英文:
Generic type is overridden by inheritance in dart
问题
我创建了一个包含通用数据类型的类。
class Buck<E> {
List<E> list;
Buck({
required this.list,
});
}
然后，我创建了一个类和一个子类：
class SomeThing {}
class SomethingOther extends SomeThing {}
我创建了一个对象 Buck，其中 E 是 SomeThing 类：
var buck = Buck<SomeThing>(list: List<SomeThing>.empty());
此时，我假设 buck.list 的类型是 List<SomeThing>。然后，我将其赋值为 List<SomethingOther>：
buck.list = List<SomethingOther>.empty();
此时，没有什么异常情况。我假设 buck.list 的类型仍然是 List<SomeThing>。
然而，当我尝试将 SomeThing 对象添加到 buck.list 时，我得到了一个错误：
buck.list.add(SomeThing()); // 这会导致错误
以下是完整的代码：
void main() {
var buck = Buck<SomeThing>(list: List<SomeThing>.empty());
buck.list = List<SomethingOther>.empty();
buck.list.add(SomeThing());
}
class Buck<E> {
List<E> list;
Buck({
required this.list,
});
}
class SomeThing {}
class SomethingOther extends SomeThing {}
为什么会发生这种情况？这是预期的行为吗？
英文:
I created a class that contains a generic data type.
class Buck<E> {
List<E> list;
Buck({
required this.list,
});
}
Then, I created a class and a subclass:
class SomeThing {}
class SomethingOther extends SomeThing {}
I created an object Buck with E is SomeThing class.
var buck = Buck<SomeThing>(list: List<SomeThing>.empty());
At this point, I assumed that the type of buck.list is List<SomeThing>. Then, I assign it with List<SomethingOther>.
buck.list = List<SomethingOther>.empty();
At this point, there is nothing unusual. I assume that type of buck.list is still List<SomeThing>.
However, when I try to add SomeThing object to buck.list, I get an error:
buck.list.add(SomeThing()); // this will be error
Here is the complete code:
void main() {
var buck = Buck<SomeThing>(list: List<SomeThing>.empty());
buck.list = List<SomethingOther>.empty();
buck.list.add(SomeThing());
}
class Buck<E> {
List<E> list;
Buck({
required this.list,
});
}
class SomeThing {}
class SomethingOther extends SomeThing {}
Why does this happen? Is this expected behavior?
答案1
得分: 2
If Derived is a subtype of Base, then Dart also treats Generic<Derived> to be a subtype of Generic<Base>. Consequently, a Generic<Derived> is assignable to (or returnable as) a Generic<Base>.
对于大多数泛型情况，这通常不是问题，它通常提供了便利性。对于集合而言，这可能导致你遇到的问题。List<Derived> 只能存储 Derived 元素。因此，将 List<Derived> 分配给 List<Base> 实际上是不安全的，如果后续代码尝试添加其他 Base 元素。
这很不幸。
为了避免这种情况，你应该确保构建一个 List<Base>（在你的示例中是 List<Something>），即使它初始化为 Derived（在你的情况下是 SomethingOther）元素。例如：
var list = <Something>[SomethingOther(), SomethingOther()];
var buck = Buck(list: list);
为了更好地保护，你可以让 Buck 创建其自己的内部副本，这样它可以确保它始终具有一个 List<Something> 而不是 List<SomethingOther>：
class Buck<E> {
List<E> _list = <E>[];
List<E> get list => _list;
set list(List<E> newList) {
_list.clear();
_list.addAll(newList);
}
Buck({required List<E> list}) {
this.list = list;
}
}
还请参阅：
英文:
If Derived is a subtype of Base, then Dart also treats Generic<Derived> to be a subtype of Generic<Base>. Consequently, a Generic<Derived> is assignable to (or returnable as) a Generic<Base>.
For most generics, that isn't a problem, and it usually provides convenience. For collections, it can lead to the problem you encountered. A List<Derived> can store only Derived elements. Therefore assigning a List<Derived> to a List<Base> is not actually safe if code later tries to add other Base elements.
It's unfortunate.
To avoid this, you should ensure that you construct a List<Base> (which in your example, is List<Something>), even if it's initialized with Derived (SomethingOther in your case) elements. For example:
var list = <Something>[SomethingOther(), SomethingOther()];
var buck = Buck(list: list);
For better protection, you can make Buck create its own internal copy of the input List, and that way it can ensure that it always has exactly a List<Something> and not a List<SomethingOther>:
class Buck<E> {
List<E> _list = <E>[];
List<E> get list => _list;
set list(List<E> newList) {
_list.clear();
_list.addAll(newList);
}
Buck({required List<E> list}) {
this.list = list;
}
}
Also see:
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