dongshi3818
2019-01-30 15:21 阅读 81
已采纳

为什么`fmt.Println(“%T ”,e)`中的`e`是错误变量会打印出<nil>而不是其类型?

I was recently playing with the Go language and I bumped into something a bit weird to say the least, let's consider a very simple function:

func main() {
    n, e := fmt.Println(`He said: "Hello"`)
    fmt.Printf("%T
", n)
}

Which outputs what I was expecting:

He said: "Hello"
int

Now if I want to display the type of e:

func main() {
    n, e := fmt.Println(`He said: "Hello"`)
    fmt.Printf("%T
", e)
}

and this time prints out:

He said: "Hello"
<nil>

I get the part that there is no error so e is an empty pointer: nil but I would have not expected to be a ~~type~~ on its own.

Why am I not getting the actual type?

If so, is there a workaround? (not saying my use case is a realistic one but curious if there is any possibility)

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2条回答 默认 最新

  • 已采纳
    doufan6886 doufan6886 2019-01-30 15:25

    The Go Programming Language Specification

    Errors

    The predeclared type error is defined as

    type error interface {
      Error() string
    }
    

    It is the conventional interface for representing an error condition, with the nil value representing no error.

    Interface types

    An interface type specifies a method set called its interface. A variable of interface type can store a value of any type with a method set that is any superset of the interface. Such a type is said to implement the interface. The value of an uninitialized variable of interface type is nil.

    The zero value

    When storage is allocated for a variable, either through a declaration or a call of new, or when a new value is created, either through a composite literal or a call of make, and no explicit initialization is provided, the variable or value is given a default value. Each element of such a variable or value is set to the zero value for its type: nil for interfaces.


    A zero-value error type, an interface, has no type.Its value is nil.

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  • douhaodang0403 douhaodang0403 2019-01-30 15:32

    e is an empty pointer

    Nope, error itself - interface, so you won't have type here.

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