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Can we access class members before class is instantiated?

Posted on 2013-06-11
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Last Modified: 2013-06-11
class AlarmTask
{
  public:
    AlarmTask();
    virtual ~AlarmTask();
    ...
    ...
       
    static AlarmTask * getInstance()
    {
      if(_instance == NULL)
      {
        _instance = new AlarmTask();
      }
      return _instance;
    }
   
  private:
   ...
    static  AlarmTask* _instance;
};


// A call to static member of class from init function
__task void init (void)
{
      ...
      // Do all the power up intialization       
      _usb = new UsbSocket();
       pdpTask = new PdpTask(_usb);
      // Get the clock driver constructed.
      AlarmTask::getInstance();
       ...


How can we call class member getInstance() when AlarmTask class hasn't been instantiated yet?
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Question by:naseeam
2 Comments
 
LVL 86

Accepted Solution

by:
jkr earned 1700 total points
ID: 39239194
That works because both 'getInstance()' and '_instance' are static class members. Static methods like 'getInstance()' don't recceive a hidden 'this' pointer and therefore are not associated with a particular instance of the class. And you will notice that '_instance' is instantiated (no pun intended) like

AlarmTask* AlarmTask::_instance = NULL;

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somewhere else in that code, thus providing storage space for that static variable outside of any instance of 'AlarmTask'. Read more at http://www.learncpp.com/cpp-tutorial/811-static-member-variables/
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LVL 60

Assisted Solution

by:Julian Hansen
Julian Hansen earned 300 total points
ID: 39239219
In this case the AlarmTask class is a singleton which means that only one instance of the object should ever be created.

You can create multiple instances with the new operator but in terms of how it has been designed the static function getInstance is static precisely for the purpose of either instantiating an instance of AlarmTask if one has not been created or returning the instantiated object.

The static AlarmTask * _instance - which is declared at the class level but could also have been declared inside getInstance - is a persistant (instance independant - think global) variable that is used to store the instantiated instance of AlarmTask.

Read here for more info on Singleton classes http://en.wikipedia.org/wiki/Singleton_pattern
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