go - 将 interface{} 转换为 int

Question

我正在尝试从 JSON 中获取一个值并将其转换为 int，但它不起作用，我不知道如何正确执行。

这是错误信息:

...cannot convert val (type interface {}) to type int: need type assertion

还有代码:

    var f interface{}
    err = json.Unmarshal([]byte(jsonStr), &f)
    if err != nil {
        utility.CreateErrorResponse(w, "Error: failed to parse JSON data.")
        return
    }

    m := f.(map[string]interface{})

    val, ok := m["area_id"]
    if !ok {
        utility.CreateErrorResponse(w, "Error: Area ID is missing from submitted data.")
        return
    }

    fmt.Fprintf(w, "Type = %v", val)   // <--- Type = float64
    iAreaId := int(val)                // <--- Error on this line.
    testName := "Area_" + iAreaId      // not reaching here

Answer 1

代替

iAreaId := int(val)

你想要一个 type assertion :

iAreaId := val.(int)
iAreaId, ok := val.(int) // Alt. non panicking version 

你不能convert的原因接口(interface)类型值是引用规范部分中的这些规则:

Conversions are expressions of the form T(x) where T is a type and x is an expression that can be converted to type T.

...

A non-constant value x can be converted to type T in any of these cases:

1. x is assignable to T.
2. x's type and T have identical underlying types.
3. x's type and T are unnamed pointer types and their pointer base types have identical underlying types.
4. x's type and T are both integer or floating point types.
5. x's type and T are both complex types.
6. x is an integer or a slice of bytes or runes and T is a string type.
7. x is a string and T is a slice of bytes or runes.

但是

iAreaId := int(val)

不是任何一种情况 1.-7.