[并发并行]_[线程同步]_[C/C++实现单例模式分析]_static modulemanager& getinstance()-优快云博客

本文介绍在C/C++中实现单例模式的多种方法，包括使用pthread_once、Windows原子锁及静态存储区特性等，并通过具体示例代码演示不同实现方式的特点。

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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
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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.youkuaiyun.com/detail/infoworld/9263465