singleton pattern in C++?
 

hiho@ll

i had the same problem like this poster: http://coding.derkeiler.com/Archive/...4-01/1176.html

and i did it like posted there

so, i have a solution to my problem
but i want to know why it is such a problem to have a pointer for a singleton pattern in C++?
i did the same stuff in Visual C++ and it worked great
but with gcc i get "undefined reference" for my instance pointer

it's exactly like the post above

but why?

now i'm using the reference operator and a "local" static instance of my class and it works

but why is it such a problem to do

MyClass::getInstance(){
if(MyClass::instance==NULL)
MyClass::instance=new MyClass();
return instance;
}

instead of

MyClass::getInstance(){
static MyClass theonlyone;
return theonlyone;
}

thx@ll

What exactly is your function definition?

Is it static MyClass* MyClass::getInstance(); or is it MyClass MyClass::getInstance();?

the getinstance is static

i would post a code
but i alredy changed it so i don't have it in this form

maybe i can reproduce it in the next few hours (cause i don't have much time)

If you can't at least post the actual function prototype (Because both of the ones in your original post won't work) then I can only make stabs in the dark...

It really would be helpful to have the entire class definition as you created it.

I have done a singleton for C++ in gcc like this

In the header you do something like this
class MyClass
{
public:
static MyClass* getInstance();

private:

MyClass();
~MyClass();

static MyClass* _pInstance;
};

Then in the cc file do this
MyClass* MyClass::_pInstance = NULL;

MyClass::MyClass(){}
MyClass::~MyClass(){}

MyClass* MyClass::getInstance()
{
if(_pInstance == NULL)
{
_pInstance = new MyClass();
}

return _pInstance;
}

It is also a good idea to but a mutex around the creation of the object if more the one thread is going to be getting this object. This singleton should work with either gcc or the Visual C++ compiler.

Im not sure making the destructor private is correct, i always put it in public, plus your comment about the mutex is not really correct. singleton classes are not thread safe and it takes more than introducing a mutex to try and make it safe, i did read a paper on this but at the moment i can't seem to find it. ;( basically the compiler is the problem and does things which you wouldnt think, not really a good explanation but hey ho.

If you don't make the destructor private then anyone that gets the pointer to the object can delete it.

Would like to see paper on the thread safe issue.

here you are http://www.aristeia.com/Papers/DDJ_J...04_revised.pdf it made me laugh when i first read it, it seems the best way is to create a wraper for the class which has a mutex in it.