Mon, 02 Dec 2013 11:27:48 GMT

Sorry, the bug report contained bad code: lambda<Predicate, _2, _1> (not sure why this even compiled since lambda only takes 2 arguments). The (now hopefully) correct fix should be to write bind<typename lambda<Predicate>::type, _2, _1> instead.

Because max_element takes a lambda expression (metafunction class or placeholder expression), and because bind takes only a metafunction class, we need lambda<Predicate>::type to convert a placeholder expression Predicate into a metafunction class.

Corresponding modified test program at: http://coliru.stacked-crooked.com/a/f8658230325c9dd8

#include <boost/mpl/begin_end.hpp>
#include <boost/mpl/bind.hpp>
#include <boost/mpl/distance.hpp>
#include <boost/mpl/lambda.hpp>
#include <boost/mpl/less_equal.hpp>
#include <boost/mpl/max_element.hpp>
#include <boost/mpl/min_element.hpp>
#include <boost/mpl/placeholders.hpp>
#include <boost/mpl/vector_c.hpp>
#include <iostream>
using namespace boost::mpl;
template<class Sequence, class Predicate = less<_1, _2>>
struct stable_min_element
:
    // mpl::min_element uses max_element<Sequence, not_<Predicate>>
    // max_element takes a lambda expression (metafunction class or placeholder expression)
    // bind takes only a metafunction class, so we need lambda<Predicate>::type 
    // to convert a placeholder expression Predicate into a metafunction class
    max_element<Sequence, bind<typename lambda<Predicate>::type, _2, _1>>
{};
int main()
{
    using V = vector_c<int, 1, 1, 1>;
    using MinIdx   = distance<begin<V>::type, min_element<V>::type>;
    using LEMinIdx = distance<begin<V>::type, min_element<V, less_equal<_1, _2> >::type>;
    using SMinIdx  = distance<begin<V>::type, stable_min_element<V>::type>;
    using MaxIdx   = distance<begin<V>::type, max_element<V>::type>;
    std::cout << MinIdx::value;     // ERROR: prints 2 instead of 0
    std::cout << LEMinIdx::value;   // 0
    std::cout << SMinIdx::value;    // 0
    std::cout << MaxIdx::value;     // 0
}

Boost C++ Libraries: Ticket #9449: Boost.MPL min_element bug for equal elements (fix included)