Basics

Dynamic and Object

Object is Dart's safe top type: any non-null value fits, but only Object members are callable without a cast. dynamic opts out of static checking, so members and reassignments are accepted at compile time and resolved at runtime.

Program

Play the program to hold a String as Object, do arithmetic on a dynamic int, and print both.

dynamic_object.dart
Replay: real traced execution (multi-file project)
void main() {
  Object boxed = 'Ada';
  dynamic flexible = 3;
  flexible = flexible + 2;
  var label = boxed.toString();
  print('$label $flexible');
}

  1. boxed ← Ada (as Object)

    1void main() {2  Object boxed = 'Ada';3  dynamic flexible = 3;
    values this stepAda (as Object)boxed

  2. flexible ← 3 (dynamic)

    2Object boxed = 'Ada';3dynamic flexible = 3;4flexible = flexible + 2;
    values this step3 (dynamic)flexible

  3. flexible ← 5

    3dynamic flexible = 3;4flexible = flexible + 2;5var label = boxed.toString();
    values this step3 → 5flexible

  4. label ← Ada (String)

    4flexible = flexible + 2;5var label = boxed.toString();6print('$label $flexible');
    values this stepAda (String)labelAdaboxed

  5. print('$label $flexible');

    5  var label = boxed.toString();6  print('$label $flexible');7}
    outputAda 5
    values this stepAdalabel5flexible

Follow the Values

  1. boxed holds Ada as an Object.
  2. flexible starts as 3 using dynamic.
  3. flexible becomes 5.
  4. label is Ada as a String.
  5. The program prints Ada 5. | name | visible value | | --- | --- | | boxed | Ada (as Object) | | starting flexible | 3 (dynamic) | | updated flexible | 5 | | label | Ada (String) | | stdout | Ada 5 |
Object top type `Object boxed = 'Ada'` keeps the value safe; only `Object` members like `toString()` are callable without a cast.
dynamic `dynamic flexible = 3` skips static checks; `flexible + 2` is resolved at runtime against the underlying `int`.
rebind A `dynamic` binding can be reassigned to anything; here the value moves from `3` to `5` and stays an `int`.

Exercise: dynamic_object.dart

Reproduce Ada 5, then identify which value changes from 3 to 5.