Go: Passing a return value via an interface{} pointer

Question

I have a function that looks like this:

func Foo(result interface{}) error {
     ...
     json.Unmarshal([]byte(some_string), result)
     ...
}

Which is called like this:

var bar Bar
Foo(&bar)

Generally, Foo fetches a string that is then unmarshalled into the result. However, now I need to update it so that Foo sometimes loads data from another source and returns that instead.

type Loader func() (interface{})

func Foo(result interface{}, Loader load) error {
     ...
     data := load()
     // result = data ???
     ...
}

Is there any way for me to assign this new value to result? I found that I can marshal the data into a string and then unmarshal it into result which works, but I can't imagine that's the best way.

Aruna Herath · Accepted Answer · 2016-04-07 03:28:20Z

5

You can do,

p := result.(*Bar)
*p = data

The first line is a type assertion.

The second assigns data to the dereferenced pointer. Assigning a value to a dereferenced pointer changes the value at the referenced address.

Since you don't know the underlying types of result or data the best you can do is using a type assertion switch.

switch v := data.(type) {
case Bar:
    // If 'data' is of type 'Bar'
    p, ok := result.(*Bar)
    if(!ok){
        // This means inputs were bad.
        // 'result' and 'data' should both have the same underlying type.
        fmt.Println("underlying types mismatch")
        break;
    }

    *p = v

case Baz:
    // If 'data' is of type 'Baz'
    // Equivalent of above for 'Baz'
}

See Type Switches from switch statement

edited Apr 7, 2016 at 3:28

answered Apr 6, 2016 at 10:24

Aruna Herath

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3 Comments

Bill Over a year ago

Only problem is that I don't know the concrete type of result inside of Foo. That's why I'm using interface{}. Do I need to use reflect to figure that out?

Aruna Herath Over a year ago

Bill, sorry I overlooked it. Updated the answer with the best solution I could come up with. I hope you have thought through using this API. Maybe you could change it to just return data instead of assigning it to a pointer?

Bill Over a year ago

I need to pass in a referenced struct for the deserialization case anyways and no matter how I refactor I'm always going to eventually get down to a point where I'm either loading a struct or deserializing a string. It ends up being something like foo(result interface{}, load Loader) (interface{}, error) which is fine but not as clean as I was hoping for.

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