为什么在Dart的null安全中无法访问List()构造函数? [英] Why List() constructor is not accessible in Dart's null safety?
问题描述
使用NNBD，不允许使用默认构造函数初始化列表:
With NNBD, you're not allowed to initialise the list using the default constructor:
List<int> foo = List(); // Compile time error
但是，您仍然可以:
List<int> foo = []; // No error
那么，两者之间有什么区别?它们要么都不显示错误，要么都不显示.
So, what's the difference between the two? Either both of them should show the error or none of them.
推荐答案
List 构造函数有两种用途:
The List constructor had two uses:
-
new List()创建一个空的可增长列表，等效于[]. -
新列表(n)，以创建长度为n的固定长度列表，并填充null值
new List()to create an empty growable list, equivalent to[].new List(n)to create a fixed-length list of lengthnfilled withnullvalues
出于安全考虑，大多数情况下第二次使用都是不安全的，并且没有很好的方法来修复它.可以将类型参数强制为不可为空，但是 List< T>(4)仅在 T 为 nullable 时有效.没有办法强制执行.
With null safety, the second use was unsound most of the time, and there was no good way to fix it. It's possible to force a type argument to be non-nullable, but List<T>(4) only works when T is nullable. There is no way to enforce that.
因此，需要进入 List(n)模式(由 List.filled(n，value)取代，这迫使您提供填充值).剩下的 List()并没有真正发挥自己的作用.您可以改为使用 [] (并且 应该 ！)，因此决定完全删除该构造函数-对其的所有使用都不安全或无用.(此外，它已经是一个奇怪的构造函数，因为如果我们想使其正确为null安全，它将有一个带有非空类型且没有默认值的可选参数.)
So, the List(n) mode needed to go (replaced by List.filled(n, value) which forces you to provide a fill-value).
That left List(), which doesn't really carry its own weight. You can just use [] instead (and you should!), so it was decided to remove the constructor entirely - all uses of it was either unsafe or useless.
(Also, it was a weird constructor already, because if we wanted to properly make it null safe, it would have an optional parameter with a non-nullable type and no default value.)
通过完全删除它，有可能在将来引入新的 List 构造函数，也许作为 List.filled 的较短别名.一个人可以希望.
By removing it completely, it makes it possible to, potentially, introduce a new List constructor in the future, perhaps as a shorter alias for List.filled. One can hope.
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