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