Go Base64软件包中的“ StdEncoding”是什么

Question

I'm an experienced programmer, but new to go, so apologies in advance for what I'm sure is an easy question.

The base64 example code on the goland.org site includes the following code.

package main

import (
    "encoding/base64"
    "fmt"
)

func main() {
    msg := "Hello, 世界"
    encoded := base64.StdEncoding.EncodeToString([]byte(msg))
    fmt.Println(encoded)
    decoded, err := base64.StdEncoding.DecodeString(encoded)
    if err != nil {
        fmt.Println("decode error:", err)
        return
    }
    fmt.Println(string(decoded))
}

I can copy and use this code without issue, but as a new go programmer, I don't understand what that StdEncoding part of the following lines does

 decoded, err := base64.StdEncoding.DecodeString(encoded)

I understand the above program imports the encode/base64 functions into my program. However, if I look at the documentation, the base64 package consists of the following types and functions

func NewDecoder(enc *Encoding, r io.Reader) io.Reader
func NewEncoder(enc *Encoding, w io.Writer) io.WriteCloser
type CorruptInputError
    func (e CorruptInputError) Error() string
type Encoding
    func NewEncoding(encoder string) *Encoding
    func (enc *Encoding) Decode(dst, src []byte) (n int, err error)
    func (enc *Encoding) DecodeString(s string) ([]byte, error)
    func (enc *Encoding) DecodedLen(n int) int
    func (enc *Encoding) Encode(dst, src []byte)
    func (enc *Encoding) EncodeToString(src []byte) string
    func (enc *Encoding) EncodedLen(n int) int
    func (enc Encoding) WithPadding(padding rune) *Encoding

I would expect to see a type of StdEncoding -- but I don't see any mention of a StdEncoding in this list.

So, my immediate question is -- what is StdEncoding?

My higher level question is probably -- what, exactly, is exported from a module for consumption by a user of that module, and how can I, as a go user, browse those exported things.

Bonus points if you can correct any unwitting misuse of terms like module, export, import above and put them into a go context.

Answer 1

To try and answer all of your questions at once... You can find the definition of StdEncoding in the file base64.go it's contained here;

65  // StdEncoding is the standard base64 encoding, as defined in
66  // RFC 4648.
67  var StdEncoding = NewEncoding(encodeStd)

and up on line 33 that const is defined as such;

33  const encodeStd = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"

So to answer your main question, it is just base64 encoding as defined in RFC 4648. The type is *Encoding and you can even view the implementation of NewEncoding in the same location if you'd like. https://golang.org/src/encoding/base64/base64.go

To address some of the auxiliary questions about packages/modules and exporting... In the go docs you'll see everything that is exported and nothing that is unexported (if you look at the actual source you'll usually find a good deal of unexported helper methods). If you were looking at the source you can see this distinction in the names of methods/types/consts ect. The basic rule is, if the identifier starts with a capital letter then it is exported, if it's lower cased then it is not exported.

One other thing worth pointing out is that is that the language is open source. In the docs it will typically list the packages main file (possibly others as well) so you can open it up and have a look. You'll find the code is well commented and usually pretty easy to understand. Often times you'll find answers to questions about how things work internally just by reading the comments in these files.