C++: Static member function returning an object of self static for a class with private constructor

Question

I've a C++ snippet as below. The "getInstance()" function is trying to return a static object of the same class "CAbc". This class has a private constructor so that objects of this class can't be created.

While calling this function in main(), like in below snippet, the returned object is not being collected anywhere in main() using any reference.

I'm trying to understand the below 2 points:

What is this way of creating a static object & returning it from within the static member function of same class type ? There is no object created when a constructor is private. Then how does this object return work here ?
And how does this way of not collecting the object in main() work ?

This is being called in the main() function.

class CAbc
{
private:
CAbc() {} // HAS A PRIVATE CONSTRUCTOR
.....
public:
static CAbc& getInstance()
    {
        static CAbc _self;
            return _self;
    }
    // what does this returning a static self object mean in C++ ?
}

main()
{
CAbc::getInstance();
// not collectig this anywhere ?
}

`This class has a private constructor so that objects of this class can't be created.` and `There is no object created when a constructor is private.`: This is wrong. The class itself (or, methods of the class) can call private methods, which means, the static class method _can_ (and does) create a instance of the class. — tkausl, Jul 18 '20 at 17:05
No object is being returned by `getInstance`, instead a reference to the object is being returned. — john, Jul 18 '20 at 17:09
And for your other question, it's not an error not to 'collect' a return value. There's nothing to work, the returned value (or reference) is just thrown away. You might ask what the point is, but that's a different question. — john, Jul 18 '20 at 17:10
FYI, What you have is called a [Meyer's singleton](https://stackoverflow.com/questions/17712001/how-is-meyers-implementation-of-a-singleton-actually-a-singleton/17713799) — PaulMcKenzie, Jul 18 '20 at 17:12
Possible duplicate of: https://stackoverflow.com/questions/1008019/c-singleton-design-pattern/1008289#1008289 — OrenIshShalom, Jul 18 '20 at 17:51

score 1 · Accepted Answer · answered Jul 18 '20 at 17:48

As @tkausi says, the static member function can create an object, as it is a member (thus having access to private methods.)

What this code does is having an instance of the class, only one object (_self) and returning it for use.

Why nobody gets the return value? Because the call is there only to create the instance. If you don't call the function no _self object will be created.

class CAbc
{
private:
    CAbc() { cout << "creating" << endl; }
public:
static CAbc& getInstance()
    {
        static CAbc _self;
            return _self;
    }
};

int main() {
    cout << "Begin" << endl;

    //CAbc::getInstance();
}

With the call to getInstance commented out you won't see the "creating" output.

If you uncomment the call, you'll see:

Begin
creating

Once created, the function will always return the same object.

C++: Static member function returning an object of self static for a class with private constructor

1 Answers1