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l-singleton.h
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/* Copyright 2011 ZAO "Begun".
*
* This library is free software; you can redistribute it and/or modify it under
* the terms of the GNU Lesser General Public License as published by the Free
* Software Foundation; either version 3 of the License, or (at your option)
* any later version.
* This library 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 Lesser General Public License for more
* details.
* You should have received a copy of the GNU Lesser General Public License
* along with this library. If not, see <http://www.gnu.org/licenses/>.
*/
#ifndef __LOCKFREE_SINGLETON_H
#define __LOCKFREE_SINGLETON_H 1
#include <new>
#include <atomic>
namespace lockfree
{
struct Once
{
constexpr Once() : l(2) {}
std::atomic <int> l;
bool
lock_once ()
{
int c = l.load(std::memory_order_relaxed);
for ( ; ; )
{
if (c == 0)
return false;
if (c == 2) {
if (l.compare_exchange_weak(c, 1, std::memory_order_release, std::memory_order_acquire)) {
return true;
}
} else {
c = l.load(std::memory_order_acquire);
}
}
}
void
unlock ()
{
l.store(0, std::memory_order_release);
}
};
template <typename T>
struct Raw_
{
char data [sizeof (T)];
} __attribute__ ((aligned (16)));
template <typename T>
struct Holder
{
static Raw_ <T> value;
static Once once;
};
template <typename T> Raw_ <T> Holder <T>::value = {{}}; //TODO
template <typename T> Once Holder <T>::once;
template <typename T>
T &
singleton ()
{
typedef Holder <T> H;
if (H::once.lock_once ())
{
new (&H::value) T ();
H::once.unlock ();
}
return (T &) H::value;
}
} //namespace lockfree
#endif //__LOCKFREE_SINGLETON_H