const constructor

Learn compile-time objects with const constructors.

const constructor

A const constructor produces a compile time constant instance. The constructor is prefixed with const and all fields must be final.

Example: class Point { final double x; final double y; const Point(this.x, this.y); }. Here const Point(0, 0) returns the same canonical instance on every call; this saves memory and speeds up equality checks. The constructor can be called without const too; then no compile time constant is produced. This shape is used heavily in the Flutter widget tree.

Canonical instances

Two const Point(0, 0) calls refer to the same instance; Dart makes them canonical. The identical(a, b) function returns true; the == operator also returns true.

Example: const a = Point(0, 0); const b = Point(0, 0); print(identical(a, b)); prints true. This means no new object is created for the same constant value; it brings memory and speed gains in large object graphs. If you omit const at the call site, a new instance is produced each time and canonicalness is lost.

Exercise Add a const constructor to Point

Define the Point class; add final double x and final double y fields. Write a const Point(this.x, this.y) constructor. In main, create an instance with const p = Point(0, 0); and call print(p.x).

Show example solution

// Define the Point class
class Point {
  final double x;
  final double y;
  const Point(this.x, this.y);
}

void main() {
  const p = Point(0, 0);
  print(p.x);
}

Test yourself

What fields does a const constructor require?
- Only static fields
- Only late fields
- Any field type
- Only final fields
Show answer

Only final fields — A const constructor requires all fields to be final.
Match the const constructor parts

const Point

Constructor prefix
final

Field keyword
identical(a, b)

Same instance query

Add the const keyword

class Point {
  final double x;
  final double y;
  ____ Point(this.x, this.y);
}

Show answer

class Point {
  final double x;
  final double y;
  const Point(this.x, this.y);
}

Next lesson: factory

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies