title: how to Unmarshal json in golang
body: <p>I have a json:</p>
  
  <pre><code>{"code":200,
   "msg":"success",
   "data":{"url":"https:\/\/mp.weixin.qq.com\/cgi-bin\/showqrcode?ticket=gQHQ7jwAAAAAAAAAAS5odHRwOi8vd2VpeGluLnFxLmNvbS9xLzAyX3pqS0pMZlA4a1AxbEJkemhvMVoAAgQ5TGNYAwQsAQAA"}}
  </code></pre>
  
  <p>and i define a struct :</p>
  
  <pre><code>type Result struct {
      code int
      msg  string                 `json:"msg"`
      data map[string]interface{} `json:"data"`
  }
  </code></pre>
  
  <p>for this code:</p>
  
  <pre><code>var res Result
  json.Unmarshal(body, &amp;res)
  fmt.Println(res)
  </code></pre>
  
  <p>the output is: <code>{0  map[]}</code></p>
  
  <p>i want to get <code>url</code> in <code>data</code>, how to get it?</p>