[Boost-users] [hash][vc-7_1] warning on boost::hash<std::size_t> [was Re: [multi_index] function object as key extractor?]

From: Joaquín Mª López Muñoz (joaquin_at_[hidden])
Date: 2007-03-15 11:23:03

Filip Konvi?ka ha escrito:

> > Looks like an overzealous or simply misguided warning. Can you please
> > try the following?
> >
> >   #include <boost/functional/hash/hash.hpp>
> >   #include <cstddef>
> >
> >   int main()
> >   {
> >     std::size_t x=0;
> >     boost::hash<std::size_t>()(x);
> >   }
> >
> > Same warning? If so, you might ask for inclusion of a suitable
> > #pragma warning(disable...) to the Boost.Hash author.
> >
> Yes...in fact,
>
>     boost::hash<unsigned int>()(x);
>
> works fine, since the function being called (via boost::hash and
> boost::call_hash) is defined as
>
>     inline std::size_t hash_value(unsigned int v)
>     {
>         return static_cast<std::size_t>(v);
>     }

Do you mean hash_value(unsigned int) is also invoked when using
boost::hash<std::size_t>, right?

> There are other variants of this function, taking bool, int, etc. as the
> argument, but no size_t.

This is correct, because std::size_t is a typedef to some builtin
unsigned integer type.

>  I'm not sure what is the official interface of boost::hash, so I don't
> know whether it's being used correctly in multi_index or not.

I hope Boost.Hash is used correctly by Boost.MultiIndex :) but anyway
the issue has little to do with the last lib since the snippet I provided in

my previous post (involving only Boost.Hash) replicates the warning.

Are you compiling with /Wp64 (Detect 64-Bit Portability Issues)? If
so, could you please:

a) look in your stdlib headers (most likely in stddef.h) how size_t is
defined?
b) try the following snippet and report whether the warning persists?

  #include <cstddef>

  void foo(unsigned int){}

  int main()
  {
    std::size_t x=0;
    foo(x);
  }

Thank you!

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo