英文:
Scope visibility of struct members while passing as a argument?
问题
在调用json.Decoder.Decode时，通过传递一个结构体，例如：
type _Sample struct {
first string // 这个字段不会被填充，因为它以小写字母开头
Second string // 这个字段是可以的
}
...
var sample _Sample
err := decoder.Decode(&sample)
根据语言规范的说明：
导出的标识符：
为了允许从另一个包中访问标识符，可以将其导出。如果满足以下两个条件，则标识符被导出：
- 标识符的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，或者它是字段名或方法名。
所有其他标识符都不会被导出。
问题是_Sample结构体并没有被导出，为什么它可以被json包访问到呢？
英文:
While calling json.Decoder.Decode by passing a struct, for instance
type _Sample struct {
first string // this will not be filled because it starts with lower case letter
Second string // it is OK.
}
...
var sample _Sample
err := decoder.Decode(&sample)
According to the Language Specification that writes:
> 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.
The question is the struct _Sample is not exported, why it is visible by the package json?
答案1
得分: 3
访问权限 在以下情况下进行检查：
- 当你创建/声明一个变量时，编译器会检查类型标识符的名称的可见性。
- 使用
reflect包来修改一个变量。 - 调用一个常量、全局变量或函数。将检查常量/变量/函数名称的可见性。
一旦变量被创建/定义，并且当你将这个变量作为函数/方法的参数传递时，副本（如果是值类型，则复制值；如果是指针类型，则复制地址）将被传递给函数/方法，并且始终可以通过参数名称和类型从函数体中访问。在 encoding/json 包中，Decoder.Decode 方法被定义为：
func (dec *Decoder) Decode(v interface{}) error
因此，你传递给 Decoder.Decode 方法的任何内容都可以通过参数 v 在方法体内部访问。请注意，v 的类型是 interface{} 而不是 _Sample 结构体。可见性在你定义变量时进行检查，例如：
var sample _Sample
这是可以的，因为它是在同一个包内部完成的。甚至匿名结构体（即结构体没有定义类型名称标识符）也可以正常工作，例如：
aSample := struct {
first string // 这个字段不会被填充，因为它以小写字母开头
Second string // 这是可以的
}{}
//...
err := decoder.Decode(&aSample)
接下来，当 decoder 填充结构体（通过反射），将会检查结构体成员的可见性，而 _Sample.first 在 json 包内部是不可见的。
英文:
Access permission is checked when
- You're creating/declaring a variable. The compiler will check the visibility of the name of the type identifier.
- Using
reflectpackage to modify a variable. - Calling a constant, global variable, or a function. The visibility of constant/variable/function name will be checked.
Once the variable is created/defined, and when you pass this variable as a function/method argument, the copy (If it is a value, the value is being copied. If it is a pointer, the address is being copied) will be passed to the function/method, and always accessible from the function body through the argument name and type. In encoding/json package, the Decoder.Decode method is defined as,
func (dec *Decoder) Decode(v interface{}) error
thus, anything you passed to the Decoder.Decode method, is always accessible from inside the method body through argument v. Note that the type of v is interface{} not _Sample struct. The visibility is checked when you defined the variable as
var sample _Sample
which was OK since it is done from within the same package. Even an anonymous struct (i.e. no type name identifier is defined for the struct)
aSample := struct {
first string // this will not be filled because it starts with lower case letter
Second string // it is OK.
}{}
//...
err := decoder.Decode(&aSample)
will works. Next, when the decoder fills the struct (through reflection), the visibility of struct members will be checked, and _Sample.first is not visible from inside the json package.
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