How do I compare two objects to see if they are the same instance, in Dart?

Question

Say I have a class that has many instance variables,. I want to overload the == operator (and hashCode) so I can use instances as keys in maps.

class Foo {
  int a;
  int b;
  SomeClass c;
  SomeOtherClass d;
  // etc.

  bool operator==(Foo other) {
    // Long calculation involving a, b, c, d etc.
  }
}

The comparison calculation may be expensive, so I want to check if other is the same instance as this before making that calculation.

How do I invoke the == operator provided by the Object class to do this ?

score 52 · Accepted Answer · edited Nov 18 '19 at 03:16

52

You're looking for "identical", which will check if 2 instances are the same.

identical(this, other);

A more detailed example?

class Person {
  String ssn;
  String name;

  Person(this.ssn, this.name);

  // Define that two persons are equal if their SSNs are equal
  bool operator ==(Person other) {
    return (other.ssn == ssn);
  }
}

main() {
  var bob = new Person('111', 'Bob');
  var robert = new Person('111', 'Robert');

  print(bob == robert); // true

  print(identical(bob, robert)); // false, because these are two different instances
}

edited Nov 18 '19 at 03:16

Erik Campobadal

725
6
13

answered Aug 25 '13 at 12:04

Christophe Herreman

15,649
7
57
86

3

if you override `operator ==` you will also need to override `hashCode`. See this answer [here](https://stackoverflow.com/a/22999113/9449426) for how to do this – NearHuscarl Dec 02 '19 at 06:14
1

What if i want to check If a list of model already contains a particular object ? – minato Jan 07 '20 at 11:21

Alexandre Ardhuin · Answer 2 · 2020-02-05T07:04:30.333

6

You can use identical(this, other).

edited Feb 05 '20 at 07:04

answered Aug 25 '13 at 12:05

Alexandre Ardhuin

62,400
12
142
126

score 2 · Answer 3 · answered Oct 11 '21 at 07:01

For completeness, this is a supplemental answer to the existing answers.

If some class Foo does not override ==, then the default implementation is to return whether they are the same object. The documentation states:

The default behavior for all Objects is to return true if and only if this object and other are the same object.

score 0 · Answer 4 · answered Jan 18 '21 at 17:56

On a different yet similar note, in cases where the framework calls to check the equality among the objects e.g. in case of list.toSet() to get the unique elements from a list, identical(this, other) may not be a choice. That time the class must override the == operator and the hasCode() methods.

However for this case another way could be to use the equatable package. This saves a lot of boiler plate code and is especially handy when you have lot of model classes.

How do I compare two objects to see if they are the same instance, in Dart?

4 Answers4

Linked