Create slice of unknown type?

Question

I'm trying to fetch multiple values using this function (from go-couchbase). ``` func (b *Bucket) Gets(k string, rv interface{}, caso *uint64) error { data, _, cas, err := b.GetsRaw(k) if err != nil { return err } if caso != nil { *caso = cas } return json.Unmarshal(data, rv) } ``` The problem is that I don't know how to create a slice of rv, since I don't know its type in my function, and I have no idea how big it will be so I can't really access the `rv` slice through indexes, right? I want to be as general as possible, so creating a function per `rv` struct isn't really optimal and the GetBulk function only returns a `[]byte`. Is it even possible to do what I want in go?

Accepted Answer

You can inspect the type information using the `reflect` package. If we have a function that has an argument `rv interface{}` where we expect a pointer to a slice to be passed in, we can create a `reflect.Value` that refers to that slice:

```
slice := reflect.ValueOf(rv).Elem()
```

We can reinitialise the slice as follows:

```
slice.Set(reflect.MakeSlice(slice.Type(), length, capacity))
```

We can determine the element type of this slice with:

```
elemType := slice.Type().Elem()
```

And then create another `reflect.Value` that represents a pointer to a newly allocated instance of that type:

```
v := reflect.New(elemType)
```

Now `v.Interface()` would be appropriate to pass to a function like `json.Unmarshal`. Once it has been filled, it can be appended to the slice:

```
slice.Set(reflect.Append(slice, v.Elem()))
```

For a full example that decodes a slice of JSON documents into an arbitrary slice type, see http://play.golang.org/p/G-SHQO2MAT

Create slice of unknown type?

Solution

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