“圆括号"的含义是什么?句法? [英] What is the meaning of "dot parenthesis" syntax?

问题描述

我正在研究一个示例Go应用程序，该应用程序将数据存储在mongodb中.此行的代码( https://github.com/zeebo/gostbook/blob/master/context.go#L36 )似乎正在访问存储在大猩猩会话中的用户ID:

I am studying a sample Go application that stores data in mongodb. The code at this line (https://github.com/zeebo/gostbook/blob/master/context.go#L36) seems to access an user ID stored in a gorilla session:

if uid, ok := sess.Values["user"].(bson.ObjectId); ok {
  ...
}

有人可以在这里给我解释一下语法吗?我知道 sess.Values ["user"] 从会话中获取值，但是接下来是什么呢?为什么括号中的点后面是表达式?这是函数调用吗?

Would someone please explain to me the syntax here? I understand that sess.Values["user"] gets a value from the session, but what is the part that follows? Why is the expression after the dot in parentheses? Is this a function invocation?

推荐答案

sess.Values ["user"] 是 interface {} ，括号之间是什么被称为 类型断言 .它检查 sess.Values ["user"] 的值是否为 bson.ObjectId 类型.如果是，则 ok 将为 true .否则，它将为 false .

sess.Values["user"] is an interface{}, and what is between parenthesis is called a type assertion. It checks that the value of sess.Values["user"] is of type bson.ObjectId. If it is, then ok will be true. Otherwise, it will be false.

例如:

var i interface{}
i = int(42)

a, ok := i.(int)
// a == 42 and ok == true

b, ok := i.(string)
// b == "" (default value) and ok == false

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