Dart Initializer List

ClassName(parameterList) : initializer1, initializer2 {
  // constructor body
}

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Execution Order

1. Initializer list expressions (evaluated from left to right).
2. Superclass constructor.
3. Main class’s constructor body.

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Scope and Restrictions

• No this Access: The right-hand side of an initializer expression does not have access to the this keyword. You cannot invoke instance methods or read instance variables within the initializer list because the object is in an uninitialized state.
• Final Variables: It is the primary mechanism for initializing final instance variables that require computation or parameter mapping before the constructor body executes.
• Redirection Isolation: If an initializer list contains a redirecting constructor call (this(...)), it cannot contain any other initializers.

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Syntax Mechanics

import 'dart:math';

class Vector {
  final double x;
  final double y;
  final double magnitude;

  Vector(double x, double y)
      : x = x,
        y = y,
        magnitude = sqrt(x * x + y * y),
        assert(x >= 0 && y >= 0) {
    // Body executes after x, y, and magnitude are set
  }
}

class Vector3D extends Vector {
  final double z;

  Vector3D(double x, double y, double z)
      : z = z,
        super(x, y) {
    // Body executes after z is set and Vector(x, y) completes
  }
}

class Vector {
  final double x;
  final double y;

  Vector(this.x, this.y);

  // Redirecting constructor
  Vector.zero() : this(0.0, 0.0); 
}