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;
}