设计模式<c++> （2）单例模式

#include <iostream>
#include <thread>
#include <mutex>

class Singleton
{
private:
    static Singleton* pinstance_;
    static std::mutex mutex_;

protected:
    Singleton(const std::string value) : value_(value)
    {
    }
    ~Singleton() {}
    std::string value_;

public:
    /**
     * Singletons should not be cloneable.
     */
    Singleton(Singleton& other) = delete;
    /**
     * Singletons should not be assignable.
     */
    void operator=(const Singleton&) = delete;
    /**
     * This is the static method that controls the access to the singleton
     * instance. On the first run, it creates a singleton object and places it
     * into the static field. On subsequent runs, it returns the client existing
     * object stored in the static field.
     */

    static Singleton* GetInstance(const std::string& value);
    /**
     * Finally, any singleton should define some business logic, which can be
     * executed on its instance.
     */
    void SomeBusinessLogic()
    {
        // ...
    }

    std::string value() const {
        return value_;
    }
};




/**
 * Static methods should be defined outside the class.
 */

Singleton* Singleton::pinstance_{ nullptr };
std::mutex Singleton::mutex_;

/**
 * The first time we call GetInstance we will lock the storage location
 *      and then we make sure again that the variable is null and then we
 *      set the value. RU:
 */
Singleton* Singleton::GetInstance(const std::string& value)
{
    std::lock_guard<std::mutex> lock(mutex_);
    if (pinstance_ == nullptr)
    {
        pinstance_ = new Singleton(value);
    }
    return pinstance_;
}

void ThreadFoo() {
    // Following code emulates slow initialization.
    //std::this_thread::sleep_for(std::chrono::milliseconds(1000));
    Singleton* singleton = Singleton::GetInstance("FOO");
    std::cout << singleton->value() << "\n";
}

void ThreadBar() {
    // Following code emulates slow initialization.
    //std::this_thread::sleep_for(std::chrono::milliseconds(1000));
    Singleton* singleton = Singleton::GetInstance("BAR");
    std::cout << singleton->value() << "\n";
}

int main()
{
    std::cout << "If you see the same value, then singleton was reused (yay!\n" <<
        "If you see different values, then 2 singletons were created (booo!!)\n\n" <<
        "RESULT:\n";
    std::thread t1(ThreadFoo);
    std::thread t2(ThreadBar);
    t1.join();
    t2.join();

    return 0;
}

单例模式如果不加互斥锁，则是线程不安全的（多个线程会出现多个实例的情况）；

参考C++ 单例模式讲解和代码示例 (refactoringguru.cn)