访问地图使用其引用

huangapple go评论78阅读模式
英文:

Accessing a map using its reference

问题

我尝试循环遍历一个作为指针传递给函数的映射,但我找不到访问元素的方法。以下是代码:

func refreshSession(sessions *map[string]Session) {
    now := time.Now()
    for sid := range *sessions {
        if now.After((*sessions)[sid].timestamp.Add(sessionRefresh)) {
            delete(*sessions, sid)
        }
    }
}

在这个例子中,第4行返回以下编译错误:

./controller.go:120: invalid operation: sessions[sid] (type *map[string]Session does not support indexing)

我尝试使用方括号,但没有效果。如果我去掉所有的引用操作符(* &),那么它可以编译通过。

我应该如何编写这段代码?

英文:

I try to loop through a map, that I pass as a pointer to a function, but I can't find a way to access the elements. This is the code:

func refreshSession(sessions *map[string]Session) {
    now := time.Now()
    for sid := range *sessions {
        if now.After(*sessions[sid].timestamp.Add(sessionRefresh)) {
            delete( *sessions, sid )
        }
    }
}

Line 4 in this example return following compile error:

./controller.go:120: invalid operation: sessions[sid] (type *map[string]Session does not support indexing)

I tried brackets, but it had no effect. If I take away all reference operators (* &) then it compiles fine.

How must I write this?

答案1

得分: 53

你不需要在map中使用指针。

map类型是引用类型,类似于指针或切片

如果你需要更改Session,你可以使用指针:

map[string]*Session

英文:

You don't need to use a pointer with a map.

> Map types are reference types, like pointers or slices

If you needed to change the Session you could use a pointer:

map[string]*Session

答案2

得分: 18

首先取消引用地图,然后访问它(在play上的示例):

(*sessions)[sid]

值得注意的是,地图实际上是引用类型,因此使用指针的用例非常有限。只是将地图值传递给函数不会复制内容。在play上的示例

英文:

De-reference the map first and then access it (Example on play):

(*sessions)[sid]

It's also noteworthy that maps are actually reference types and therefore there is a very limited use-case of using pointers. Just passing a map value to a function will not copy the content. Example on play.

答案3

得分: 11

你没有考虑到 * 的优先级。

*session[sid] 实际上表示 *(session[sid]),也就是先对指向映射的指针进行索引(因此出现错误),然后再对其进行解引用。

你应该使用 (*session)[sid].timestamp,先对指向映射的指针进行解引用,然后再使用键访问它。

英文:

You are not taking into account the precedence of *.

*session[sid] really means *(session[sid]), that is, first indexing the pointer to map (hence the error), then dereferencing it.

You should use (*session)[sid].timestamp to first dereference the pointer to the map and then access it using the key.

huangapple
  • 本文由 发表于 2015年2月8日 00:11:55
  • 转载请务必保留本文链接:https://go.coder-hub.com/28384343.html
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