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

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 > struct equal;

This works fine most of the time, but I was recently bitten by the following:

template struct find_vector : find_if< VectorOfVectors, equal > { };

typedef find_vector< vector< vector, vector >, vector >::type ThisWillBreak;

What happens here is that the `equal` expression inside `find_vector` really is `equal >` 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 >::type> struct equal;

or equivalently

template > 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