1. 單例模式 （C++實現）

單例模式：保證一個類只有一個物件例項，並提供一個訪問該物件例項的全域性訪問點。

單例模式有兩種實現方法:懶漢模式和餓漢模式。

1.1 懶漢模式

懶漢模式：故名思義，不到萬不得已就不會去例項化類，也就是說在第一次用到類例項時候才會去例項化它。

在訪問量較小時，採用懶漢模式，這裡是以時間換空間。

執行緒安全的懶漢實現:

Singleton.cpp

#include <iostream>
#include <pthread.h>
#include <unistd.h>

using namespace std;

class Singleton
 
;

class Singleton {

private:
	static Singleton *gInstance;
	static pthread_mutex_t g_tMutex;

public:
	static Singleton *getInstance()
	{
		if (NULL == gInstance)//雙重鎖定，提高多執行緒效能
		{
			pthread_mutex_lock(&g_tMutex);
			if (NULL == gInstance)
				gInstance = new Singleton;
			pthread_mutex_unlock
 
(&g_tMutex);
		}

		return gInstance;
	}

	void printInfo(){ cout<<"This is singleton"<<endl; }

private:
	Singleton()
	{
		cout<<"Singleton()"<<endl;
	}

};

/* 懶漢模式 */
Singleton *Singleton::gInstance;
pthread_mutex_t Singleton::g_tMutex  = PTHREAD_MUTEX_INITIALIZER;

/* ------------------------------- for test --------------------------------------- */
 

void *start_routine_thread1(void *arg)
{
	cout<<"this is thread 1 ..."<<endl;

	Singleton *s = Singleton::getInstance();
	s->printInfo();	

	return NULL;
}

void *start_routine_thread2(void *arg)
{
	cout<<"this is thread 2 ..."<<endl;

	Singleton *s = Singleton::getInstance();
	s->printInfo();	

	return NULL;
}

int main()
{
	Singleton *s = Singleton::getInstance();
	s->printInfo();	

	Singleton *s2 = Singleton::getInstance();
	s2->printInfo();

	Singleton *s3 = Singleton::getInstance();
	s3->printInfo();

	/* 建立執行緒,線上程裡也去呼叫Singleton::getInstance */
	pthread_t thread1ID;
	pthread_t thread2ID;

	pthread_create(&thread1ID, NULL, start_routine_thread1, NULL);
	pthread_create(&thread2ID, NULL, start_routine_thread2, NULL);

	sleep(3);

	return 0;
}

測試:

$ g++ Singleton.cpp -pthread
$ ./a.out
Singleton()
This is singleton
This is singleton
This is singleton
this is thread 1 ...
This is singleton
this is thread 2 ...
This is singleton

1.2 餓漢模式

餓漢模式:餓了肯定要飢不擇食，所以在單例類定義的時候就進行例項化。

在訪問的執行緒比較多時，採用餓漢模式，可以實現更好的效能，這裡是以空間換時間。餓漢模式執行緒是安全的，因為一開始已經對單例類進行的例項化。

餓漢模式的實現:

Singleton2.cpp

#include <iostream>
#include <pthread.h>
#include <unistd.h>

using namespace std;

class Singleton;

class Singleton {

private:
	static Singleton *gInstance;

public:
	static Singleton *getInstance()
	{
		return gInstance;
	}

	void printInfo(){ cout<<"This is singleton"<<endl; }

private:
	Singleton()
	{
		cout<<"Singleton()"<<endl;
	}

};

/* 餓漢模式 */
Singleton *Singleton::gInstance = new Singleton;

/* ------------------------------- for test --------------------------------------- */
void *start_routine_thread1(void *arg)
{
	cout<<"this is thread 1 ..."<<endl;

	Singleton *s = Singleton::getInstance();
	s->printInfo();	

	return NULL;
}

void *start_routine_thread2(void *arg)
{
	cout<<"this is thread 2 ..."<<endl;

	Singleton *s = Singleton::getInstance();
	s->printInfo();	

	return NULL;
}

int main()
{
	Singleton *s = Singleton::getInstance();
	s->printInfo();	

	Singleton *s2 = Singleton::getInstance();
	s2->printInfo();

	Singleton *s3 = Singleton::getInstance();
	s3->printInfo();

	/* 建立執行緒,線上程裡也去呼叫Singleton::getInstance */
	pthread_t thread1ID;
	pthread_t thread2ID;

	pthread_create(&thread1ID, NULL, start_routine_thread1, NULL);
	pthread_create(&thread2ID, NULL, start_routine_thread2, NULL);

	sleep(3);

	return 0;
}

除錯:

$ g++ Singleton2.cpp -pthread
$ ./a.out
Singleton()
This is singleton
This is singleton
This is singleton
this is thread 1 ...
this is thread 2 ...
This is singleton
This is singleton