Extension operator

Learn extension operator overloading.

Extension operator

Extension operator is operator overloading within an extension. The extension VectorOps on Vector { Vector operator +(Vector other) => ... } declaration redefines the + operator for Vector.

Operator overloading allows custom operations with natural syntax: v1 + v2 adds two Vectors. Dart allows overloading many operators like + - * / == < >. Each operator has a fixed signature.

Operator rules

In Dart, each operator has a fixed signature. operator + takes one parameter and the return type is free. operator == must return bool and should be consistent with hashCode when possible.

Some operators cannot be overloaded: =, !, ??, ., ~/ (except ~/=). operator [] and operator []= are used for indexing. Logical operators (&&, ||) cannot be overloaded.

Exercise A Vector + operator

Define a Vector class with final int x; final int y; const Vector(this.x, this.y); field and constructor. Define a VectorOps extension on Vector with a Vector operator +(Vector other) => Vector(x + other.x, y + other.y); method. In main create final v = Vector(1, 2) + Vector(3, 4);; print with print('${v.x},${v.y}');.

Show example solution

class Vector {
  final int x;
  final int y;
  const Vector(this.x, this.y);
}

extension VectorOps on Vector {
  Vector operator +(Vector other) => Vector(x + other.x, y + other.y);
}

void main() {
  final v = Vector(1, 2) + Vector(3, 4);
  print('${v.x},${v.y}');
}

Test yourself

Match the operators

operator +

Addition
operator ==

Equality
operator <

Less than
operator *

Multiplication
What must operator == return?
- int
- bool
- String
- void
Show answer

bool — operator == must return bool.

An operator + declaration

Vector operator +(____ other) => Vector(x + other.x, y + other.y);

Show answer

Vector operator +(Vector other) => Vector(x + other.x, y + other.y);

Next lesson: Generic extension

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