在Golang中执行JSON Unmarshal时如何设置默认值到映射值？

Question

I have a struct like this:

package main

import (
    "encoding/json"
    "fmt"
)

type request struct {
    Version    string               `json:"version"`
    Operations map[string]operation `json:"operations"`
}
type operation struct {
    Type   string `json:"type"`
    Width  int    `json:"width"`
    Height int    `json:"height"`
}

func main() {
    jsonStr := "{\"version\": \"1.0\", \"operations\": {\"0\": {\"type\": \"type1\", \"width\": 100}, \"1\": {\"type\": \"type2\", \"height\": 200}}}"
    req := request{
         Version: "1.0",
    }
    err := json.Unmarshal([]byte(jsonStr), &req)
    if err != nil {
        fmt.Println(err.Error())
    } else {
        fmt.Println(req)
    }
}

I can set Version = "1.0" as its default value, but how can I set default value to Width and Height?