CODEWITHSUNDEEP
dart
iDecode
iDecode

Reputation: 28916

What's the significance of having no members in an abstract class?

Exception class looks like:

abstract class Exception {
  factory Exception([var message]) => _Exception(message);
}

_Exception class is the implementation of it. As you can see there are no fields or functions declared in this class other than a factory constructor, so what's the significance of having such classes?

Upvotes: 0

Views: 39

Answers (1)

lrn
lrn

Reputation: 71693

It's like a marker interface which tells the reader that a subclass is intended to be used as an exception (thrown in a situation where the user is expected to catch and handle the exceptional situation, rather than an error).

Dart, the language, does not treat Exception specially. It's completely unnecessary from a technical point of view since all objects can be thrown. Making your class implement Exception signifies that it's purpose is to be thrown, even though it is not an Error, but you should see it as documentation, not something your program can meaningfully use.

Exceptions, unlike Errors, are intended to be caught and handled, just as much as a return value is. An exception should contain enough information to make the receiver able to do something meaningful. Because of that, you should never throw a plain Exception in production code (so the constructor is really deprecated except during development, until you have created a proper exception class).

Similarly, you should never catch a plain Exception (no on Exception catch (e) {...}) because then you are not handling the actual problem. If you just want to catch anything, use on Object, because code can throw any class, and not all exception classes need to implement Exception.

Upvotes: 2

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