105 lines
3.5 KiB
C++
105 lines
3.5 KiB
C++
|
/*
|
||
|
|
HASH-STANDARD.hpp - allow use both of std and boost hasher implementations
|
||
|
|
|
||
|
|
Copyright (C) Lumiera.org
|
||
|
|
2014, Hermann Vosseler <Ichthyostega@web.de>
|
||
|
|
|
||
|
|
This program is free software; you can redistribute it and/or
|
||
|
|
modify it under the terms of the GNU General Public License as
|
||
|
|
published by the Free Software Foundation; either version 2 of
|
||
|
|
the License, or (at your option) any later version.
|
||
|
|
|
||
|
|
This program is distributed in the hope that it will be useful,
|
||
|
|
but WITHOUT ANY WARRANTY; without even the implied warranty of
|
||
|
|
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
|
||
|
|
GNU General Public License for more details.
|
||
|
|
|
||
|
|
You should have received a copy of the GNU General Public License
|
||
|
|
along with this program; if not, write to the Free Software
|
||
|
|
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
|
||
|
|
|
||
|
|
*/
|
||
|
|
|
||
|
|
|
||
|
|
/** @file hash-standard.hpp
|
||
|
|
** Helper to use a single extension point for specialised hash functions.
|
||
|
|
** With the switch to C++11, there are now two competing quasi standard frameworks
|
||
|
|
** for defining individual hasher functions: the \c std::hash functor and \c <boost/functional/hash.hpp>.
|
||
|
|
** The standard library hasher is used by the (new) STL hashtables and requires an explicit
|
||
|
|
** specialisation of \c std::hash<TYPE>, while the boost hash automatically picks up an
|
||
|
|
** free function `hash_value(MyType const&)`. Additionally, Boost ships with a large collection
|
||
|
|
** of predefined hash functions for various combinations of standard containers and custom types.
|
||
|
|
** To add to this venerable mess, the standard hash defines a default specialisation which
|
||
|
|
** triggers a static assertion and halts compilation, in case no custom specialisation
|
||
|
|
** can be found.
|
||
|
|
**
|
||
|
|
** In the Lumiera code base, the habit is to define a free function \c hash_value alongside with
|
||
|
|
** custom data types. This helper uses metaprogramming to generate a bridge from the standard hasher
|
||
|
|
** to use this boost-style custom hash function if applicable.
|
||
|
|
**
|
||
|
|
** @todo 4/2014 doesn't work as expected. My suspicion is that in the actual use case (PlacementIndex),
|
||
|
|
** the type providing the hasher is mixed in through inheritance, and the template specialisation
|
||
|
|
** for this base type is not considered on lookup. ///////TICKET #722
|
||
|
|
**
|
||
|
|
** @see HashIndexed
|
||
|
|
** @see LUID
|
||
|
|
**
|
||
|
|
*/
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
#ifndef LIB_HASH_STANDARD_H
|
||
|
|
#define LIB_HASH_STANDARD_H
|
||
|
|
|
||
|
|
#include "lib/hash-value.h"
|
||
|
|
#include "lib/meta/util.hpp"
|
||
|
|
|
||
|
|
#include <boost/utility/enable_if.hpp>
|
||
|
|
//#include <boost/functional/hash.hpp>
|
||
|
|
#include <functional>
|
||
|
|
|
||
|
|
namespace lib {
|
||
|
|
namespace meta {
|
||
|
|
|
||
|
|
|
||
|
|
template <typename T>
|
||
|
|
struct has_boost_hash_subst { typedef void* Type; };
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
template<typename TY>
|
||
|
|
struct provides_BoostHasher
|
||
|
|
{
|
||
|
|
template<class X>
|
||
|
|
static Yes_t check(typename has_boost_hash_subst<decltype(hash_value(X()))>::Type);
|
||
|
|
template<class>
|
||
|
|
static No_t check(...);
|
||
|
|
|
||
|
|
enum{ value = (sizeof(Yes_t)==sizeof(check<TY>(nullptr))) };
|
||
|
|
};
|
||
|
|
|
||
|
|
}}
|
||
|
|
|
||
|
|
namespace std {
|
||
|
|
|
||
|
|
/////////////////////////////////////////////////////////////////////////TICKET #722 : attempt to provide a generic bridge to use hash_value
|
||
|
|
//
|
||
|
|
// this was my first attempt 4/2014, but doesn't work, probably because of the actual ID being inherited
|
||
|
|
// see also the attempt at the bottom of hash-indexed.hpp
|
||
|
|
|
||
|
|
|
||
|
|
template<typename TY>
|
||
|
|
struct hash< boost::enable_if<lib::meta::provides_BoostHasher<TY>,
|
||
|
|
TY > >
|
||
|
|
{
|
||
|
|
size_t
|
||
|
|
operator() (TY const& val) const noexcept
|
||
|
|
{
|
||
|
|
return hash_value(val);
|
||
|
|
}
|
||
|
|
};
|
||
|
|
|
||
|
|
|
||
|
|
}
|
||
|
|
#endif
|