切片结构vs指向结构的指针 [英] Slice of structs vs a slice of pointers to structs

问题描述

我不理解以下代码的行为.在创建匹配结构的列表作为结构指针的一部分时，代码始终会打印原始数组的最后一个元素(实际上不是匹配项)，它会打印12和12.但是，如果我将匹配更改为[]小部件，而不是[] * Widget，它将显示10和11.

I don't understand the behavior of the following piece of code. In creating a list of matching structs as a slice of struct pointers, the code always prints the last element of original array (which actually wasn't a match)—it prints 12 and 12. However, if I change matches to be []Widget instead of []*Widget, then it will print 10 and 11.

这是为什么?

package main

import (
    "fmt"
)

func main() {

    type Widget struct {
        id    int
        attrs []string
    }

    widgets := []Widget{
        Widget{
            id:    10,
            attrs: []string{"blah", "foo"},
        },
        Widget{
            id:    11,
            attrs: []string{"foo", "bar"},
        },
        Widget{
            id:    12,
            attrs: []string{"xyz"},
        },
    }

    var matches []*Widget
    for _, w := range widgets {
        for _, a := range w.attrs {
            if a == "foo" {
                matches = append(matches, &w)
                break
            }
        }
    }

    for _, m := range matches {
        fmt.Println(m.id)
    }

}

推荐答案

这是因为当您使用指针时，会将& w 添加到数组中.

That's because when you use the pointers you are adding &w to the array.

请注意， w 实际上是循环中使用的局部变量，因此，这不是您要添加到 matches 数组中的地址.

Note that w is actually the local variable used in the loop, so that's not the address you want to add to the matches array.

(即使变量 w 的 value 在循环中发生变化，其地址也保持不变)

(even though the value of the variable w changes through the loop, its address stays the same)

当循环结束时， w 以最后一个值结束，这就是为什么它两次打印 12 的原因.

When the loop ends, w ends up with the last value so that's why it prints 12 two times.

您需要添加匹配的元素的地址.

You need to add the address of the element that matched instead.

如果您这样做:

matches = append(matches, &widgets[i])

然后它也可以很好地与指针配合使用.

Then it'd work fine with pointers as well.

改良的Go游乐场供您测试:

Modified Go playground for you to test it:

https://play.golang.org/p/YE-cokyEHu

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