C / C++ / MFC

5-Jul-12 7:17

How can I provide one instance only for an object ? And here I'm not refer to CMyWinApp (one instance of an application), but a custom object ? It's an interview question ... Smile | :)

David Crow5-Jul-12 7:22

David Crow

5-Jul-12 7:22

Hint: singleton.

"One man's wage rise is another man's price increase." - Harold Wilson

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

"Show me a community that obeys the Ten Commandments and I'll show you a less crowded prison system." - Anonymous

fat_boy5-Jul-12 8:13

5-Jul-12 8:13

Create a static count in the object and increment it in the constructor?

_Flaviu5-Jul-12 20:07

5-Jul-12 20:07

Ok, and when second instance is about to be executed, how can I stop it then ?

fat_boy5-Jul-12 21:26

5-Jul-12 21:26

Use a get instance static member func to do that.

Stefan_Lang5-Jul-12 22:35

Stefan_Lang

5-Jul-12 22:35

In header file:

C++

class myUniqueClass {
private:
   myUniqueClass();                        // default constructor is private!
   myUniqueClass(const myUniqueClass&);    // copy constructor is private, too!
   static myUniqueClass* myUniqueInstance;
public:
   static myUniqueClass* getInstance();
   // put the real functionality below
   // ...
};

In cpp file:

C++

myUniqueClass* myUniqueClass::myUniqueInstance = 0;
myUniqueClass::getInstance() {
   if (myUniqueInstance == 0) {
      myUniqueInstance = new myUniqueClass;
   }
   return myUniqueInstance;
}

Explanation:
The two private constructors prevent the uncontrolled creation of a new instance, or the copying of the one, valid instance. The getInstance function creates the one instance if necessary: as a member function it can use the private constructor.

_Flaviu5-Jul-12 23:08

5-Jul-12 23:08

And when I want to work with this object, I write :

C++

myUniqueClass* pUniqueClass = myUniqueClass::GetInstance();

I'm right ? And what is happen when object goes out of scope ? I have memory leak ?

Stefan, I thank you very much !!

modified 6-Jul-12 6:38am.

fat_boy6-Jul-12 0:14

6-Jul-12 0:14

Of course there is nothing to stop someone totally ignoring the GetInstance func and just creating their own, so it depends on your question, do you want to *force* users to have only one instance.

_Flaviu6-Jul-12 0:39

6-Jul-12 0:39

I think that is impossible to create a new instance of myUniqueClass because have private constructor ...

fat_boy6-Jul-12 1:31

6-Jul-12 1:31

Good point. Smile | :)

Stefan_Lang6-Jul-12 5:33

Stefan_Lang

6-Jul-12 5:33

Correct. getInstance is used to access the single object.

You are correct that the code above produces a memory leak. The problem is that the only way to avoid it is to create and destroy it from another singleton! In a windows application, you could implement that in your WinApp class. In a console program you could do it in main().

Also, as Luc Pattyn mentioned in his response, my code can not prevent the creation of a second instance in multithreaded code: the creation of the instance (and the preceeding test of the pointer) has to be locked. However that is only a problem until the first instance is created, so you can solve this by creating the instance at the start of the program in main() or your WinApp class.

Luc Pattyn6-Jul-12 0:47

Luc Pattyn

6-Jul-12 0:47

And what about thread safety? If you're unlucky two threads calling getInstance() at the same time may cause two objects being created. There are many ways to create a Singleton pattern, lots of the attempts are just wrong, only a few are correct without showing too much of a performance hit. Look at several articles on the subject here at CP, or elsewhere.

Smile | :)

Luc Pattyn [My Articles] Nil Volentibus Arduum

fat_boy6-Jul-12 1:30

6-Jul-12 1:30

Yep, of course it needs lock protecting.