why cannot static function reference static variable during singleton class creation in c++?

Question

I was trying to understand singleton design pattern and created a simplest one:

#include <iostream>


class mySingleton{

private:
   static mySingleton *ptr;
   mySingleton(){ }    

public:
   static mySingleton* getInstance(){
     if(!ptr){
        ptr = new mySingleton();
        return ptr;
     } else return ptr;
   }

   void msg(){
     std::cout << " Hello World!! " << std::endl;
   }

};


int main(){

mySingleton* obj = mySingleton::getInstance();
mySingleton* obj2 = mySingleton::getInstance();

return 0;
}

When I try to compile I get :

Undefined symbols for architecture x86_64:
"mySingleton::ptr", referenced from:
    mySingleton::getInstance()       in ccm822LI.o
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status

Why can't I use ptr inside a static function, since ptr is also a static variable? Am I missing something here?

score 7 · Accepted Answer · edited May 23 '17 at 11:58

7

Am I missing something here?

Yes, several things:

As mentioned you are missing the definition of the static mySingleton pointer variable.
Your code isn't thread safe
The correct way to implement it is to use a local static variable in the getInstance() function (aka. Scott Meyer's Singleton):
```
static mySingleton* getInstance(){
    static mySingleton theInstance;
    return &theinstance;
}
```
This implementation is guaranteed to be thread safe, and you don't need to bother with memory allocation.

Using a pointer probably isn't what you want as a return type

static mySingleton& getInstance(){
               // ^
    static mySingleton theInstance;
    return theinstance;
}

edited May 23 '17 at 11:58

Community

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answered Jun 23 '16 at 17:53

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Thank you for the answer .. one last thing I want to wrap my head around is .. so getInstance() can be called multiple times ... but how come the object it instantiate is always the same .. I know its because of static but I am still missing some thing that I need to understand – pokche Jun 23 '16 at 18:08
@pokche The 1st call to `getInstance()` triggers to create the instance, any following will get the same results as from the 1st call. – πάντα ῥεῖ Jun 23 '16 at 18:10
awesome .. I knew some thing like that has to happen ... but u explained clearly ... thank you :) – pokche Jun 23 '16 at 18:11

score 3 · Answer 2 · answered Jun 23 '16 at 17:43

3

static mySingleton *ptr;

inside the class definition is just a declaration. It is not a definition. You need to define it using:

mySingleton * mySingleton::ptr = nullptr;

outside the class definition.

answered Jun 23 '16 at 17:43

R Sahu

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@R Sahu So I cannot define the ptr inside the class static mySingleton *ptr = nullptr; .. cause when I do that I get compilation error. – pokche Jun 23 '16 at 17:47
2

@pokche But it doesn't even have to be a static *member*. It could be a static variable inside of the `getInstance` function. – juanchopanza Jun 23 '16 at 17:52
4

@pokche And it doesn't even have to be a pointer to a dynamically allocated object either :-) – juanchopanza Jun 23 '16 at 17:54

why cannot static function reference static variable during singleton class creation in c++?

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