c++实现单例模式的三种方案

第一种：单线程（懒汉）

第二种：多线程（互斥量实现锁+懒汉）

第三种：多线程（const static+饿汉）（还要继续了解）

//单线程解法 //这种解法在多线程的情况下，可能创建多个实例。
class Singleton1 { private: static Singleton1* m_pInstance1;//需要的时候才创建，懒汉 //利用static关键字的特性，不属于任何类，整个类只有一个
 Singleton1(); public: static Singleton1* GetInstance1(); static void DestroyInstance1(); }; Singleton1::Singleton1() { cout << "创建单例" << endl; } Singleton1* Singleton1::GetInstance1() { return m_pInstance1; } void Singleton1::DestroyInstance1() { if (m_pInstance1 != nullptr) { delete m_pInstance1; m_pInstance1 = nullptr; } } //初始化一个对象
Singleton1* Singleton1::m_pInstance1 = new Singleton1(); //单线程下多次获取实例
void test1() { Singleton1* singletoObj1 = Singleton1::GetInstance1(); cout << singletoObj1 << endl; Singleton1* singletoObj2 = Singleton1::GetInstance1(); cout << singletoObj2 << endl; //上面的两个对象会指向同一个地址
 Singleton1::DestroyInstance1(); }

 

//多线程+加锁（互斥量）
class Singleton2 { private: Singleton2(); static Singleton2* m_pInstance2; static mutex m_mutex;//互斥量
public: static Singleton2* GetInstance2(); static void DestroyInstance2(); }; Singleton2::Singleton2() { cout << "创建单例2" << endl; } Singleton2* Singleton2::GetInstance2() { if (m_pInstance2 == nullptr) { cout << "加锁中" << endl; m_mutex.lock(); if (m_pInstance2 == nullptr) { m_pInstance2 = new Singleton2(); } cout << "解锁中" << endl; m_mutex.unlock(); } return m_pInstance2; } void Singleton2::DestroyInstance2() { if (m_pInstance2 != nullptr) { delete m_pInstance2; m_pInstance2 = nullptr; } } //静态成员变量的定义
Singleton2* Singleton2::m_pInstance2 = nullptr;//懒汉式的写法
mutex Singleton2::m_mutex; //常见一个实例对象，给下面的多线程调用
void print_singleton_instance() { Singleton2* singletonObj2 = Singleton2::GetInstance2(); cout << "新的一个实例对象" << singletonObj2 << endl; } void test2() { vector<thread> threads; for (int i = 0; i < 10; i++) { //十个线程都指向同一个静态变量的地址
 threads.push_back(thread(print_singleton_instance)); } for (auto& thr : threads) { thr.join(); } }

 

//方案三：使用const特性，来替换方案二的加锁操作
class Singleton3 { private: Singleton3(){} static const Singleton3* m_pInstance3; public: static Singleton3* GetInstance3(); static void DestroyInstance3(); }; Singleton3* Singleton3::GetInstance3() { //这个函数的返回值如果变化曾const static属性，就不用进行const_cast
    return const_cast<Singleton3*> (m_pInstance3); } void Singleton3::DestroyInstance3() { if (m_pInstance3 != nullptr) { delete m_pInstance3; m_pInstance3 = nullptr; } } //静态成员变量的定义
const Singleton3* Singleton3::m_pInstance3 = new Singleton3();//饿汉式的写法 //常见一个实例对象，给下面的多线程调用
void print_singleton_instance3() { Singleton3* singletonObj3 = Singleton3::GetInstance3(); cout << "新的一个实例对象" << singletonObj3 << endl; } void test3() { vector<thread> threads; for (int i = 0; i < 10; i++) { //十个线程都指向同一个静态变量的地址
 threads.push_back(thread(print_singleton_instance3)); } for (auto& thr : threads) { thr.join(); } }