英文:
What is the meaning of "dot parenthesis" syntax?
问题
这段代码是一个Go应用程序，它将数据存储在mongodb中。在这行代码（https://github.com/zeebo/gostbook/blob/master/context.go#L36）中，它似乎访问了存储在gorilla session中的用户ID：
if uid, ok := sess.Values["user"].(bson.ObjectId); ok {
...
}
请问有人可以解释一下这里的语法吗？我知道sess.Values["user"]从session中获取一个值，但是后面的部分是什么意思？为什么点号后面的表达式要用括号括起来？这是一个函数调用吗？
英文:
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 a user ID stored in a gorilla session:
if uid, ok := sess.Values["user"].(bson.ObjectId); ok {
...
}
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?
答案1
得分: 145
sess.Values["user"] 是一个 interface{}，括号中的内容被称为 类型断言。它检查 sess.Values["user"] 的值是否为 bson.ObjectId 类型。如果是，ok 将为 true。否则，将为 false。
例如：
var i interface{}
i = int(42)
a, ok := i.(int)
// a == 42，ok == true
b, ok := i.(string)
// b == ""（默认值），ok == false
英文:
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.
For instance:
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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