Re: [boost] [MPL] Bug with default lambda expressions?

Subject: Re: [boost] [MPL] Bug with default lambda expressions?
From: Larry Evans (cppljevans_at_[hidden])
Date: 2013-05-21 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