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Hello ,

I have the following code , which implements the singleton pattern:

class Singleton{
private:
static Singleton* uniqueInstance;
//other useful instance variables here

Singleton(){
cout<<"Object created!"<<endl;
}

public:
static Singleton* getInstance(){
if(uniqueInstance == NULL)
uniqueInstance = new Singleton();
return uniqueInstance;
}
};

int main(){
Singleton::getInstance();
return 0;
}

But when i try to compile it , i get the following error:

dan@sea:~/school/dp$ g++ -o singleton singleton.cpp
/tmp/ccHL1rvi.o: In function `Singleton::getInstance()':
singleton.cpp:
(.text._ZN9Singleton11getInstanceEv[Singleton::getInstance()]+0x:
undefined reference to `Singleton::uniqueInstance'
singleton.cpp:
(.text._ZN9Singleton11getInstanceEv[Singleton::getInstance()]+0x2e):
undefined reference to `Singleton::uniqueInstance'
singleton.cpp:
(.text._ZN9Singleton11getInstanceEv[Singleton::getInstance()]+0x54):
undefined reference to `Singleton::uniqueInstance'
collect2: ld returned 1 exit status

Can you enlighten me

Thanks,
Daniel.

On Fri, 18 Jul 2008 04:09:03 -0700, daniel wrote:

> Hello ,
>
> I have the following code , which implements the singleton pattern:
>
> class Singleton{
> private:
> static Singleton* uniqueInstance;
> //other useful instance variables here
>
> Singleton(){
> cout<<"Object created!"<<endl;
> }
>
> public:
> static Singleton* getInstance(){
> if(uniqueInstance == NULL)
> uniqueInstance = new Singleton();
> return uniqueInstance;
> }
> };
>
> int main(){
> Singleton::getInstance();
> return 0;
> }
>
> But when i try to compile it , i get the following error:
>
> dan@sea:~/school/dp$ g++ -o singleton singleton.cpp /tmp/ccHL1rvi.o: In
> function `Singleton::getInstance()': singleton.cpp:
> (.text._ZN9Singleton11getInstanceEv[Singleton::getInstance()]+0x:
> undefined reference to `Singleton::uniqueInstance' singleton.cpp:
> (.text._ZN9Singleton11getInstanceEv[Singleton::getInstance()]+0x2e):
> undefined reference to `Singleton::uniqueInstance' singleton.cpp:
> (.text._ZN9Singleton11getInstanceEv[Singleton::getInstance()]+0x54):
> undefined reference to `Singleton::uniqueInstance' collect2: ld returned
> 1 exit status
>
> Can you enlighten me

The linker is telling you that it can't find a definition for
Singleton::uniqueInstance. You have declared it, but my guess is that you
haven't *defined* it; static member variables must be defined in some
compilation unit. See:

http://www.parashift.com/c++-faq-lit...html#faq-10.11


--
Lionel B

daniel a écrit :
> I have the following code , which implements the singleton pattern:
>
> class Singleton{
> private:
> static Singleton* uniqueInstance;
> //other useful instance variables here
>
> Singleton(){
> cout<<"Object created!"<<endl;
> }
>
> public:
> static Singleton* getInstance(){
> if(uniqueInstance == NULL)
> uniqueInstance = new Singleton();
> return uniqueInstance;
> }
> };

You must instantiate Singleton::uniqueInstance.
Add the following line somewhere in a cpp:

Singleton* Singleton::uniqueInstance;


>
> int main(){
> Singleton::getInstance();
> return 0;
> }
>
> But when i try to compile it , i get the following error:
>
> dan@sea:~/school/dp$ g++ -o singleton singleton.cpp
> /tmp/ccHL1rvi.o: In function `Singleton::getInstance()':
> singleton.cpp:
> (.text._ZN9Singleton11getInstanceEv[Singleton::getInstance()]+0x:
> undefined reference to `Singleton::uniqueInstance'
[snip]

--
Michael

On Jul 18, 2:18*pm, Lionel B <m...@privacy.net> wrote:
> On Fri, 18 Jul 2008 04:09:03 -0700, daniel wrote:
> > Hello ,
>
> > I have the following code , which implements the singleton pattern:
>
> > class Singleton{
> > private:
> > * * static Singleton* uniqueInstance;
> > * * //other useful instance variables here
>
> > * * Singleton(){
> > * * * * cout<<"Object created!"<<endl;
> > * * }
>
> > public:
> > * * static Singleton* getInstance(){
> > * * * * if(uniqueInstance == NULL)
> > * * * * * * uniqueInstance = new Singleton();
> > * * * * return uniqueInstance;
> > * * }
> > };
>
> > int main(){
> > * * Singleton::getInstance();
> > * * return 0;
> > }
>
> > But when i try to compile it , i get the following error:
>
> > dan@sea:~/school/dp$ g++ *-o singleton singleton.cpp /tmp/ccHL1rvi.o: In
> > function `Singleton::getInstance()': singleton.cpp:
> > (.text._ZN9Singleton11getInstanceEv[Singleton::getInstance()]+0x:
> > undefined reference to `Singleton::uniqueInstance' singleton.cpp:
> > (.text._ZN9Singleton11getInstanceEv[Singleton::getInstance()]+0x2e):
> > undefined reference to `Singleton::uniqueInstance' singleton.cpp:
> > (.text._ZN9Singleton11getInstanceEv[Singleton::getInstance()]+0x54):
> > undefined reference to `Singleton::uniqueInstance' collect2: ld returned
> > 1 exit status
>
> > Can you enlighten me
>
> The linker is telling you that it can't find a definition for
> Singleton::uniqueInstance. You have declared it, but my guess is that you
> haven't *defined* it; static member variables must be defined in some
> compilation unit. See:
>
> http://www.parashift.com/c++-faq-lit...html#faq-10.11
>
> --
> Lionel B

got it , thanks a lot

Hi daniel,

while writing the singleton class you can avoid using pointer and pass
your class as reference
that will be more effective.

Rewriting ur code :

class Singleton{

private:
static Singleton uniqueInstance;
//other useful instance variables here

private:
Singleton();
~Singleton();

public:
static Singleton& getInstance(){
return uniqueInstance;
}

};


int main(){
Singleton::getInstance();
return 0;
}

+suresh

On Jul 18, 9:06*am, vsuresh...@gmail.com wrote:
> Hi daniel,
>
> while writing the singleton class you can avoid using pointer and pass
> your class as reference
> that will be more effective.
>
> Rewriting ur code :
>
> class Singleton{
>
> private:
> * * static Singleton uniqueInstance;
> * * //other useful instance variables here
>
> private:
> * *Singleton();
> * *~Singleton();
>
> public:
> * * static Singleton& getInstance(){
> * * * * return uniqueInstance;
> * * }
>
> };
>
> int main(){
> * * Singleton::getInstance();
> * * return 0;
>
> }
>
> +suresh

Not going to work, for you forgot to define a uniqueInstance.

>
> Not going to work, for you forgot to define a uniqueInstance.- Hide quoted text -
>
> - Show quoted text -

Hi Cracker,

Can i know the reason ?

On Fri, 18 Jul 2008 06:46:11 -0700, vsuresh.cs wrote:

>> Not going to work, for you forgot to define a uniqueInstance.
>
> Hi Cracker,
>
> Can i know the reason ?

For the same reason the OP's code didn't compile in the first place. Go
back and re-read this thread from the beginning.

--
Lionel B

On Jul 18, 2:06*pm, vsuresh...@gmail.com wrote:
> class Singleton{
>
> private:
> * * static Singleton uniqueInstance;
> * * //other useful instance variables here
>
> private:
> * *Singleton();
> * *~Singleton();
>
> public:
> * * static Singleton& getInstance(){
> * * * * return uniqueInstance;
> * * }
>
> };

Instead of private static member for the uniqueInstance try static
variable in the scope of the getInstance() function.

static Singleton& getInstance(){
static Singleton uniqueInstance;
return uniqueInstance;
}

The singleton will be constructed on the first access.

Kind regards,
Vladimir