status changed; resolution set

Joaquín M López Muñoz — Fri, 05 Dec 2014 10:45:56 GMT

status new → closed
resolution → wontfix

Hi Maxim,

std::equal_to<void>: in order to deal with compatible keys for hashed indices, you'd need not only transparent equality comparison but also some sort of transparent hash functor such as

struct generic_hash
{
  template<typename T>
  std::size_t operator()(const T& x)const
  {
     boost::hash<T> h;
     return h(x);
  }
};

but using this is tricky (and dangerous):

multi_index_container<
  std::string,
  indexed_by<
    hashed_unique<identity<std::string>,generic_hash,std::less<void>>
  >
> c{"hello"};
std::cout<<*(c.find("hello"))<<"\n"; // crash

The reason for the problem is: hashing a std::string does not yield the same value has hashing a const char*, so c.find("hello") does not find the string "hello". This is why N3657 applies only to associative containers and has not been extended to unordered associative containers.

As for std::less<void>, I'm sympathetic to your proposal but would prefer to go in line with the standard, which decided for std::less<void> to be explicitly provided by the user rather than the default.

bugs@… — Mon, 08 Dec 2014 10:19:47 GMT

@joaquin

Thanks for the insightful explanations.

I love boost multi index containers and I'm continuously surprised just how much effort went into not creating arbitrary limitations.

Boost C++ Libraries: Ticket #10678: search by a compatible key and default predicates

status changed; resolution set