c++设计模式-单例模式

单例模式

单例模式

适用情况

保证一个类只有一个实例， 并提供一个访问该实例的全局节点，不仅考虑实例的个数，还要考虑线程安全问题

优缺点

优点：保证全局只有一个实例
缺点：该模式在多线程环境下需要进行特殊处理， 避免多个线程多次创建单例对象

简易类图

代码实例

基础单例（线程不安全）

/**
 * The Singleton class defines the `GetInstance` method that serves as an
 * alternative to constructor and lets clients access the same instance of this
 * class over and over.
 */
class Singleton
{

    /**
     * The Singleton's constructor should always be private to prevent direct
     * construction calls with the `new` operator.
     */

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

    static Singleton* 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_;
    } 
};

Singleton* Singleton::singleton_= nullptr;;

/**
 * Static methods should be defined outside the class.
 */
Singleton *Singleton::GetInstance(const std::string& value)
{
    /**
     * This is a safer way to create an instance. instance = new Singleton is
     * dangeruous in case two instance threads wants to access at the same time
     */
    if(singleton_==nullptr){
        singleton_ = new Singleton(value);
    }
    return singleton_;
}

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;
}

线程安全版

/**
 * The Singleton class defines the `GetInstance` method that serves as an
 * alternative to constructor and lets clients access the same instance of this
 * class over and over.
 */
class Singleton
{

    /**
     * The Singleton's constructor/destructor should always be private to
     * prevent direct construction/desctruction calls with the `new`/`delete`
     * operator.
     */
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;
}

真正的完全安全版，使用volatile，保证编译器不使用优化处理，导致直接把内存赋给未实例对象

/*
* The Singleton class defines the `GetInstance` method that serves as an
* alternative to constructor and lets clients access the same instance of this
* class over and over.
*/
#include<iostream>
#include<string>
#include<mutex>


class Singleton
{

    /**
     * The Singleton's constructor/destructor should always be private to
     * prevent direct construction/desctruction calls with the `new`/`delete`
     * operator.
     */
private:
    static volatile Singleton * pinstance_; //完全安全的操作
    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;
}