[并发并行]_[线程同步]_[C/C++实现单例模式分析]

场景:
1. 很多情况下,我们需要实现一个类的单例模式,比如XXManager, 于是提供一个GetInstance()的函数. Java可以使用双锁来处理.C/C++也类似.
2. 提供GetInstance()的一个必要条件是必须是线程安全的, 否则会出现创建多个实例的情况. 更简单的做法是在程序初始化时(Main线程)
调用一个CreateInstance()函数来创建, 但是如果这种单例对象多了的话, 这来CreateInstance调用也会增多, 如果忘记了调用还会直接崩溃.
3. 这里实现了C/C++ 创建单例模式的方法, 一种是使用pthread库创建的, 一种是使用win32 api创建.

4. Objc可以使用 dispatch_once来实现单例模式.
+(DhTaskManager*)getInstance
{
static DhTaskManager *manager = nil;
static dispatch_once_t predicate;
dispatch_once(&predicate, ^{
manager = [DhTaskManager new];
});
return manager;
}

test.cpp
#include <Windows.h>
#include <stdint.h>
#include "pthread.h"
#include <iostream>
#include <assert.h>

static int THREADCOUNT = 100;

class CSLock
{
public:
CSLock()
{
InitializeCriticalSection(&cs_);
std::cout << "init lock!" << std::endl;
}

~CSLock()
{
DeleteCriticalSection(&cs_);
}
operator PCRITICAL_SECTION() throw()
{
return &cs_;
}
private:

CRITICAL_SECTION cs_;
};

// 实用场景1: 创建单例对象.
class UiDeviceManager
{
public:
static void Destroy()
{
delete singleton_;
}

// 1.使用pthread_once来实现单例模式.
static UiDeviceManager* GetInstance()
{
pthread_once(&random_is_initialized,init_instance);
return singleton_;
}

// 1.使用Windows的原子锁实现单例模式,不需要pthread库
static UiDeviceManager* GetInstance2()
{
static LONG done = 0;
if (!InterlockedExchangeAdd(&done, 0))
{
EnterCriticalSection(cs_lock_);
if(!done)
{
init_instance();
done = 1;
}

LeaveCriticalSection(cs_lock_);
}
return singleton_;
}

private:
static pthread_once_t random_is_initialized;
static CSLock cs_lock_;

static void init_instance ()
{
//测试,Sleep 3 seconds,验证这个函数只被执行一次.
//看有没有线程进入这里.
//这里只是测试
Sleep(3000);
std::cout << "Sleep End" << std::endl;
static LONG locki_; //1. 这个变量只是测试用.
InterlockedExchangeAdd(&locki_,1);
assert(locki_ == 1);

//只需要new
singleton_ = new UiDeviceManager();
}
UiDeviceManager()
{

}
static UiDeviceManager* singleton_;

};

CSLock UiDeviceManager::cs_lock_;
pthread_once_t UiDeviceManager::random_is_initialized = PTHREAD_ONCE_INIT;
UiDeviceManager* UiDeviceManager::singleton_ = NULL;
static pthread_barrier_t barrier = NULL;

void* ThreadFunc(void* data)
{
for (int i = 0; i < 10000; ++i)
{
// UiDeviceManager* udm = UiDeviceManager::GetInstance();
UiDeviceManager* udm = UiDeviceManager::GetInstance2();
}

pthread_barrier_wait(&barrier);
std::cout << "ThreadFunc Finish" << std::endl;
return NULL;
}

void TestPthreadOnce()
{
pthread_barrier_init(&barrier,NULL, THREADCOUNT + 1);

for (int i = 0; i < THREADCOUNT; ++i)
{
pthread_t t;
pthread_create(&t,NULL,ThreadFunc,NULL);
pthread_detach(t);
}
std::cout << "TestPthreadOnce Begin" << std::endl;
pthread_barrier_wait(&barrier);
std::cout << "TestPthreadOnce Finish" << std::endl;
pthread_barrier_destroy(&barrier);

UiDeviceManager::Destroy();
}

int main(int argc, char const *argv[])
{
TestPthreadOnce();
return 0;
}

输出:
init lock!
TestPthreadOnce Begin
Sleep End
ThreadFunc Finish
ThreadFunc Finish
ThreadFunc Finish
TestPthreadOnce Finish
ThreadFunc Finish
ThreadFunc Finish
ThreadFunc Finish
ThreadFunc Finish
ThreadFunc Finish
ThreadFunc Finish
ThreadFunc Finish
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ThreadFunc Finish
ThreadFunc Finish
ThreadFunc Finish
ThreadFunc Finish
ThreadFunc Finish

5. 其实还有一个更简单的方法实现单例模式,利用了C/C++的静态存储区特性,因为类的静态实例是在程序开始运行时main运行前加载的,所以就保证了只加载一次. 2015.12.10
h头文件
class ARLModuleManager
{
public:

static ARLModuleManager* GetModuleManager();

private:
static ARLModuleManager manager_;
ARLModuleManager(){}
virtual ~ARLModuleManager();
cpp文件:

ARLModuleManager ARLModuleManager::manager_;

ARLModuleManager* ARLModuleManager::GetModuleManager()
{
return &manager_;
}

参考:
http://www.douban.com/note/269039110/  这个其实是有缺陷的!

pthread_once 函数源码:
int
pthread_once (pthread_once_t * once_control, void (*init_routine) (void))
{
if (once_control == NULL || init_routine == NULL)
{
return EINVAL;
}

if (!InterlockedExchangeAdd((LPLONG)&once_control->done, 0)) /* MBR fence */
{
ptw32_mcs_local_node_t node;

ptw32_mcs_lock_acquire((ptw32_mcs_lock_t *)&once_control->lock, &node);

if (!once_control->done)
{

#ifdef _MSC_VER
#pragma inline_depth(0)
#endif

pthread_cleanup_push(ptw32_once_on_init_cancel, (void *)&node);
(*init_routine)();
pthread_cleanup_pop(0);

#ifdef _MSC_VER
#pragma inline_depth()
#endif

once_control->done = PTW32_TRUE;
}

ptw32_mcs_lock_release(&node);
}

return 0;

} /* pthread_once */
完整项目下载地址:
http://download.csdn.net/detail/infoworld/9263465