Double curly braces in structures when used in Decode(&struct{}{})

In PHP, we can use Decode(&struct{}{}) to decode a structure. However, there is a special case when we use a structure in the Decode function, we need to use double curly braces in the structure. This usage can be confusing, so I'll explain it in detail here. When using a structure in the Decode function, the double curly braces are used to indicate the initialization of the structure. In this way, we can define and initialize a structure in one statement, making the code more concise and easier to understand. Therefore, remember to pay attention to the use of double braces when using structures in the Decode function!

Question content

I have this function in some code. What are the double braces in the struct that help make sure it's not two JSONs? How does it work?

func readJSON(w http.ResponseWriter,r *http.Request,data interface{}) error {
    maxBytes := 1024 * 1024 
    r.Body = http.MaxBytesReader(w,r.Body,int64(maxBytes))
    dec := json.NewDecoder(r.Body)
    dec.DisallowUnknownFields()
    err := dec.Decode(data)

    if err != nil {
        return err
    }

    err = dec.Decode(&struct{}{})
    if err != io.EOF {
        return errors.New("Body must Only contain 1 json ")
    }
    
    return nil
}

Solution

Let’s break it down:

• struct{} is a type: a structure without fields.
• struct{}{} is a literal value: a new instance of the above type.
• &struct{}{} is a pointer to the literal value above.

By trying to decode the JSON a second time, it confirms that the body does not have a second JSON document after the first one, for example:

{
  "foo": "bar"
}
{
  "foo": "qux"
}

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