Abstract Properties

[denvned]

This is a proposal to extend #3578 to allow abstract properties in abstract classes. So the following should be legal:

abstract class A {
  abstract p;
}
class B extends A {
  get p() {...}
  set p(v) {...}
}

[Pajn]

+1

Also abstract getters should be full-fillable by properties:

abstract class A {
  abstract get p();
}
class B extends A {
  p;
}

This is a pretty common pattern in Dart when the abstract class just need some values by it's implementers.

[alexeagle]

+1 angular has been wishing for this as well:
https://github.com/angular/angular/blob/a88e6f31063ac3abb439f93cbbc3a46c065f7614/modules/angular2/src/core/compiler/element_ref.ts#L60
cc @tbosch

[mhegazy]

declaring the property as abstract does not change anything different from just declaring it. the type is going to always have the property.

As for getters and setters., the type system does not differentiate between getter/setter and a property declaration.

[denvned]

@mhegazy Actually, there is an important difference. Using an abstract property I could specify that the property must be specialized (usually with getters and/or setters) in a derived class, and the compiler should rise an error if it's not specialized.

Please reconsider closing this proposal.

[raveclassic]

@mhegazy I totally agree with @denvned about enforcing getter declaration in child classes.

[bykbtzr]

The lack of abstract properties allows child classes to simply ignore some important properties in my contracts.

Please reopen this for further discussion...

[craigbroadman]

+1

Especially as lambda/arrow functions are actually properties as I found out when I was trying to implement the following functionality but it wouldn't compile due to....

"Class 'Base' defines instance member function 'def', but extended class 'Concrete' defines it as instance member property"...

abstract class Base {
    abstract abc(): void;
    abstract def(): void;
}

class Concrete extends Base {
    private setting: boolean;

    public abc(): void  {
        this.setting = true;
    }

    public def = (): void => {
        this.setting = false;
    }
}