Private static member set to NULL directly?

spaghetticode · 12-01-2011

Hey people,

found the following in the book I work with:

Code:

class Singleton {

    private:

        static Singleton *instanz;
        Singleton() { }

    public:

        static Singleton* getInstanz() {

            if (instanz == NULL) instanz = new Singleton();
            return instanz;

        }

};

Singleton* Singleton::instanz = NULL;

int main() {

    Singleton* obj1 = Singleton::getInstanz();
    Singleton* obj2 = Singleton::getInstanz();

    cout << " obj1: " << &(*obj1) << endl;
    cout << " obj2: " << &(*obj2) << endl << endl;

    return 0;

}

Why can I set

Code:

Singleton::instanz

directly to

Code:

NULL

above

Code:

main()

when it's private?

(May be a stupid question, but I worked through a whole chapter of classes, constructors, members, and now I feel a bit overwhelmed with all the new things.)

**manasij7479** · 12-01-2011

Of course you can .
Just #include<cstdlib>

Btw...Consider using nullptr instead of NULL when writing C++ code.

spaghetticode · 12-01-2011

But what I have learned by now is that you always need a member function to access private members. And what's that about cstdlib? The book just includes iostream, and with gcc this code perfectly compiles (-Wall -pedantic). What am I missing here?

(Thanks btw for that hint to nullptr - since my textbook uses NULL, I will research about that on this very website.)

**manasij7479** · 12-01-2011

Well.. NULL is defined in <cstdlib>

Sorry.. I misread your question(halfway)...and inferred that it wasn't compiling, when it was just the opposite!

In that case, whether you can define it outside, using the scoped notation does not depend on whether it is private or not.
The visibility comes into play when you try it to access it from a function which is not a friend.
(Following your trend of thought...it would have been impossible to define private methods in an implementation file if this wasn't allowed)

I trust that you understand why the definition for a static data member is needed..so skipping that.

spaghetticode · 12-01-2011

Ahhhhhhhh! *head -> table*

I seem to have misinterpreted the whole line. That's what I meant when I said I felt confused after all the new stuff.

I see, that's just the definition. Now I feel clearer about that, thanks a lot. I guess I will have to re-read the whole chapter at least one more time to have it all sink in...

**manasij7479** · 12-01-2011

>I guess I will have to re-read the whole chapter at least one more time to have it all sink in...
Or you could write a nice program.

spaghetticode · 12-01-2011

I don't think I already know enough to find any problem where I would need a code like the above. Keep in mind that I just *started* working on OOP, classes and stuff in C++.

**MK27** · 12-01-2011

Originally Posted by dennis.cpp

I don't think I already know enough to find any problem where I would need a code like the above.

Singleton's have a pretty limited use value IMO, but a more general lesson you can take from that is how to use non-const static class members. Try this:

Code:

#include <iostream>

class Thing {
    public:
        static int n;
        Thing(int a) { n = a; };
};
 
int Thing::n = 5;

using namespace std;
 
int main() {
	Thing a(2), b(6);
	cout << a.n << endl;
	return 0;
}

Notice the output is 6. Static members allow one object to set a value for all other instances of a class, but they must be initialized in class::scope outside of a (non-friend) function (which, as manasij7479 points out, is why you are allowed to do this even with private members).

spaghetticode · 12-01-2011

Thanks you too. I remember the use of class members from Java, actually. But having to define them in the global scope is something I'll need to get used to.

**manasij7479** · 12-01-2011

>But having to define them in the global scope is something I'll need to get used to.
You are NOT defining them in global scope.
It is still in the class scope(the :: operator is there for that reason).

For methods, there is no difference between :

Code:

class foo
{
     int bar()
    { 
         return 0;
    }
};

and

Code:

class foo
{
    int bar();
};
int foo::bar()
{ 
    return 0;
}

For static data members ... you must define them outside for the common memory to be set aside from the beginning.

spaghetticode · 12-01-2011

You're right! Thanks again. Wah, Java was so much easier there.

I will revise the whole chapter tomorrow and maybe, in addition, work through this page's tutorial section for classes.

**Acorn** · 12-01-2011

I dont know how much it explains static members and static functions but a static function can only modify static members or only call other static functions.

however non static functions can call static functions and static members can be returned from a non static function.

spaghetticode · 12-01-2011

That's where Java takes the same road again.

**manasij7479** · 12-01-2011

Originally Posted by Acorn

I dont know how much it explains static members and static functions but a static function can only modify static members or only call other static functions.

..Unless you give the static function an object to refer to.

**Acorn** · 12-01-2011

thanks mana i didnt know that. i just tested it out and your right.

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