c++单例三种实现

c++实现一：

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

class Singleton
{
public:
static Singleton* getInstance();

private:
static Singleton* Instance;
Singleton(){}
Singleton(const Singleton&);
Singleton& operator=(const Singleton&);
};

Singleton* Singleton::Instance = NULL;
Singleton* Singleton::getInstance()
{
if(Instance == NULL)
{
Instance = new Singleton();
}
return Instance;
}

int main(int argc, char *argv[])
{
Singleton* instance_a = Singleton::getInstance();
Singleton* instance_b = Singleton::getInstance();

if(instance_a == instance_b)
std::cout << "OK" << std::endl;

return 0;
}

第二种实现方式：

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

class Singleton
{
public:
static Singleton* Instance;
static Singleton* getInstance()
{
return Instance;
}
private:
Singleton(){}
Singleton(const Singleton&);
Singleton& operator=(const Singleton&);
};

Singleton* Singleton::Instance = new Singleton();

int main(int argc, char *argv[])
{
Singleton* instance_a = Singleton::getInstance();
Singleton* instance_b = Singleton::getInstance();

if(instance_a == instance_b)
std::cout << "OK" << std::endl;

return 0;
}

第三种实现方式：

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

class Singleton
{
public:
Singleton(){}
static Singleton* Instance;
static void InitSingleton() { Instance = new Singleton();}
static Singleton* getInstance();
private:
Singleton(const Singleton&);
Singleton& operator=(const Singleton&);
};

static pthread_once_t once = PTHREAD_ONCE_INIT;
Singleton* Singleton::Instance = NULL;

Singleton* Singleton::getInstance()
{
pthread_once(&once, Singleton::InitSingleton);
return Instance;
}

int main(int argc, char *argv[])
{
Singleton* instance_a = Singleton::getInstance();
Singleton* instance_b = Singleton::getInstance();

if(instance_a == instance_b)
std::cout << "OK" << std::endl;

return 0;
}

pthread_once函数：

int pthread_once(pthread_once_t *once_control, void (*init_routine) (void))

这个函数使用初值为PTHREAD_ONCE_INIT的once_control变量保证init_routine()函数在本进程执行序列中仅执行一次。