此行“ binTag：= field.Tag.Get（“ binary”）”之后分配给“ bintag”的值是什么，其中field是GOLang中的struct字段之一

Question

I was trying to analyse the GO program, when I encountered this line
"binTag := field.Tag.Get("binary")"
I was confused with value that "binTag" will be assigned.

I searched in the GO reflect Package for the syntax explanation and I found this,

func (tag StructTag) Get(key string) string

Get returns the value associated with key in the tag string. If there is no such key in the tag, Get returns the empty string. If the tag does not have the conventional format, the value returned by Get is unspecified. To determine whether a tag is explicitly set to the empty string, use Lookup.

Then I searched what is Tag mean in Golang, as an example I got this

Tag

A field declaration may be followed by an optional string literal (tag) which becomes an attribute of all the fields in the corresponding field declaration.

type T struct {
    f1     string "f one"
    f2     string
    f3     string `f three`
    f4, f5 int64  `f four and five`
}

So, now I am bit confused on exact value assigned to "binTag" after execution.

Thanks in Advance.

Answer 1

See the StructTag documentation for a description of the tag format.

See the StructTag example and the StructTag.Lookup examples examples of tags.

The value of binTag is "" because the tag does not contain a value for the key "binary", nor does the tag following the convention for formatting struct tags.

This code shows how to access the tag:

v := reflect.TypeOf(T{})
sf, _ := v.FieldByName("f1")
fmt.Println(sf.Tag)               // prints "f one"
fmt.Println(sf.Tag.Get("binary")) // prints blank line

Here's an example with a valid tag with key "binary":

type U struct {
    g1 string `binary:"hello"`
}

v := reflect.TypeOf(U{})
sf, _ := v.FieldByName("g1")
fmt.Println(sf.Tag)               // prints binary:"hello"
fmt.Println(sf.Tag.Get("binary")) // prints hello