Dart Constant Constructor

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Syntax and Declaration

class Vector2 {
  final double x;
  final double y;

  // Constant constructor declaration
  const Vector2(this.x, this.y);
}

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Structural Constraints

1. Immutable State: All instance variables within the class must be declared as final. Furthermore, fields cannot be declared as late; even late final instance variables are strictly prohibited.
2. No Execution Body: The constructor cannot possess a block body ({}). State initialization must occur exclusively through formal parameters (this.field) or an initializer list.
3. Potentially Constant Initializers: Expressions within the initializer list must be potentially constant expressions. Because constant constructors can be invoked at runtime, these expressions only evaluate as compile-time constants when the constructor itself is invoked in a constant context. Otherwise, they evaluate at runtime using the provided non-constant arguments.
4. Inheritance Chain: If the class extends a superclass, the superclass must also expose a constant constructor. The subclass must invoke it, either explicitly via a super call in the initializer list, or implicitly if the superclass provides an unnamed, no-argument constant constructor.

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Instantiation and Canonicalization

const Vector2 v1 = Vector2(1.0, 2.0);
const Vector2 v2 = Vector2(1.0, 2.0);

print(identical(v1, v2)); // true: Both reference the same canonical instance

Vector2 v3 = Vector2(1.0, 2.0);
Vector2 v4 = Vector2(1.0, 2.0);

print(identical(v3, v4)); // false: Distinct instances allocated at runtime

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Initializer List Mechanics

class Circle {
  final double radius;
  final double diameter;

  // Valid: 'radius * 2' is a potentially constant expression.
  // It evaluates at compile-time if invoked with 'const', 
  // or at runtime if invoked without 'const'.
  const Circle(this.radius) : diameter = radius * 2;
}

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Factory Constant Constructors

class Logger {
  final String name;

  const Logger._internal(this.name);

  // Redirecting factory constant constructor
  const factory Logger(String name) = Logger._internal;
}