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

问题

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?

回答1:

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;
}

回答2:

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.

来源：https://stackoverflow.com/questions/37998408/why-cannot-static-function-reference-static-variable-during-singleton-class-crea

标签

c++

design-patterns

singleton