Making a struct before the unmarshal in golang - json

Question

Im new in golang and i have a question.
I have 5 structs where i use Json , but the JSON file can have more structs than the ones i have predetermined BUT... the structures of the JSON satisfies the structures in my programing ( lets say i have 5 structs "struct1" , 6 structs "struct 2" , 1 struct "struct 3", and so on...)
My question is , i want to make a function were i take the JSON FILE , read the structs of it and have as an output the number of structs of the JSON file.
I think i could use the map[string]interface{} but i dont understand it.
I hope i have explain myself
Thank u very much!

Answer 1

Without example JSON or structs, the exact question you are asking is a bit hard to decipher, specifically the "output the number of structs" bit in the question, as it could be interpreted several different ways. I will do my best to answer what I think are the most probable questions you are asking.
Interfaces
First off, some basic go knowledge that might be useful, but outside JSON marshaling itself. The interface{} type appears special, but is not a hardwired keyword as it first might appear. What the interface keyword does is describe the requirements that an object must have to fulfill that interface. Because interface{} has no requirements, and because everything is automatically interfaced in go, everything satisfies the interface{} type.
Because of this implementation of interfaces, map[string]interface{} is really map[string]. This allows for the JSON un/marshal to not care about what is on the value side of the map. This exactly lines up with the format of JSON, where you have a string key on one side, and a value that could be any of the JSON datatypes on the other.
How many different objects are in the base JSON object?
let us take an example json
{
"debug": "on",
"window": {
"title": "Sample Konfabulator Widget",
"name": "main_window",
"width": 500,
"height": 500
},
"image": {
"src": "Images/Sun.png",
"name": "sun1",
"hOffset": 250,
"vOffset": 250,
"alignment": "center"
},
"text": {
"data": "Click Here",
"size": 36,
"style": "bold",
"name": "text1",
"hOffset": 250,
"vOffset": 100,
"alignment": "center",
"onMouseUp": "sun1.opacity = (sun1.opacity / 100) * 90;"
}
}
The answer to the question in this circumstance would be four. debug, window, image, and text.
The process for determining the number would then be:
Load the json into a byte array.
Marshal into an interface{}
Determine type (array vs object etc.) using type switch. see this A Tour of Go page
If you know the type, you can skip this step
Convert to desired type
Get length, or perform any other operation as desired.
package main
import (
"encoding/json"
"fmt"
)
func main() {
myJSON := `<see above>`
var outStruct *interface{}
json.Unmarshal([]byte(myJSON), &outStruct)
outMap := (*outStruct).(map[string]interface{})
fmt.Printf("Num Structs: %d", len(outMap))
}
Go Playground
How many json objects that I do not have structs for are present?
This answer has a very similar answer to the first one, and is really about manipulation of the output map and the struct
Taking almost the entire code from the first one to the second, let us assume that you have the following structs set up
type Image struct {
Name string
//etc
}
type Text struct {
Name string
//etc
}
type Window struct {
Name string
//etc
}
type Base struct {
Image Image
Window Window
Text Text
}
In this case, in addition to the previous steps, you would have to
5. Marshal the json into a base object
6. Go through the map[string]interface{}, and for each key
7. Determine if the key is one of the objects in your base struct
total := 0
for k, _ := range outMap {
if k != "image" && k != "text" && k != "window" && k != "other" {
total++
}
}
fmt.Printf("Total unknown structs: %d\n", total)
How many of my structs are empty?
This last question is also rather simple, and could be done by checking the map for a value given the input key, but for completion's sake, the example code marshals the JSON into a struct, and uses that.
Marshal JSON into base
For each of Window, Item, Text, Other in base, determine if empty.
total = 0
if (Image{}) == outBase.Image {
total++
}
if (Window{}) == outBase.Window {
total++
}
if (Text{}) == outBase.Text {
total++
}
if (Other{}) == outBase.Other {
total++
}
fmt.Printf("Total empty structs: %d\n", total)
Go Playground
See this go blog post for more information on golang JSON.