Create slice of pointers using reflection in Go

Question

I've seen a few examples of reflection around this topic, but I can't find anything that solves this issue. It feels a little convoluted, but the alternative is a massive amount of repetition so I thought I'd give it a try.

I have a function that returns a function (handler). The wrapping function passes in an instance of a struct. I need the inner function to create a slice of pointers to that struct type:

func createCollectionHandler(app *appSession, record interface{}, name string) func(res http.ResponseWriter, req *http.Request) {
    return func(res http.ResponseWriter, req *http.Request) {
        res.Header().Set("Content-Type", "application/json")

        // This line needs to be dynamic:
        var result []*Person

        err := meddler.QueryAll(app.MysqlDB, &result, "select * from "+name)
        if err != nil {
            log.Fatal(err)
        }
        json, err := json.MarshalIndent(result, "", " ")
        if err != nil {
            log.Println(err)
        }
        res.Write([]byte(json))
        return
    }
}

That should work fine, maybe meddler is doing something wrong? — OneOfOne
– OneOfOne, Commented Oct 27, 2015 at 16:35

Mr_Pink · Accepted Answer · 2015-10-27 17:41:31Z

5

You can create a slice using reflect and an example of the type like so:

var t *MyType

typeOfT := reflect.TypeOf(t)
sliceOfT := reflect.SliceOf(typeOfT)

s := reflect.MakeSlice(sliceOfT, 0, 0).Interface()

In order to pass a pointer to the slice without knowing the type, you can create the pointer first, then set the slice value:

ptr := reflect.New(sliceOfT)
ptr.Elem().Set(reflect.MakeSlice(sliceOfT, 0, 0))
s := ptr.Interface()

http://play.golang.org/p/zGSqe45E60

edited Oct 27, 2015 at 17:41

answered Oct 27, 2015 at 16:41

Mr_Pink

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5 Comments

Mr_Pink Over a year ago

@thwd: I know ;) just showing the type assertion in case the OP wasn't sure how to get the concrete type out of the reflect.MakeSlice.

i0n Over a year ago

Hey there. I tried that already, but thought that perhaps I was missing something because I get the following error: ScanAll called with pointer to non-slice: *interface {} which is being emitted from meddler. I'm not married to using meddler, just want a quick and simple way of querying MySQL. I'd happily remove it.

Mr_Pink Over a year ago

@i0n: you have to assert the type before you take the address, since you can't use a pointer to an interface. I'll add an example about creating a pointer with reflect if that's what you're looking for.

i0n Over a year ago

Hey, thanks for the help! I think what I need to do is create a slice of pointers to a struct type that is passed in through the functions interface. The resulting slice needs to have type slice etc when it is introspected, because I think this is what is causing meddler to blow up. Is this possible?

Mr_Pink Over a year ago

@i0n: The first example makes a []*MyType, the second makes a *[]*MyType. Those are the actual types which will come out of the interface{}. I think the second example is what you're looking for, since you want the pass the pointer to meddler.QueryAll immediately.

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