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investigate the hijacking trick proposed by "enobayram"

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...push away the definition from the standard library
and plant our own definition instead -- with a marker
typedef for metaprogramming
This commit is contained in:
Fischlurch 2014-08-16 02:04:29 +02:00
parent 79b19b270f
commit 7391d02c35
1 changed files with 95 additions and 7 deletions
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102
research/try.cpp
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@ -1,7 +1,7 @@
/* try.cpp - for trying out some language features....
* scons will create the binary bin/try
*
*/
*/
// 8/07 - how to control NOBUG??
// execute with NOBUG_LOG='ttt:TRACE' bin/try
@ -35,17 +35,100 @@
** \c boost::hash. The latter has the benefit of being simpler and less verbose
** to write, since a simple ADL function is sufficient as extension point
**
** Now it would be desirable to bridge automatically between the two systems of
** defining hash functions. Unfortunately, the standard library since GCC 4.7
** contains a default implementation of std::hash to trigger a static assertion.
** Probably this was meant to educate people, but has the adverse side effect to
** prohibit any metaprogramming through SFINAE
**
** This mistake was corrected somewhere in the 4.8.x series, but in the meantime
** we'll use the dirty trick proposed by "enobayram" to hijack and fix the problematic
** definition from the standard library.
** http://stackoverflow.com/questions/12753997/check-if-type-is-hashable
**
*/
#include <cstddef>
namespace std {
template<typename Result, typename Arg>
struct __hash_base;
/**
* Primary class template for std::hash.
* We provide no default implementation, but a marker type
* to allow detection of custom implementation through metaprogramming
*/
template<typename TY>
struct hash
: public __hash_base<size_t, TY>
{
/////// deliberately NOT defined to allow for SFINAE...
//
// static_assert (sizeof(TY) < 0, "std::hash is not specialized for this type");
// size_t operator()(const _Tp&) const noexcept;
typedef int NotHashable;
};
}
#define hash hash_HIDDEN
#define _Hash_impl _Hash_impl_HIDDEN
#include <functional>
#undef hash
#undef _Hash_impl
namespace std {
struct _Hash_impl
: public std::_Hash_impl_HIDDEN
{
template<typename ... ARGS>
static auto
hash (ARGS&&... args) -> decltype(hash_HIDDEN (std::forward<ARGS>(args)...))
{
return hash_HIDDEN (std::forward<ARGS>(args)...);
}
};
#define STD_HASH_IMPL(_TY_) \
template<> struct hash<_TY_> : public hash_HIDDEN<_TY_> { };
STD_HASH_IMPL (bool)
STD_HASH_IMPL (char)
STD_HASH_IMPL (signed char)
STD_HASH_IMPL (unsigned char)
STD_HASH_IMPL (wchar_t)
STD_HASH_IMPL (char16_t)
STD_HASH_IMPL (char32_t)
STD_HASH_IMPL (short)
STD_HASH_IMPL (int)
STD_HASH_IMPL (long)
STD_HASH_IMPL (long long)
STD_HASH_IMPL (unsigned short)
STD_HASH_IMPL (unsigned int)
STD_HASH_IMPL (unsigned long)
STD_HASH_IMPL (unsigned long long)
STD_HASH_IMPL (float)
STD_HASH_IMPL (double)
STD_HASH_IMPL (long double)
#undef STD_HASH_IMPL
}
#include <boost/functional/hash.hpp>
//#include <type_traits>
#include <iostream>
#include <string>
#include <vector>
#include <boost/functional/hash.hpp>
#include <functional>
using std::vector;
using std::string;
using std::cout;
@ -94,7 +177,7 @@ class V
};
int
int
main (int, char**)
{
string p("Путин"), pp(p);
@ -107,10 +190,15 @@ main (int, char**)
std::hash<S> std_customHasher;
boost::hash<V> boo_customHasher;
cout << "raw hash(std) = " << std_stringHasher(p) <<"|"<< std_stringHasher(pp)
<< " (boost) = " << boo_stringHasher(p) <<"|"<< boo_stringHasher(pp)
std::hash<V> boo2std_crossHar;
boost::hash<S> std2boo_crossHar;
cout << "raw hash(std) = " << std_stringHasher(p) <<"|"<< std_stringHasher(pp)
<< "\n (boost) = " << boo_stringHasher(p) <<"|"<< boo_stringHasher(pp)
<< "\n custom hash (std) " << std_customHasher(s) <<"|"<< std_customHasher(ss)
<< "\n custom hash (boost) " << boo_customHasher(v) <<"|"<< boo_customHasher(vv)
// << "\n use std from boost: " << std2boo_crossHar(s) <<"|"<< std2boo_crossHar(ss)
// << "\n use boost from std: " << boo2std_crossHar(v) <<"|"<< boo2std_crossHar(vv)
;
cout << "\n.gulp.\n";