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The [] operator, formally known as the subscript operator, is a special instance method that enables indexed read access to an object. It allows an instance to be queried using bracket notation, functioning as syntactic sugar for a method call that accepts a single argument (the index or key) and returns a value.

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Syntax Definition

To implement the subscript operator in a class, define a method named [] with the operator keyword. 
ReturnType operator [](IndexType index) {
  // Logic to retrieve value based on index
  return value;
}
  • ReturnType: The data type of the value returned by the operation.
  • IndexType: The data type of the key used to look up the value (commonly int for sequences or Object for hash maps).

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Operational Mechanics

When the Dart compiler encounters an expression like object[key], it translates the syntax into a direct method invocation on object.
  1. Expression: var x = myObject[i];
  2. Translation: var x = myObject.[](i);
The operator is strictly unary regarding the index; it accepts exactly one argument within the brackets.

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Implementation Example

The following example demonstrates how to overload the [] operator within a custom class to expose internal data. 
class ByteBuffer {
  final List<int> _data;

  ByteBuffer(this._data);

  // Operator definition
  // Accepts an integer index and returns the integer at that position
  int operator [](int index) {
    if (index < 0 || index >= _data.length) {
      throw RangeError.index(index, _data);
    }
    return _data[index];
  }
}

void main() {
  final buffer = ByteBuffer([10, 20, 30, 40]);

  // Invoking the [] operator via bracket notation
  final byte = buffer[2]; 
  
  print(byte); // Output: 30
}

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Relationship with []=

The [] operator is exclusively for read access. To enable indexed write access (assignment), the class must also implement the corresponding []= (index assignment) operator.
  • Read: var val = obj[i]; invokes operator [](i)
  • Write: obj[i] = val; invokes operator []=(i, val)
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