Need &quot;simple&quot; singleton class example

100

Author

November 09, 2003 04:28 AM

I want to start using singleton classes, but wherever i look the examples include other stuff that i cannot differentiate from the actual singleton class code. So if someone could provide an example or modify this code here to be a singleton class, would be really great.


class Apple {
public:
   Apple()
   {
   }

   ~Apple()
   {
   }
   
   void myMethod()
   {
   }

private:
};

thanks,

Fruny

1,658

November 09, 2003 05:01 AM

class Apple{private:   // Prevent creation of Apple instances from client code   Apple() { /* constructor code */ }      // Disable copying (and pass-by-value)   Apple(const Apple&);            // no implementation    Apple& operator=(const Apple&); // no implementation  // The instance will have to be destroyed when the  // program finally terminates.  ~Apple() { /* destructor code */ }public:  // This is where you get your singleton instance from  static Apple* GetInstance()  {     static Apple instance;     return &instance  }};Apple* ptr = Apple::GetInstance();

"Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it." — Brian W. Kernighan

SiCrane

11,840

November 09, 2003 05:05 AM

Ack, no. If you use a Meyer''s Singleton to manage lifetime and you return a pointer from your instance function, make the destructor non-public so that the client isn''t tempted to delete the returned pointer.

Fruny

1,658

November 09, 2003 05:08 AM

quote:Original post by SiCrane
Ack, no. If you use a Meyer''s Singleton to manage lifetime and you return a pointer from your instance function, make the destructor non-public so that the client isn''t tempted to delete the returned pointer.

"Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it." — Brian W. Kernighan

johnnyBravo

100

Author

November 09, 2003 05:42 AM

quote:Original post by SiCrane
Ack, no. If you use a Meyer's Singleton to manage lifetime and you return a pointer from your instance function, make the destructor non-public so that the client isn't tempted to delete the returned pointer.

i get this error message if i do that

quote:
error C2248: 'g::~g' : cannot access private member declared in class 'g'
see declaration of 'g::~g'

so i put the destructor in the public part and it did, is there anything else i should do instead?

edit:

just one more thing, how would i use the class,
i got this error when i tried doing it like this
Apple myApple;

quote:
error C2248: 'D3D::D3D' : cannot access private member declared in class 'D3D'
see declaration of 'D3D::D3D'

[edited by - johnnyBravo on November 9, 2003 6:44:49 AM]

Fruny

1,658

November 09, 2003 05:51 AM

quote:Original post by johnnyBravo
just one more thing, how would i use the class,
i got this error when i tried doing it like this
Apple myApple;

"Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it." — Brian W. Kernighan

SiCrane

11,840

November 09, 2003 05:54 AM

What compiler are you using? All the ones I tried will let the destructor be private.

With the version of the Singleton that Fruny described, you always access the Apple through Apple::GetInstance(). You don''t need to create it or allocate it. The Apple will construct itself the first time you call Apple::GetInstance().

johnnyBravo

100

Author

November 09, 2003 06:08 AM

Thankyou both!

This is exactly what i needed for my program.

I have visual c++ 6.
Its kinda annoying that i cant have the destructor in the private, as you said, to stop people from calling it.

Fruny

1,658

November 09, 2003 06:12 AM

Well, you could always do

class Apple{private:   // Prevent creation of Apple instances from client code   Apple() { /* constructor code */ }      // Disable copying (and pass-by-value)   Apple(const Apple&);            // no implementation    Apple& operator=(const Apple&); // no implementationpublic:  // The instance will have to be destroyed when the  // program finally terminates.  ~Apple() { /* destructor code */ }  // This is where you get your singleton instance from  static Apple& GetInstance()  {     static Apple instance;     return instance;  }};Apple& ref = Apple::GetInstance();

"Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it." — Brian W. Kernighan

johnnyBravo

100

Author

November 09, 2003 06:17 AM

hmm isnt that just the same as the other one? cept with the destructor in public which i had to do anyway?

one more question, is there a singleton way that i can call it like

Apple myApple;
?

Its for other things...

Need "simple" singleton class example

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Need "simple" singleton class example

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