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C++ 单例模式详解

backend 2025/5/7 14:20:04

单例模式是一种创建型设计模式，它确保一个类只有一个实例，并提供一个全局访问点来访问该实例。

概念解析

单例模式的核心思想是：

限制类的实例化次数，确保在整个应用程序中只有一个实例存在
提供对该实例的全局访问点
控制共享资源的访问

主要特点

私有构造函数：防止外部通过new来创建实例
静态实例成员：保存唯一的实例
静态访问方法：提供全局访问点（通常命名为getInstance()）
线程安全：需要考虑多线程环境下的安全性

代码示例

下面是一个完整的单例模式示例，包含详细注释和多种实现方式：

#include <iostream>
#include <mutex>
#include <atomic>// 版本1：基础单例（非线程安全）
class SingletonV1 {
private:static SingletonV1* instance; // 静态实例指针// 私有构造函数，防止外部实例化SingletonV1() {std::cout << "SingletonV1 实例创建" << std::endl;}// 防止拷贝和赋值SingletonV1(const SingletonV1&) = delete;SingletonV1& operator=(const SingletonV1&) = delete;public:// 获取单例实例的静态方法static SingletonV1* getInstance() {if (instance == nullptr) {instance = new SingletonV1();}return instance;}void doSomething() {std::cout << "SingletonV1 正在执行操作" << std::endl;}// 用于测试的析构函数~SingletonV1() {std::cout << "SingletonV1 实例销毁" << std::endl;}
};// 初始化静态成员
SingletonV1* SingletonV1::instance = nullptr;// 版本2：线程安全的单例（使用锁）
class SingletonV2 {
private:static SingletonV2* instance;static std::mutex mtx; // 互斥锁SingletonV2() {std::cout << "SingletonV2 实例创建" << std::endl;}SingletonV2(const SingletonV2&) = delete;SingletonV2& operator=(const SingletonV2&) = delete;public:// 线程安全的获取实例方法static SingletonV2* getInstance() {std::lock_guard<std::mutex> lock(mtx); // 加锁if (instance == nullptr) {instance = new SingletonV2();}return instance;}void doSomething() {std::cout << "SingletonV2 正在执行操作" << std::endl;}~SingletonV2() {std::cout << "SingletonV2 实例销毁" << std::endl;}
};// 初始化静态成员
SingletonV2* SingletonV2::instance = nullptr;
std::mutex SingletonV2::mtx;// 版本3：更高效的线程安全单例（双重检查锁定）
class SingletonV3 {
private:static SingletonV3* instance;static std::mutex mtx;SingletonV3() {std::cout << "SingletonV3 实例创建" << std::endl;}SingletonV3(const SingletonV3&) = delete;SingletonV3& operator=(const SingletonV3&) = delete;public:// 双重检查锁定模式static SingletonV3* getInstance() {if (instance == nullptr) { // 第一次检查std::lock_guard<std::mutex> lock(mtx); // 加锁if (instance == nullptr) { // 第二次检查instance = new SingletonV3();}}return instance;}void doSomething() {std::cout << "SingletonV3 正在执行操作" << std::endl;}~SingletonV3() {std::cout << "SingletonV3 实例销毁" << std::endl;}
};// 初始化静态成员
SingletonV3* SingletonV3::instance = nullptr;
std::mutex SingletonV3::mtx;// 版本4：C++11最推荐的实现（Meyer's Singleton）
class SingletonV4 {
private:SingletonV4() {std::cout << "SingletonV4 实例创建" << std::endl;}SingletonV4(const SingletonV4&) = delete;SingletonV4& operator=(const SingletonV4&) = delete;public:// C++11保证局部静态变量的初始化是线程安全的static SingletonV4& getInstance() {static SingletonV4 instance; // 线程安全的初始化return instance;}void doSomething() {std::cout << "SingletonV4 正在执行操作" << std::endl;}~SingletonV4() {std::cout << "SingletonV4 实例销毁" << std::endl;}
};// 版本5：使用atomic的现代C++实现
class SingletonV5 {
private:static std::atomic<SingletonV5*> instance;static std::mutex mtx;SingletonV5() {std::cout << "SingletonV5 实例创建" << std::endl;}SingletonV5(const SingletonV5&) = delete;SingletonV5& operator=(const SingletonV5&) = delete;public:// 使用atomic的现代实现static SingletonV5* getInstance() {SingletonV5* tmp = instance.load(std::memory_order_acquire);if (tmp == nullptr) {std::lock_guard<std::mutex> lock(mtx);tmp = instance.load(std::memory_order_relaxed);if (tmp == nullptr) {tmp = new SingletonV5();instance.store(tmp, std::memory_order_release);}}return tmp;}void doSomething() {std::cout << "SingletonV5 正在执行操作" << std::endl;}~SingletonV5() {std::cout << "SingletonV5 实例销毁" << std::endl;}
};// 初始化静态成员
std::atomic<SingletonV5*> SingletonV5::instance{nullptr};
std::mutex SingletonV5::mtx;int main() {std::cout << "=== 单例模式演示 ===" << std::endl;// 测试基础单例std::cout << "\n测试 SingletonV1 (基础单例):" << std::endl;SingletonV1::getInstance()->doSomething();SingletonV1::getInstance()->doSomething();// 测试线程安全单例std::cout << "\n测试 SingletonV2 (线程安全-锁):" << std::endl;SingletonV2::getInstance()->doSomething();SingletonV2::getInstance()->doSomething();// 测试双重检查锁定单例std::cout << "\n测试 SingletonV3 (双重检查锁定):" << std::endl;SingletonV3::getInstance()->doSomething();SingletonV3::getInstance()->doSomething();// 测试Meyer's单例std::cout << "\n测试 SingletonV4 (Meyer's Singleton):" << std::endl;SingletonV4::getInstance().doSomething();SingletonV4::getInstance().doSomething();// 测试atomic单例std::cout << "\n测试 SingletonV5 (atomic实现):" << std::endl;SingletonV5::getInstance()->doSomething();SingletonV5::getInstance()->doSomething();return 0;
}