Re: [boost] Boost.Variant is not EqualityComparable

Subject: Re: [boost] Boost.Variant is not EqualityComparable
From: Eric Friedman (ebf_at_[hidden])
Date: 2009-05-27 13:50:53

I think operator!= (and operator<=, operator>, operator>=) were left out for
simplicity, but no good reason other than that.

The main issue I considered at the time was that I felt variant should
forward to the underlying operator. That is, it should call operator!=,
rather than assume operator!= is equivalent to the negation of operator==.

Maybe there's no need to be this pedantic though. Do others have an opinion?

Eric

On Mon, May 25, 2009 at 6:56 PM, Brook Milligan <brook_at_[hidden]>wrote:

> I just ran into the case that Boost Variant does not satisfy the
> EqualityComparable concept.  While it provides operator==() and
> operator<() (and therefore is LessThanComparable), it does not provide
> operator!=(), which is required for the EqualityComparable concept.
>
> Is this by design or oversight?  It seems reasonable to expect that a
> Boost Variant model this concept, but perhaps I am missing something.
>
> In case it is by oversight, the following patch seems to provide the
> missing operator.
>
> Cheers,
> Brook
>
> --- boost/variant/variant.hpp.orig      2009-05-13 21:21:59.000000000 -0600
> +++ boost/variant/variant.hpp   2009-05-25 19:21:40.000000000 -0600
> @@ -1681,6 +1681,8 @@
>         return rhs.apply_visitor(visitor);
>     }
>
> +    bool operator!=(const variant& rhs) const { return !(*this == rhs); }
> +
>     bool operator<(const variant& rhs) const
>     {
>         //
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