Re: [Boost-users] Turning Boost.Multi-Index ordered_non_unique index into ordered_unique one, with custom comparator

Subject: Re: [Boost-users] Turning Boost.Multi-Index ordered_non_unique index into ordered_unique one, with custom comparator
From: Joaquin M LópezMuñoz (joaquin_at_[hidden])
Date: 2016-03-03 04:21:26

Dominique Devienne <ddevienne <at> gmail.com> writes:

> 
> I would like to support a slightly unusual use-case.
> We have a class with a integer member, and a BMI container for
> instances of this class.
> 
> That member is supposed to be unique, *except* for a special "invalid"
> value (-1), which several instances can use.
> 
> Right now, we use an ordered_non_unique index, because of the -1's,
> but that leaves the door open for other duplicate "valid" values,
> which should be forbidden.
> 
> So my question is whether one can index this member uniquely, thanks to
> a custom comparator, such that the comparator does not violate the
> usual strict-weak-ordering requirement of ordered containers? 

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documentation).

Lookup for values other than -1 behaves perfectly, as it is only when
invoking operator()(-1,-1) that strict weak ordering properties are
violated. If you're able to live in a state of undefined behavior and
cope with the oddities described above, well, you might take advantage of
this.

Joaquín M López Muñoz