CareerCup

I explained about the multithreading problem. Interviewer asked me to show the piece of the code in the singleton class that is not thread safe, then he asked how to solve the problem

class Singleton {

public: static Singleton *getInstance();
Singleton();
~Singleton();
Singleton(Singleton &another);

private: static Singleton *instance;

};

Singleton *Singleton::instance = 0;

Singleton *Singleton::getInstance() {

//here: obtain_lock for mutual exclusion.

if (!instance) {
instance = new Singleton();
return instance;
}
else
return instance;
}

the problem is that in multi-threaded environment, if two threads fight for a singleton object, we may have two threads with different data values. the singleton object itself exists as one, but the values within the object may be manipulated by the two threads by having its local copy and manupulation, etc...

Thus, a mutex problem. this can be solved by having lock mechnism at the location shown above.

Yes without lock, the second process will cause the istance variable to be overwritten with the new singleton object created in heap. Memory leak.

Just for completion:

Also need to implement/override Clone method of the Object Class. This would avoid multiple copies of Singleton being available.

class Singleton{

private static final objSingleton= new Singleton();

private Singleton(){
}

public static getInstance(){
return objSingleton;
}

...

}

this is no synchronization problem with this kind of singleton creation in java.