将interface {}转换为int

Question

I'm trying to get a value from a JSON and cast it to int but it doesn't work, and I don't know how to do it properly.

Here is the error message:

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

And the code:

    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

Instead of

iAreaId := int(val)

you want a type assertion:

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

The reason why you cannot convert an interface typed value are these rules in the referenced specs parts:

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.

But

iAreaId := int(val)

is not any of the cases 1.-7.