英文:
private/public fields in structs.. acting differently
问题
为什么我可以这样做
package main
import "fmt"
func main() {
c := Circle{x: 0, y: 0, r: 5}
fmt.Println(c.r)
}
type Circle struct {
x float64
y float64
r float64
}
http://play.golang.org/p/0ypcekVDV9
但是如果我尝试在一个包中访问一个具有小写字段的结构体，会返回编译器错误。
英文:
Why can I do this
package main
import "fmt"
func main() {
c := Circle{x: 0, y: 0, r: 5}
fmt.Println(c.r)
}
type Circle struct {
x float64
y float64
r float64
}
http://play.golang.org/p/0ypcekVDV9
When I can't do the same with a struct in a package?
If I would try to access a struct with a field with lowercase a compiler error is returned..
答案1
得分: 4
如已经说明的那样，需要将字段导出以便从另一个包中访问。请参阅规范。
导出的标识符
为了允许从另一个包中访问，可以将标识符导出。如果满足以下两个条件，则标识符被导出：
- 标识符名称的第一个字符是一个 Unicode 大写字母（Unicode 类别 "Lu"）；
- 标识符在包块中声明，或者是字段名或方法名。其他所有标识符都不会被导出。
如果你想保持字段的私有性，需要使用访问器（set/get）方法，你可以在这里了解更多信息。
Getter 方法
Go 语言不提供自动支持 getter 和 setter 方法。自己提供 getter 和 setter 方法没有问题，而且通常是合适的，但在 getter 的名称中加入 Get 既不符合惯例，也不是必需的。如果你有一个名为 owner 的字段（小写，未导出），getter 方法应该被命名为 Owner（大写，导出），而不是 GetOwner。使用大写名称导出字段提供了区分字段和方法的方法。如果需要的话，setter 函数可能被命名为 SetOwner。这两个名称在实践中都很好读：
owner := obj.Owner() if owner != user { obj.SetOwner(user) }
英文:
As already stated, the fields need to be exported to be accessible from another package. See the spec
> Exported identifiers
>
> An identifier may be exported to permit access to it from another
> package. An identifier is exported if both:
>
> - the first character of the identifier's name is a Unicode upper case
> letter (Unicode class "Lu");
> - and the identifier is declared in the
> package block or it is a field name or method name. All other
> identifiers are not exported.
If you want to keep the fields private, you need to use accessor (set/get) methods which you can read about here
> Getters
>
> Go doesn't provide automatic support for getters and setters. There's
> nothing wrong with providing getters and setters yourself, and it's
> often appropriate to do so, but it's neither idiomatic nor necessary
> to put Get into the getter's name. If you have a field called owner
> (lower case, unexported), the getter method should be called Owner
> (upper case, exported), not GetOwner. The use of upper-case names for
> export provides the hook to discriminate the field from the method. A
> setter function, if needed, will likely be called SetOwner. Both names
> read well in practice:
>
> owner := obj.Owner()
> if owner != user {
> obj.SetOwner(user)
> }
答案2
得分: 2
如果结构体位于与main函数不同的包中，那么你无法从该main函数中访问结构体的私有字段。
这就是“private”的含义。
英文:
If the struct is in a different package than the main function, then you cannot access the struct's private fields from that main function.
That is what "private" means.
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