nil在golang中是什么意思?

很多情况下在golang中使用 nil .例如:

There are many cases using nil in golang. For example:

func (u *URL) Parse(ref string) (*URL, error) {
    refurl, err := Parse(ref)
    if err != nil {
        return nil, err
    }
    return u.ResolveReference(refurl), nil
}

但是我们不能像这样使用它:

but we can't use it like this:

var str string //or var str int
str = nil

golang编译器将抛出can't use nil as type string in assignment错误.

the golang compiler will throw a can't use nil as type string in assignment error.

类似nil的外观只能用于struct和interface的指针.如果是这样，那么这是什么意思? 和当我们使用它与另一个对象进行比较时，它们如何进行比较，换句话说，golang如何确定一个对象为零?

Looks like nil can only be used for a pointer of struct and interface. If that is the case, then what does it mean? and when we use it to compare to the other object, how do they compare, in other words, how does golang determine one object is nil?

例如，如果接口为nil，则其类型和值必须同时为nil. golang如何做到这一点?