单例模式的懒汉模式与饿汉模式之间的对比C++

    单例模式，是GOF 23种设计模式中的一种，有2种方法可以实现单例模式，分为懒汉式、饿汉式，它们的区别如下：

1、懒汉式单例

    这里采用可调用函数std:call_one()来保证单例只创建一次，从而保证线程安全。
//singleton.cpp

#include
#include
#include using namespace std;static std::once_flag g_flag;
//--------------------- A:懒汉式 -----------------------------------
class Singelton {
public:static Singelton* getInstance() {if (m_instance == NULL) {//m_instance = new Singelton;std::call_once(g_flag, initSingleton);}m_count++;return m_instance;}int getCount(){return m_count;}~Singelton(){if (m_instance != NULL) {delete m_instance;m_instance = NULL;}}private:Singelton(){m_instance = NULL;m_count = 0;cout << "call Singleton()..." << endl;}static void initSingleton(){m_instance = new Singelton;}static Singelton* m_instance;static int m_count;
};//初始化
//懒汉式，在程序调用getInstance时，才创建对应的实例
Singelton *Singelton::m_instance = NULL;
int Singelton::m_count = 0;

2、饿汉式单例

    通过静态初始化，在进入main()函数之前，单例就已经创建了，天然就具有线程安全性。
//singleton2.cpp


//--------------------- B:饿汉式 -----------------------------------
class Singelton2 {
public:static Singelton2* getInstance(){m_count++;return m_instance;}int getCount(){return m_count;}~Singelton2(){if (m_instance != NULL) {delete m_instance;m_instance = NULL;}}private:Singelton2(){m_instance = NULL;m_count = 0;}static Singelton2* m_instance;static int m_count;
};//饿汉式的单例，在程序编译的时候，就已经创建好了
Singelton2* Singelton2::m_instance = new Singelton2;
int Singelton2::m_count = 0;

3、完整代码

//single.cpp

#include
#include
#include using namespace std;static std::once_flag g_flag;
//--------------------- A:懒汉式 -----------------------------------
class Singelton {
public:static Singelton* getInstance() {if (m_instance == NULL) {//m_instance = new Singelton;std::call_once(g_flag, initSingleton);}m_count++;return m_instance;}int getCount(){return m_count;}~Singelton(){if (m_instance != NULL) {delete m_instance;m_instance = NULL;}}private:Singelton(){m_instance = NULL;m_count = 0;cout << "call Singleton()..." << endl;}static void initSingleton(){m_instance = new Singelton;}static Singelton* m_instance;static int m_count;
};//初始化
//懒汉式，在程序调用getInstance时，才创建对应的实例
Singelton *Singelton::m_instance = NULL;
int Singelton::m_count = 0;//--------------------- B:饿汉式 -----------------------------------
class Singelton2 {
public:static Singelton2* getInstance(){m_count++;return m_instance;}int getCount(){return m_count;}~Singelton2(){if (m_instance != NULL) {delete m_instance;m_instance = NULL;}}private:Singelton2(){m_instance = NULL;m_count = 0;}static Singelton2* m_instance;static int m_count;
};//饿汉式的单例，在程序编译的时候，就已经创建好了
Singelton2* Singelton2::m_instance = new Singelton2;
int Singelton2::m_count = 0;int main(){//---------------------------- 1) 懒汉式的单例模式 ------------------------Singelton *singer = Singelton::getInstance();cout << "call times:"<<singer->getCount() << endl;Singelton *singer2 = Singelton::getInstance();cout << "call times:" << singer2->getCount() << endl;if (singer == singer2) {cout << "1.1 These two singes are the same!" << endl;}else {cout << "1.2 These two singes are not the same!" << endl;}//---------------------------- 2) 饿汉式的单例模式 ------------------------Singelton2 *singer3 = Singelton2::getInstance();cout << "call times:" << singer3->getCount() << endl;Singelton2 *singer4 = Singelton2::getInstance();cout << "call times:" << singer4->getCount() << endl;if (singer3 == singer4) {cout << "2.1 These two singes are the same!" << endl;}else {cout << "2.2 These two singes are not the same!" << endl;}system("pause");return 0;
}

    效果如下：