C++笔记之单例模式

参考笔记：C++笔记之call_once和once_flag
code review

1.返回实例引用

代码

#include <iostream>class Singleton {public:static Singleton &getInstance() {static Singleton instance;return instance;}void printMessage() {std::cout << "hello world!" << std::endl;}private:Singleton() {}                          // 禁止外部创建实例Singleton(const Singleton &);           // 禁止复制实例Singleton operator=(const Singleton &); // 禁止赋值实例
};int main() {Singleton &instance = Singleton::getInstance();instance.printMessage();return 0;
}

2.返回实例指针

代码

#include <iostream>class Singleton {public:static Singleton *getInstance() {if (instance == nullptr) {instance = new Singleton();}return instance;}void printMessage() {std::cout << "hello world!" << std::endl;}private:static Singleton *instance;Singleton() {}                          // 禁止外部创建实例Singleton(const Singleton &);           // 禁止复制实例Singleton operator=(const Singleton &); // 禁止赋值实例
};Singleton *Singleton::instance = nullptr;int main() {Singleton *instance = Singleton::getInstance();instance->printMessage();return 0;
}

3.单例和智能指针share_ptr结合

代码

#include <iostream>
#include <memory>class Singleton {public:static std::shared_ptr<Singleton> getInstance() {if (!instance) {instance = std::shared_ptr<Singleton>(new Singleton());}return instance;}void printMessage() {std::cout << "hello world!" << std::endl;}private:Singleton() {}                          // 禁止外部创建实例Singleton(const Singleton &);           // 禁止复制实例Singleton operator=(const Singleton &); // 禁止赋值实例static std::shared_ptr<Singleton> instance;
};std::shared_ptr<Singleton> Singleton::instance = nullptr;int main() {std::shared_ptr<Singleton> instance = Singleton::getInstance();instance->printMessage();return 0;
}

4.单例和std::call_once结合

编译运行：

代码

#include <iostream>
#include <mutex>class Singleton {
public:static Singleton *getInstance() {std::call_once(initFlag, [](){instance = new Singleton();});return instance;}void printMessage() {std::cout << "hello world!" << std::endl;}private:static Singleton *instance;static std::once_flag initFlag;Singleton() {} // 禁止外部创建实例Singleton(const Singleton &); // 禁止复制实例Singleton operator=(const Singleton &); // 禁止赋值实例
};Singleton *Singleton::instance = nullptr;
std::once_flag Singleton::initFlag;int main() {Singleton *instance = Singleton::getInstance();instance->printMessage();return 0;
}

5.单例和std::call_once、unique_ptr结合

编译运行

代码

#include <iostream>
#include <mutex>
#include <memory>class Singleton {
public:static Singleton *getInstance() {std::call_once(initFlag, [](){instance.reset(new Singleton());});return instance.get();}void printMessage() {std::cout << "hello world!" << std::endl;}private:static std::unique_ptr<Singleton> instance;static std::once_flag initFlag;Singleton() {} // 禁止外部创建实例Singleton(const Singleton &); // 禁止复制实例Singleton operator=(const Singleton &); // 禁止赋值实例
};std::unique_ptr<Singleton> Singleton::instance;
std::once_flag Singleton::initFlag;int main() {Singleton *instance = Singleton::getInstance();instance->printMessage();return 0;
}