why can't I use reflection to take the address

Question

How come this works:

slice := make([]string, 0, 10)
sliceptr := &slice

this too:

sliceptr := &[]string{"foo","bar","baz"}

But this doesn't:

sliceaddrval := reflect.ValueOf([]string{"foo","bar","baz"}).Addr()

It panics with: reflect.Value.Addr of unaddressable value

---

EDIT: Overall what I'm trying to do is take a struct that is of an unknown type, make a slice of structs of that type and return a pointer to it (I'm using github.com/jmoiron/modl which requires a pointer to slice to populate with results from a SQL query).

Answer 1

reflect.Value takes an interface{}, and an interface{} to a value can't be used to change the original. Otherwise, you could end up with code changing data in your struct when you didn't even intend to pass it a pointer. (Or, in this case, changing the length of a slice that was passed by value.) So if you take the address you'd have to do it before the ValueOf.

To make a pointer to a slice that you can to pass to a package that will append to it (like modl or Google App Engine GetMulti), you'd use something like http://play.golang.org/p/1ZXsqjrqa3, copied here:

package main

import (
    "fmt"
    "reflect"
)

type row struct { i, j int }

func main() {
    aRow := row{}

    valueType := reflect.ValueOf(aRow).Type()
    slicePtrVal := reflect.New(reflect.SliceOf(valueType))
    slicePtrIface := slicePtrVal.Interface()

    getQueryResults(slicePtrIface)
    fmt.Println(slicePtrIface)
}

// standing in for `modl` or whatever populates the slice
func getQueryResults(slicePtr interface{}) {
    sPtr := slicePtr.(*[]row)
    (*sPtr) = append((*sPtr), row{1,3}) 
}

Appending to a slice in a reflect.Value yourself takes another few lines of reflect, but it sounds like the package you're working with takes care of that part for you. For general info, code to do the append is at http://play.golang.org/p/m3-xFYc6ON and below:

package main

import (
    "fmt"
    "reflect"
)

type row struct { i, j int }

func main() {
    aRow := row{}

    // make a pointer to an empty slice
    rowType := reflect.ValueOf(aRow).Type()
    slicePtrVal := reflect.New(reflect.SliceOf(rowType))
    slicePtrIface := slicePtrVal.Interface()

    // append a zero row to it
    rowVal := reflect.Zero(rowType)
    sliceVal := reflect.Indirect(slicePtrVal)
    sliceVal.Set(reflect.Append(sliceVal, rowVal))

    fmt.Println(slicePtrIface)
}