Singleton pattern with uninitialized argument error

Question

I've applied the singleton pattern as in this issue and tried to accept a parameter using the method in this issue. But it gives me this error:

Non-nullable instance field 'id' must be initialized. Try adding an initializer expression, or add a field initializer in this constructor, or mark it 'late'

So where is the issue?

Here is my code:

  static final Singleton _inst = Singleton._internal();
  int id;
  Singleton._internal() {
    // some logic 
  }

  factory Singleton({required int id}) {
    _inst.id = id;
    return _inst;
  }

score 0 · Accepted Answer · answered Dec 23 '21 at 10:00

0

Without your full code I am going to assume that your full Singleton class looks something like this:

class Singleton {
  int id;

  static final Singleton _inst = Singleton._internal();

  Singleton._internal() {
    // some logic
  }

  factory Singleton({required int id}) {
    _inst.id = id;
    return _inst;
  }
}

The thing is that since nullsafety you can't declare a non-nullable variable without assigning its value or mark it as late (just as said in your error message).

So what you should do is simply declare id like this:

class Singleton {
  late int id;

  // ...
}

Try the full sample on DartPad

answered Dec 23 '21 at 10:00

Guillaume Roux

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If `id` is not in the `Singleton` class how can he assign it to `_inst` ? – Guillaume Roux Dec 23 '21 at 10:22
He has shared tow links in ques having Singlton class you may check – Diwyansh Dec 23 '21 at 10:25
I've checked both and the second one is exactly showing a singleton with an `id` property declared inside the singleton's class. – Guillaume Roux Dec 23 '21 at 10:27
1

yes @GuillaumeRoux is right there is int id .. sorry I've edited the code in the question – Ahmed Osama Dec 23 '21 at 10:44
@GuillaumeRoux Your answer is technically correct. But the problem with this approach is, even if he wants to access the instance, he may need to pass an id to that factory (Since it's mandatory) – Midhun MP Dec 23 '21 at 10:44
@MidhunMP you are indeed correct but it seems like it was Ahmed intention, based on the code he provided. – Guillaume Roux Dec 23 '21 at 10:48

score -2 · Answer 2 · answered Dec 23 '21 at 10:01

-2

In your People or Singlton class you have defined a field named id which is neither nullable nor initialized that is why you are getting this error. Just make your field nullable or assign some value to it to initialize. Like below

To make it nullable

   int? id;

To initialize

  int id = "";

answered Dec 23 '21 at 10:01

Diwyansh

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Using the `late` keyword is much better as it will allow `id` to be non-nullable and he won't need to have an initialized value. – Guillaume Roux Dec 23 '21 at 10:05
I think we should let the user decide their need rather then being judge and one more thing you voted down my answer but it's not wrong just you have a different point of view I'll suggest you to now to misuse your rights – Diwyansh Dec 23 '21 at 10:12

Singleton pattern with uninitialized argument error

2 Answers2