类型不可分配给类型“T"，但可以使用约束类型的不同子类型实例化“T" [英] Type is not assignable to type 'T', but 'T' could be instantiated with a different subtype of constraint Type

问题描述

我正在尝试实施活动例程&带有打字稿的存储库逻辑.但是在处理泛型类型时遇到了一些问题.

Hi I'm trying to implement active routine & repository logic with typescript. But having some problems dealing with generic types.

我有一个抽象的 BaseModel，它具有返回自身和自身子类型的 create 方法.为此，我说它将返回 BaseModel.在具有从 BaseModel 扩展的泛型类型的存储库中，我包装了我的模型方法并使用泛型类型作为返回值.但是当我这样做时，它给了我以下错误.

I have an abstract BaseModel which has create method that returns itself and subtype of itself. To do so I said it will return BaseModel. In repository which has a generic type extends from BaseModel, I wrapped my model methods and used generic type as returned value. But when I do that it gives me below error.

'BaseModel' 不能分配给类型 'T'.BaseModel"可分配给T"类型的约束，但T"可以用约束BaseModel"的不同子类型实例化.

当我用谷歌搜索时，发现了一些答案，比如在嵌套结构中重新分配泛型或复制时可能会发生这种情况.但我既没有分配一些东西，也没有一遍又一遍地定义相同的泛型

When I googled it, found some answers like it may happen when re-assigning the generic or dublicating in nested structures. But neither I'm assigning something nor define same generic over and over

为什么我会收到这种类型的错误，我该如何解决?

Why I'm getting this type error and how do I solve it ?

这是部分代码.

BaseModel.ts

BaseModel.ts

export abstract class BaseModel {
  protected constructor(private attributes: any[] = []) {
  }

  create(attributes: any[] = null): BaseModel {
    return this;
  }
  // ...
}

BaseRepository.ts

BaseRepository.ts

export class Repository<T extends BaseModel> {
  constructor(private model: T) {
  }

  create(attributes: any[]): T {
    return this.model.create(attributes);
  }
  // ...
}

推荐答案

那是因为 BaseModel 的 create 返回了不可分配给的 BaseModel派生类型(子类型).它应该返回 this:

That's because create of BaseModel returns BaseModel which is not assignable to derived type (subtype). It should return this:

create(attributes: any[] = null): this { ...

游乐场

更多关于多态这种类型这里.

More on polymorphic this types here.

这篇关于类型不可分配给类型“T"，但可以使用约束类型的不同子类型实例化“T"的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！