问题

所以我有以下两个方法：

func Marshal(in interface{}) (out []byte, err error)
func readDocument(r io.Reader) ([]byte, error)

在我的代码中，我做了以下操作：

queryDoc, err := readDocument(client) // queryDoc 是长度为 408 的切片
if something {
    queryDoc, err := bson.Marshal(something)
    newDocLen := len(queryDoc) // len 现在是 200
}
len(queryDoc) // len 是 408？？？

由于某种原因，queryDoc 在解组后没有得到更新。然而，如果我将其赋值给一个中间变量，它就可以工作：

```go
queryDoc, err := readDocument(client) // queryDoc 是长度为 408 的切片
if something {
    q, err := bson.Marshal(something)
    queryDoc = q
    newDocLen := len(queryDoc) // len 现在是 200
}
len(queryDoc) // len 是 200

由于我在第一个示例中将返回值赋给了 queryDoc，那么 queryDoc 这个变量不应该引用新的数组吗？

<details>
<summary>英文:</summary>

So I have the two following methods:

    func Marshal(in interface{}) (out []byte, err error)
    func readDocument(r io.Reader) ([]byte, error)

In my code I do the following:

    queryDoc, err := readDocument(client) // querydoc is slice of len 408
    if something {
        queryDoc, err := bson.Marshal(something) 
        newDocLen := len(queryDoc) // len is now 200
    }
    len(queryDoc) // len is 408????

For some reason, queryDoc doesn&#39;t get updated with the unmarshalling. If however, I assign to an intermediate value, it works:

    queryDoc, err := readDocument(client) // querydoc is slice of len 408
    if something {
        q, err := bson.Marshal(something)
        queryDoc = q
        newDocLen := len(queryDoc) // len is now 200
    }
    len(queryDoc) // len is 200

Since I&#39;m assigning the return value to queryDoc in the first example, shouldn&#39;t the variable queryDoc now reference the new array?  

</details>


# 答案1
**得分**: 5

在

    queryDoc, err := bson.Marshal(something)

中，你实际上使用了`:=`创建了一个新的`queryDoc`，而不是`=`。编译器没有捕捉到这个错误，因为你也使用了它。将其替换为

    var err error
    queryDoc, err = bson.Marshal(something)

应该可以按预期工作。

<details>
<summary>英文:</summary>

In

    queryDoc, err := bson.Marshal(something)

you actually created a new `queryDoc` with `:=` instead of `=`. The compiler didn&#39;t catch it because you have used it as well. Replace that with

    var err error
    queryDoc, err = bson.Marshal(something)

and it should work as intended.

</details>