Generics: Create instance from abstract class

Question

I am trying to create an instance of a class, but the compile-time error is saying:

Cannot create an instance of the abstract class 'QueueProcess'.

However, I am not creating an instance of it, I am creating an an instance of a class that extends QueueProcess. So, why am I getting this error?

export class Queue<T extends QueueProcess> {

    private _queue: T[] = []

    private async runFirstProcess() {
      let process = new this._queue[0]
    }

}

export abstract class QueueProcess {

}

The code once compiled works fine, it is just throwing that compile-time error.

Shouldn't it be,at least, export class Queue extends QueueProcess implements T. It seems really weird while you are using that "arrows" around extends — Sergey
– Sergey, Commented Oct 16, 2017 at 21:12
because Queue doesn't extend QueueProcess, it holds an array of QueueProcess's — Get Off My Lawn
– Get Off My Lawn, Commented Oct 16, 2017 at 21:13

CRice · Accepted Answer · 2017-10-16 22:36:40Z

5

So, first of all, the line T extends QueueProcess means that T is an instance of QueueProcess, which won't be newable. To access the constructor type, you need T extends typeof QueueProcess.

But that won't work anyway, since QueueProcess itself extends QueueProcess, and since it's abstract, typescript will complain about that. So instead, make T extend a newable function that returns a QueueProcess, eg:

export class Queue<T extends new () => QueueProcess> {

    private _queue: T[] = []

    private async runFirstProcess() {
        let process = new this._queue[0]()
    }

}

export abstract class QueueProcess {

}

edited Oct 16, 2017 at 22:36

answered Oct 16, 2017 at 21:34

CRice

32.6k5 gold badges63 silver badges75 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Get Off My Lawn Over a year ago

Okay, that makes sense. I didn't know that you could do <T extends new() => xxx>

artem Over a year ago

new () => SomeType is a shorthand syntax for an interface with construct signature, also called "newable type": {new(): SomeType}

rattray · Accepted Answer · 2022-06-02 20:53:44Z

0

You can use the Constructor helper from type-fest:

export class Queue<T extends Constructor<QueueProcess>> {

    private _queue: T[] = []

    private async runFirstProcess() {
        let process = new this._queue[0]()
    }

}

export abstract class QueueProcess {

}

answered Jun 2, 2022 at 20:53

rattray

6,1871 gold badge41 silver badges30 bronze badges

Comments

Sergey · Accepted Answer · 2017-10-16 21:23:04Z

-1

I rewrote your code a bit to satisfy the code analyzer. It's placed here. However, your code has issue with new this._queue[0]. Also, extension of class that is placed after current is a bad idea.

Result code is

export abstract class QueueProcess {

}

export class Queue<T> extends QueueProcess {

    private _queue: T[] = []

    private async runFirstProcess() {
      let process = new this._queue[0]
    }

}

answered Oct 16, 2017 at 21:23

Sergey

7,69014 gold badges49 silver badges97 bronze badges

Collectives™ on Stack Overflow

Generics: Create instance from abstract class

3 Answers 3

2 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related