How to initialize variables in constructor body in Dart

How to initialize variables in constructor body in Dart

In Dart, we must initialize all instance variables before a constructor body is executed.

This is the main difference and the cause of confusion between Dart and other programming languages.

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Non-nullable instance field '...' must be initialized

If you try to initialize instance variables in a constructor body, you will find out that it is not as easy as you normally do in other programming languages.

Here is an example where I try to initialize all instance variables in a constructor body.

class DoublePoint {
  double x;
  double y;

  DoublePoint(
    double x,
    double y,
  ) {
    this.x = x * 2;
    this.y = y * 2;
  }
}

You will get the following error.

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

Luckily, the error also tells us three ways to solve this.

1. Adding an initializer expression.
2. Add a field initializer in this constructor.
3. Mark it late.

Let's see what we have to do for each solution.

Try adding an initializer expression

We can silence the error by initialize all instance variables when we declare them.

This solution is straightforward, since Dart requried all fields to be initialized before entering a constructor body, we do it at the declaration time.

class DoublePoint {
  double x = 0;
  double y = 0;

  DoublePoint(
    double x,
    double y,
  ) {
    this.x = x * 2;
    this.y = y * 2;
  }
}

Add a field initializer in this constructor

I think this is the solution that most people who aren't familiar with Dart are looking for.

Dart has a unique syntax to initialize each field in a constructor.

You initialize the field in the Initializer list, which is placed before a constructor body.

It has the following syntax:

1. You put a colon (:) before a constructor body.
2. You assign each instance variable after the colon.
3. Separate each expression with a comma (,).

ClassName(param1, params2, ...) 
: field1 = ..., field2 = ...., .... 
{
  // Constructor body
}

Here is the same example with each field initialized in the initializer list.

What you need to do is basically move field initializers out of the constructor body to the special place before the body.

class DoublePoint {
  double x;
  double y;

  DoublePoint(
    double x,
    double y,
  )   : x = x * 2,
        y = y * 2 {
    print('this.x: ${this.x}');
    print('x: $x');
    print('this.y: ${this.y}');
    print('y: $y');
  }
}

DoublePoint(1, 2)
// this.x: 2.0 
// x: 1.0 
// this.y: 4.0 
// y: 2.0 

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Mark it late

Adding the late modifier on a variable tells a compiler that we will make sure it is initialized before being used.

We add the late keyword on all instance variables so we can initialize them lazily in the constructor body.

class DoublePoint {
  late double x;
  late double y;

  DoublePoint (
    double x,
    double y,
  ) {
    this.x = x * 2;
    this.y = y * 2;
  }
}

The last solution is quite dangerous, in my opinion. Adding the late is kind of skipping the Dart requirement and promising we will fulfill it later. Failing to fulfill that will cause a runtime error.