使用`encoding / json`的默认值golang结构？

Question

How to set default value for Encoding as "base64"?

type FileData struct {
    UID string `json:"uid"`                 
    Size int `json:"size"`
    Content string `json:content`
    Encoding string `json:encoding` 
    User string `json:"user"`
}

I tried

Encoding string `json:encoding`= "base64" // Not working

Answer 1

You can't because in Go types do not have constructors.

Instead, have either an explicit initializer function (or method on the pointer receiver) or a constructor/factory function (these are conventionally called New<TypeName> so yours would be NewFileData) which would return an initialized value of your type.

All-in-all, I have a feeling this looks like an XY problem. From your question, it appears you want to have a default value on one of your fields if nothing was unmarshaled. If so, just post-process the values of this type unmarshaled from JSON and if nothing was unmarshaled to Encodning set it to whatever default you want.

Alternatively you might consider this approach:

1. Have a custom type for your field.

   Something like type EncodingMethod string should do.

2. Have a custom JSON unmarshaling method for this type which would do whatever handling it wants.