英文:
Go type assert nil to pointer type
问题
为什么我不能将nil类型断言为指针类型？这背后的逻辑是什么？
package main
func main() {
var s interface{} = nil
var p *string = nil
var q *string = s.(*string)
_ = q
_ = p
}
为什么我不能将nil类型断言为指针类型？这背后的逻辑是什么？
英文:
Why can't I type-assert a nil to a pointer type? What is the logic behind this?
package main
func main() {
var s interface{} = nil
var p *string = nil
var q *string = s.(*string)
_ = q
_ = p
}
答案1
得分: 3
你无法对没有类型的内容进行类型断言。
静态类型（或者只是类型）是在变量声明中给出的类型，new调用或复合字面量中提供的类型，或者结构化变量的元素的类型。接口类型的变量也有一个独特的动态类型，在运行时分配给变量的值的具体类型（除非该值是预声明的标识符nil，它没有类型）。动态类型可能会在执行过程中变化，但存储在接口变量中的值始终可以赋值给变量的静态类型。
直接引用自规范（我强调的部分）
接口知道底层值的类型。例如，如果我有一个带有type MyType的接口，它也不能被断言为*string。你可以通过一些工作来转换它的类型，但类型断言和类型转换是不同的。
还可以参考这里
对于一个接口类型的表达式x和类型T，主表达式
x.(T)
断言x不是nil，并且存储在x中的值是类型T。表示为x.(T)。
英文:
You can't type assert something with no type.
> The static type (or just type) of a variable is the type given in its
> declaration, the type provided in the new call or composite literal,
> or the type of an element of a structured variable. Variables of
> interface type also have a distinct dynamic type, which is the
> concrete type of the value assigned to the variable at run time
> (unless the value is the predeclared identifier nil, which has no
> type). The dynamic type may vary during execution but values stored in
> interface variables are always assignable to the static type of the
> variable.
Straight from the spec (emphasis mine)
Interfaces know the type of the underlying value. For instance if I have a interface with a type MyType it can't be type asserted to *string either. You could perhaps convert its type with some work, but type assertion and type conversion are different.
Also take a look here
> For an expression x of interface type and a type T, the primary
> expression
>
> x.(T)
>
> asserts that x is not nil and that the value stored in x is of
> type T. The notation x.(T)
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