切片的零值不是零 [英] The zero value of a slice is not nil

问题描述

我按照以下示例 https://tour.golang.org/moretypes/10
我修改了希望得到相同结果的代码。我没有。这是一个错误还是文档错误？旅游状态

I was following the example https://tour.golang.org/moretypes/10 I modified the code expecting to get the same result. I did not. Is this a bug, or a documentation error? The tour states

一个零切片的长度和容量为0

A nil slice has a length and capacity of 0.

我的y变量的长度和容量都是0.

My y variable has a length and capacity of 0.

package main

import "fmt"

func myPrint(z []int) {
    fmt.Println(z, len(z), cap(z))
    if z == nil {
        fmt.Println("nil!")
    }
}

func main() {
    var z  []int 
    y := []int {}
    myPrint(z)
    myPrint(y)
}

这是我的输出。

[] 0 0
nil!
[] 0 0

我期待第二个零〜为什么没有我得到了它？

I was expecting a second "nil"~ Why didn't I get it?

推荐答案

您引用的doc指出 nil slice的长度和容量均为0 ，但不是每个长度和零容量片都是零片。

The doc you referenced to states that A nil slice has a length and capacity of 0 but not that every slice of length and capacity of zero is a nil slice.

这支持 len 和 cap 指向未指定（nil）的指针（片）。否则，我们需要首先检查非零。

This is a convenience to support len and cap on pointers (of slices) which are unassigned (nil). Otherwise we would need to check for non-nil first.

指针是理解我认为的问题的关键词。在第一种情况下，你有一个零指针，在第二种情况下你已经分配了slice，但它是空的。这可以适用于任何结构：

Pointer is key word here to understand the problem I think. In the first case you have a nil pointer, in the second case you have allocated slice, but it's empty. This would hold for any structure:

package main

import "fmt"

func main() {
    var s *struct{}
    fmt.Println(s) // <nil>
    s = &struct{}{}
    fmt.Println(s) // &{}
}

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