's own comparator (either a
>specialization of std::less or something original). However, having
>operator< available to suit the default case seems like a useful
>convenience.

I think that operator< solves the wrong problem and sends out the wrong
signals: If the problem is that you wish to have a std::set of
shared_ptr the default case solution is to provide a Boost
specialisation of std::less, not operator<. Overloading operator<
strongly suggests that there is an interesting and easily repeatable
ordering (ie across program executions), which is not the case.

Given that the standard does not assume a total ordering for pointers
based on operator<, it seems a little presumptuous to introduce it for
shared_ptr. The irony is that to implement operator< you would have to
do it in terms of std::less on the underlying pointer :->
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  Kevlin Henney                 Without art we are nothing
  kevlin_at_[hidden]                but monkeys with car keys
  kevlin_at_[hidden]
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