dart: what is the meaning of class constructor being marked as const

Question

So I have seen code as such:

class Whatever {
  final String name;
  const Whatever(this.name);
}

What does change by the fact that the constructor is marked with const? Does it have any effect at all?

I have read this:

Use const for variables that you want to be compile-time constants. If the const variable is at the class level, mark it static const. (Instance variables can’t be const.)

but it does not seem to make sense for the class constructor.

Answer 1

• The constructor can't have a constructor body.
• All members have to be final and must be initialized at declaration or by the constructor arguments or initializer list.
• You can use an instance of this class where only constants are allowed (annotation, default values for optional arguments, ...)
• You can create constant fields like static const someName = const Whatever();

If the class doesn't have a const constructor it can't be used to initialize constant fields. I think it makes sense to specify this at the constructor. You can still create instances at runtime with new Whatever() or add a factory constructor.

See also

• Dartlang const constructor - how is it different to "regular" constructor
• Dart factory constructor - how is it different to “const” constructor
• How to make a factory constructor that returns as const value
• Why does Dart have compile time constants?
• How to write abstract class constructors so that it will be flexible for extending in sub classes

The "old style" (still valid) enum is a good example how to use const https://stackoverflow.com/a/15854550/217408

Answer 2

If your class produces objects that never change, you can make these objects compile-time constants. To do this, define a const constructor and make sure that all instance variables are final.

class ImmutablePoint {
  const ImmutablePoint(this.x, this.y);

  final int x;
  final int y;

  static const ImmutablePoint origin = ImmutablePoint(0, 0);
}

Code example

Modify the Recipe class so its instances can be constants, and create a constant constructor that does the following:

• Has three parameters: ingredients, calories, and milligramsOfSodium (in that order).

• Uses this. syntax to automatically assign the parameter values to the object properties of the same name.

• Is constant, with the const keyword just before Recipe in the constructor declaration.

  class Recipe {
    final List<String> ingredients;
    final int calories;
    final double milligramsOfSodium;
  
    const Recipe(this.ingredients, this.calories, this.milligramsOfSodium);
  }