How to initialize a static reference to a variable inside the class, for use in nested classes

Question

I need to save in Class static ref to a variable for use in nested classes in a future. I have no problem in use ref to a variable (not static), but with static i have to use int & myClass::refA = ... in someware. The problem is that i can use int & myClass::refA = ... only in myClass constuctor, because only there i got the ref for refA, in myClass mc(newA) colling. How to initialize a static reference to a variable inside the class constuctor?

Code:

class myClass
{
    private:
        static int & refA;
    public:
        myClass(int &_refA);
        void updateA();
};

myClass::myClass(int &_refA)
: refA(_refA)
{
}

void myClass::updateA() {
    refA = refA++;
}

int newA = 10;
myClass mc(newA);
mc.updateA();

Error:

In constructor 'myClass::myClass(int&)':

QF:17:3: error: 'int& myClass::refA' is a static data member; it can only be initialized at its definition

 : refA(_refA)
   ^

exit status 1
'int& myClass::refA' is a static data member; it can only be initialized at its definition

The question is why do you want to do this? The reason you can't is that references cannot be uninitialised so must be initialised when they are defined. — Alan Birtles, Jul 13 '18 at 14:25
`static int* refA = nullptr;` and set it in constructor then ? — Jarod42, Jul 13 '18 at 14:27
What do you want to initialize it **to**? It is a static member, initializing it to (randomly) first argument to the constructor makes no sense. I sense XY problem. — SergeyA, Jul 13 '18 at 14:31

Kai Guther · Answer 1 · 2018-07-13T15:46:15.320

You can hack this by using a static member function that has a local static variable which is set to the argument of the constructor when first called.

static int dummy =  1;

class myClass
{
private:
    static int &refA(int &in = dummy){
        static int &buf = in;
        return buf;
    };
public:
    myClass(int &_refA){refA(_refA);};
    void updateA();
};

The static function refA() now has a static member which will be initialized when the function is first called, which is when the first myClass is constructed. refA() now behaves like a static reference to the argument of the constructor of the first myClass object constructed.

Note that this is a horrible hack, if that is really the easiest solution to the underlying problem is another story (almost certainly not), and there might be very well a solution that does not require a static reference.

How to initialize a static reference to a variable inside the class, for use in nested classes

1 Answers1