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Subject: Re: [boost] [MPL] Bug with default lambda expressions?
From: Larry Evans (cppljevans_at_[hidden])
Date: 20130521 19:04:58
On 05/21/13 15:43, Louis Dionne wrote:
> Hi,
>
> Some higher order algorithms in the MPL have a default for the lambda
> expression they accept. A good example is `boost::mpl::equal`:
>
> template <typename S1, typename S2, typename Pred = is_same<_, _> >
> struct equal;
>
> This works fine most of the time, but I was recently bitten by the following:
>
> template <typename VectorOfVectors, typename Vector>
> struct find_vector
> : find_if<
> VectorOfVectors, equal<Vector, _1>
> >
> { };
>
> typedef find_vector<
> vector<
> vector<int, int>,
> vector<char, char>
> >,
> vector<char, char>
> >::type ThisWillBreak;
>
> What happens here is that the `equal<Vector, _1>` expression inside
> `find_vector` really is `equal<Vector, _1, is_same<_1, _2> >` because
> of the default value for the predicate to `equal`. When the lambda is
> evaluated, the placholders inside the inner `is_same<_1, _2>` expression
> are replaced too, which yields unexpected results.
>
> Using
>
> template <typename S1, typename S2,
> typename Pred = typename lambda<is_same<_, _> >::type>
> struct equal;
>
> or equivalently
>
> template <typename S1, typename S2, typename Pred = quote2<is_same> >
> struct equal;
>
> fixes the issue. Also note that all of the unit tests of the MPL still pass
> with these changes. Is the current behavior intended, or should I submit
> a patch?
>
>
> Regards,
>
> Louis Dionne
>
This sounds like a problem similar to the one here:
http://article.gmane.org/gmane.comp.lib.boost.devel/227344
Maybe reading that thread would give you some ideas for
a solution.
HTH.
Larry
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