C ++不同的单例实现 [英] C++ different singleton implementations

问题描述

我通常以这种方式实现单例模式：

I usually implement the singleton pattern this way :

class Singleton
{
    public:
        virtual ~Singleton() {}

        static Singleton& GetInstance()
        {
            static Singleton instance;
            return instance;
        }

    private:
        Singleton();
        Singleton(const Singleton&);
        Singleton& operator=(const Singleton&);
}

最近我碰到了这个实现，这有点不同：

Recently, I ran into this implementation, which is slightly different :

class Singleton
{
    public:
        Singleton();
        virtual ~Singleton() {}

        static Singleton& GetInstance()
        {
            return instance;
        }

    private:
        Singleton(const Singleton&);
        Singleton& operator=(const Singleton&);

        static Singleton instance;
}

Singleton Singleton::instance;

哪个实现更好？

不要让构造函数私有（第二个实现）是危险的？

Isn't it dangerous not to make the constructor private (2nd implementation) ?

谢谢。

推荐答案

我不必重复关于在其他答案中进行的单身人士的懒惰建设的好处。

I need not repeat the good point about lazy construction of the singleton made in other answers.

让我补充一下：

public:
    Singleton();
    virtual ~Singleton() {}

设计师这个特殊的阶级感觉需要允许：

The designer of this particular class felt a need to allow:

• 从这个 singleton 类派生说派生类叫做 DerSingleton


• DerSingleton 可以有实例可以通过指向 Singleton （所以 DerSingleton 不是单例）的指针删除

• derivation from this Singleton class, say the derived class is called DerSingleton
• DerSingleton can have instances which can be deleted with a pointer to Singleton (so DerSingleton is not a singleton)

任何 DerSingleton 的实例也是一个 Singleton 实例根据定义，所以如果 DerSingleton 被实例化， Singleton 不是单例。

Any instance of DerSingleton is also a Singleton instance by definition, so it follows that if DerSingleton is instanciated, Singleton is not a singleton.

所以这个设计断言了两件事：

So this design asserts two things:

• 这个类是单例


• 这个课程是不是单身人士

• this class is a singleton
• this class is not a singleton

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