C ++ Singleton /活动对象范例 [英] C++ Singleton/Active Object Paradigm

问题描述

我想知道你如何创建一个你创建几个实例的课程，例如

I was wondering how you would make a class where you create several instances i.e

Session o1 = new Session();
Session o2 = new Session();

然后，您可以使这些会话成为活动会话。

You could then make these sessions the active session like so.

o1.makeActiveSession();
Session::setActiveSession(o2);

然后在我的代码的任何一点我可以去：

Then at any point in my code I could go:

Session::getActiveSession();

，它将返回活动会话对象，如果不存在，则创建新对象。任何时候只有一个会话可以是活动会话，所以如果一个会话被告知成为活动会话，那么旧的会话将被停用。

and it would return the active session object or create new one if one doesn't exist. Only one session can be the active session at any one time, so if a session is told to become the active session then the old one is deactivated.

所以我的问题是，我该如何做这样的事情？

So my question is, how would I make something like this ?

推荐答案

这是基于懒惰加载的单例：

This is lazy-loading based singleton:

class Session
{
public:
    static Session& getInstance() {
        static Session s;                   // <-- instantiated upon first call
        return s;
    }

private:
    Session() { }                           // <-- private constructor
    ~Session() { }
    Session(const Session&);                // <-- private copy constructor
    Session& operator=(const Session&);     // <-- private assignment operator
};

用作：

Session& s = Session::getInstance();

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