By debugging I found that calling emplace constructs the object and std::moves it. This is not consistent with std::optional. I'm using a std::vector<nonstd::optional<OBJ>> object named cms and OBJ has a constructor taking three arguments. In this code:

                               if (not cms[index]) {
                                        cms[index].emplace(e, p, m);
                                }

...the destructor of OBJ is called unexpectedly because here:

+ 46 at optional.hpp:763
   760 	    void emplace( Args&&... args )
   761 	    {
   762 	        *this = nullopt;
-> 763 	        initialize( T( std::forward<Args>(args)...) );
   764 	    }
   765 	

OBJ is apparently created and destroyed temporarily.