C++ accessing private static variable within a static function

Question

I have a class with a static function getSharedInstance, thus should give me a pointer to an already instanced version of the class.

Header:

class foo {
public:
  static foo *getSharedInstance();

private:
  static foo *sharedInstance;
}

Implementation:

foo *foo::getSharedInstance() {
    if(sharedInstance == NULL)
        sharedInstance = new foo();
    return sharedInstance;
}

The point I don't understand is, why do I get undefined reference to the variable sharedInstance?

You need to *define* static data members. You have only *declared* one. — juanchopanza, Jun 03 '13 at 08:15

score 6 · Accepted Answer · answered Jun 03 '13 at 08:14

6

You have to instanciate it somewhere (generally in you class' cpp file):

foo *foo::sharedInstance;

The compiler assumes it will find the symbol (ie the pointer representing your instance) somewhere. However, if you don't define it, it wont find it and will therefor raise undefined reference

answered Jun 03 '13 at 08:14

Xaqq

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That means, you must do something like: `foo *foo::sharedInstance = 0;` in your cpp-file. – NG_ Jun 03 '13 at 08:16
Thanks, I think I've been doing to much php :) – Christian Richter Jun 03 '13 at 08:23

C++ accessing private static variable within a static function

1 Answers1