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一、要点

0.单例模式，顾名思义只能有一个实例对象
1.私有化构造函数：不能随便调用构造函数来实例化对象
2.删除拷贝构造函数、赋值运算符函数：确保唯一实例
3.使用静态成员变量，存储指向单例对象的指针：静态方法属于类，不属于具体的实例对象。
4.提供静态方法，在堆上获取单例对象

二、代码

延迟实例化、懒加载、懒汉式：实例在第一次使用时才会被创建

1.核心代码

static Singleton* getInstance() {if (instance == nullptr) {instance = new Singleton();  // 只有在第一次调用时创建实例}return instance;
}

2.详细注释代码

#include <iostream> 
using std::cout;
using std::endl;//单例模式:要通过给的接口来创造唯一实例。
//因此，要将构造函数私有化，并且删除拷贝构造和赋值运算符函数class Singleton
{
public://静态方法,用于获取唯一的实例。静态方法属于类，而非实例对象//懒加载、懒汉式、延迟实例化static Singleton* getInstance(){if(pInstance == nullptr){pInstance = new Singleton(); }return pInstance;}void display(){cout << "This is the Singleton instance." << "\n";}
private://构造函数私有化，防止外部创建对象Singleton(){cout << "Singleton instance created.\n";}//禁用拷贝构造和赋值运算符函数Singleton(const Singleton & ) = delete;Singleton & operator=(const Singleton &) = delete;//静态成员变量，存储唯一的实例static Singleton* pInstance;
};//类外初始化静态成员变量
Singleton* Singleton::pInstance = nullptr;int main()
{//获取单例对象的唯一实例Singleton* s1 = Singleton::getInstance();s1->display();//再次获取单例对象的实例Singleton* s2 = Singleton::getInstance();s2->display();//验证两个指针是否指向同一个实例if(s1 == s2){cout << "Both pointer point to the same instance.\n";}return 0;
}

另一版

#include <iostream> 
using std::cout;
using std::endl;//单例模式:要通过给的接口来创造唯一实例。
//因此，要将构造函数私有化，并且删除拷贝构造和赋值运算符函数class Singleton
{
private://构造函数私有化，防止外部创建对象Singleton(){cout << "Singleton instance created.\n";}//禁用拷贝构造和赋值运算符函数Singleton(const Singleton & ) = delete;Singleton & operator=(const Singleton &) = delete;//静态成员变量，存储唯一的实例static Singleton* instance;
public://静态方法,用于获取唯一的实例static Singleton* getInstance(){if(instance == nullptr){instance = new Singleton(); //延迟实例化}return instance;}void display(){cout << "This is the Singleton instance." << "\n";}
};//静态成员变量初始化为nullptr
Singleton* Singleton::instance = nullptr;int main()
{//获取单例对象的唯一实例Singleton* s1 = Singleton::getInstance();s1->display();//再次获取单例对象的实例Singleton* s2 = Singleton::getInstance();s2->display();//验证两个指针是否指向同一个实例if(s1 == s2){cout << "Both pointer point to the same instance.\n";}return 0;
}

3.无注释代码 (单线程时，存在线程安全问题)

#include <iostream> 
using std::cout;
using std::endl;class Singleton
{
public:static Singleton* getInstance(){if(pInstance == nullptr){pInstance = new Singleton();}return pInstance;}private:Singleton(){};Singleton(const Singleton & rhs) = delete;Singleton & operator= (const Singleton & rhs) = delete;static Singleton* pInstance;
};Singleton* Singleton::pInstance = nullptr;int main()
{Singleton* s1 = Singleton::getInstance();Singleton* s2 = Singleton::getInstance();if(s1 == s2){cout << "s1 == s2,是单例的\n";}else{cout << "s1 != s2,不是单例的\n";}return 0;
}

4.考虑线程安全 (getInstance加锁)

#include <iostream> 
#include <mutex>
using std::cout;
using std::mutex;
using std::lock_guard;class Singleton
{
public:static Singleton* getInstance(){lock_guard<mutex> lock(mtx);if(pInstance == nullptr){pInstance = new Singleton();}return pInstance;}private:Singleton(){};Singleton(const Singleton & rhs) = delete;Singleton & operator= (const Singleton & rhs) = delete;static Singleton* pInstance;static mutex mtx; //互斥锁，确保线程安全
};Singleton* Singleton::pInstance = nullptr;
mutex Singleton::mtx;int main()
{Singleton* s1 = Singleton::getInstance();Singleton* s2 = Singleton::getInstance();if(s1 == s2){cout << "s1 == s2,是单例的\n";}else{cout << "s1 != s2,不是单例的\n";}return 0;
}

4.1 进一步减少加锁的开销，使用双重检查锁

// 线程安全的 getInstance 方法，使用双重检查锁
static Singleton* getInstance()
{if(pInstance == nullptr) {  // 第一次检查，无锁std::lock_guard<std::mutex> lock(mtx);  // 加锁if(pInstance == nullptr) {  // 第二次检查，加锁后pInstance = new Singleton();}}return pInstance;
}

5.释放单例资源以避免内存泄露(智能指针)

#include <iostream> 
#include <mutex>
#include <memory>using std::cout;
using std::mutex;
using std::lock_guard;
using std::unique_ptr;class Singleton
{
public:static Singleton* getInstance(){lock_guard<mutex> lock(mtx);if(pInstance == nullptr){/* pInstance = new Singleton(); */pInstance = unique_ptr<Singleton>(new Singleton());}return pInstance.get();}private:Singleton(){};Singleton(const Singleton & rhs) = delete;Singleton & operator= (const Singleton & rhs) = delete;/* static Singleton* pInstance; */static unique_ptr<Singleton> pInstance;static mutex mtx; //互斥锁，确保线程安全
};/* Singleton* Singleton::pInstance = nullptr; */
unique_ptr<Singleton> Singleton::pInstance = nullptr;
mutex Singleton::mtx;int main()
{Singleton* s1 = Singleton::getInstance();Singleton* s2 = Singleton::getInstance();if(s1 == s2){cout << "s1 == s2,是单例的\n";}else{cout << "s1 != s2,不是单例的\n";}return 0;
}

6.饿汉式

饿汉式：在类加载时就进行实例化

#include <iostream>
using namespace std;class Singleton{
public:// 提供全局访问点static Singleton& getInstance(){// 静态局部变量在程序启动时初始化static Singleton instance;return instance;}// 删除拷贝构造函数和赋值操作符，防止复制Singleton(const Singleton&) = delete;Singleton& operator=(const Singleton&) = delete;// 示例方法void display() const{cout << "Singleton::display() called on instance: " << this << endl;}
private:// 私有构造函数Singleton(){cout << "Singleton()" << endl;}// 私有析构函数~Singleton(){cout << "~Singleton()" << endl;}
};int main()
{// 通过getInstance获取单例对象的引用，并调用示例方法Singleton& s1 = Singleton::getInstance();s1.display();// 再次获取单例对象的引用，验证它们是同一个实例Singleton& s2 = Singleton::getInstance();s2.display();// 输出将展示s1和s2有相同的地址，说明它们是同一个实例return 0;
}