I'm trying to practice reflections with Go, I read a few articles but it seems like I'm missing some basic understanding that maybe you guys can clear out.
I wrote a simple app to demonstrate what I'm trying to achieve.
in general I want a function to receive a slice of a pointer to a struct as an interface type, and to fill it with data using reflections.
again. this example seems a bit useless but i minimized what i'm trying to achieve. I know how to find the column names of a struct, but i have no problem there so I removed it from the example.
so this is the code:
package main
import (
"log"
"reflect"
"unsafe"
)
type MyTesting struct {
MyBool bool
MyFloat float64
MyString string
}
func addRow(dst interface{}) {
iValue := reflect.ValueOf(dst)
iType := reflect.TypeOf(dst)
// getting the Struct Type (MyTesting)
structType := iType.Elem().Elem().Elem()
// creating an instance of MyTesting
newStruct := reflect.New(structType)
// getting the current empty slice
slice := iValue.Elem()
// appending the new struct into it
newSlice := reflect.Append(slice,newStruct)
// trying to set the address of the varible to the new struct ? the original var is not a pointer so something here
// is clearly wrong. I get the PANIC here, but if i remove that line, then rows stays nil
reflect.ValueOf(&dst).SetPointer(unsafe.Pointer(newSlice.Pointer()))
currentPlaceForRow := newStruct.Elem()
structField := currentPlaceForRow.FieldByName("MyString")
structField.SetString("testing")
}
func main() {
var rows []*MyTesting
addRow(&rows)
log.Print(rows)
}
so in general i the function gets an un-initialized slice of pointers to MyTesting struct. i want in the function to create the first slice element and to set the value of MyString in the 1st element to "testing".
when I try to execute it I get:
panic: reflect: reflect.Value.SetPointer using unaddressable value
so working with reflections is a bit confusing for me.. can anyone please a shed a light on what I'm missing here ? :)
You can use reflect.ValueOf(dst).Elem().Set(newSlice).
https://play.golang.org/p/ilDZxHc2H-x
Related
I've been struggling with the reflect package. This code below does what I expect:
package main
import (
"reflect"
"log"
)
type Car struct {
Model string
}
type Person struct {
Name string
Cars []Car
}
func ModifyIt(parent interface{},fieldName string, val interface{}) {
slice := reflect.ValueOf(parent).Elem()
nth := slice.Index(0)
//row := nth.Interface() // this line causes errors
row := nth.Interface().(Person)
elem := reflect.ValueOf(&row).Elem()
field := elem.FieldByName(fieldName)
log.Println(field.CanSet())
}
func main() {
p := []Person{Person{Name:"john"}}
c := []Car{Car{"corolla"},Car{"jetta"}}
ModifyIt(&p,"Cars",&c)
}
However, if I replace the line row := nth.Interface().(Person) with row := nth.Interface(), that is I remove the type assertion, then I get the error:
panic: reflect: call of reflect.Value.FieldByName on interface Value
on line "field := elem.FieldByName(fieldName)
I've tried a bunch of other things the last few hours like trying to do reflect.TypeOf(), reflect.Indirect() etc... on some of the other variables but with no success.
I've read some other questions like these:
reflect: call of reflect.Value.FieldByName on ptr Value
Set a struct field with field type of a interface
Golang reflection: Can't set fields of interface wrapping a struct
They seem to suggest that I don't have a good understanding of how pointers or interfaces work.
So my question is, how do I go about setting the field of a struct when the struct is typed as an interface?
UPDATE
I posted a solution as an answer, but I have no confidence in whether it is the proper or safe way of doing things. I hope someone can explain, or post a better solution.
Try this:
func ModifyIt(slice interface{}, fieldName string, newVal interface{}) {
// Create a value for the slice.
v := reflect.ValueOf(slice)
// Get the first element of the slice.
e := v.Index(0)
// Get the field of the slice element that we want to set.
f := e.FieldByName(fieldName)
// Set the value!
f.Set(reflect.ValueOf(newVal))
}
Call it like this:
p := []Person{Person{Name: "john"}}
c := []Car{Car{"corolla"}, Car{"jetta"}}
ModifyIt(p, "Cars", c)
Note that the call passes the slices directly instead of using pointers to slices. The pointers are not needed and add extra complexity.
Run it on the Playground.
Out of sheer luck, I finally got something to work.
I pieced together a bunch of random things I read with very little rhyme or reason. I even tried reading the Laws of Reflection on the Golang site, but I don't think I have a good grasp of how it relates to why I couldn't set variables typed as interface{}. In general, I still don't understand what I did.
My solution below is littered with comments to indicate my confusion, and lack of confidence in whether I did things properly or safely.
package main
import (
"reflect"
"log"
)
type Car struct {
Model string
}
type Person struct {
Name string
Cars []Car
}
func ModifyIt(parent interface{},fieldName string, val interface{}) {
log.Println(parent)
slice := reflect.ValueOf(parent).Elem()
nth := slice.Index(0)
row := nth.Interface()
log.Println(nth.CanSet()) // I can set this nth item
// I think I have a to make a copy, don't fully understand why this is necessary
newitem := reflect.New(reflect.ValueOf(row).Type())
newelem := newitem.Elem()
field := newelem.FieldByName(fieldName)
// I need to copy the values over from the old nth row to this new item
for c:=0; c<nth.NumField(); c++ {
newelem.Field(c).Set(reflect.Indirect(nth.Field(c)))
}
// now I can finally set the field for some reason I don't understand
field.Set(reflect.ValueOf(val).Elem())
// now that newitem has new contents in the field object, I need to overwrite the nth item with new item
// I don't know why I'm doing it, but I'll do it
// I also don't fully understand why I have to use Indirect sometimes, and not other times...it seems interchangeable with ValueOf(something).Elem(), I'm confused....
nth.Set(reflect.Indirect(newitem))
}
func main() {
p := []Person{Person{Name:"john"}}
c := []Car{Car{"corolla"},Car{"jetta"}}
ModifyIt(&p,"Cars",&c)
// now parent is up to date, although I have no idea how I got here.
log.Println(p)
}
If anyone can post a better answer that clears up my confusion, that will be great. I've been having a really hard time learning golang.
I am passing a pointer to a string, to a method which takes an interface (I have multiple versions of the method, with different receivers, so I am trying to work with empty interfaces, so that I don't end up with a ton of boilerplate madness. Essentially, I want to populate the string with the first value in the slice. I am able to see the value get populated inside the function, but then for some reason, in my application which calls it, tha value doesn't change. I suspect this is some kind of pointer arithmetic problem, but could really use some help!
I have the following interface :
type HeadInterface interface{
Head(interface{})
}
And then I have the following functions :
func Head(slice HeadInterface, result interface{}){
slice.Head(result)
}
func (slice StringSlice) Head(result interface{}){
result = reflect.ValueOf(slice[0])
fmt.Println(result)
}
and... here is my call to the function from an application which calls the mehtod...
func main(){
test := x.StringSlice{"Phil", "Jessica", "Andrea"}
// empty result string for population within the function
var result string = ""
// Calling the function (it is a call to 'x.Head' because I lazily just called th import 'x')
x.Head(test, &result)
// I would have thought I would have gotten "Phil" here, but instead, it is still empty, despite the Println in the function, calling it "phil.
fmt.Println(result)
}
*NOTE : I am aware that getting the first element doesn't need to be this complicated, and could be slice[0] as a straight assertion, but this is more of an exercise in reusable code, and also in trying to get a grasp of pointers, so please don't point out that solution - I would get much more use out of a solution to my actual problem here * :)
As you said in your NOTE, I'm pretty sure this doesn't have to be this complicated, but to make it work in your context:
package main
import (
"fmt"
"reflect"
)
type HeadInterface interface {
Head(interface{})
}
func Head(slice HeadInterface, result interface{}) {
slice.Head(result)
}
type StringSlice []string
func (slice StringSlice) Head(result interface{}) {
switch result := result.(type) {
case *string:
*result = reflect.ValueOf(slice[0]).String()
fmt.Println("inside Head:", *result)
default:
panic("can't handle this type!")
}
}
func main() {
test := StringSlice{"Phil", "Jessica", "Andrea"}
// empty result string for population within the function
var result string = ""
// Calling the function (it is a call to 'x.Head' because I lazily just called th import 'x')
Head(test, &result)
// I would have thought I would have gotten "Phil" here, but instead, it is still empty, despite the Println in the function, calling it "phil.
fmt.Println("outside:", result)
}
The hard part about working with interface{} is that it's hard to be specific about a type's behavior given that interface{} is the most un-specific type. To modify a variable that you pass as a pointer to a function, you have to use the asterisk (dereference) (for example *result) on the variable in order to change the value it points to, not the pointer itself. But to use the asterisk, you have to know it's actually a pointer (something interface{} doesn't tell you) so that's why I used the type switch to be sure it's a pointer to a string.
Is it possible to dynamically cast a value generated by reflect.Zero/New back to an arbitrary type?
https://blog.golang.org/laws-of-reflection seems to suggest not (as go is statically typed). That pretty much seems to limit the uses of reflection as far as I can see, as you always need to be aware of the type you are working with.
Here's an example of what I mean:
package main
import (
"fmt"
"reflect"
)
type A struct {
Name string
}
func main() {
a := &A{Name: "Dave"}
fmt.Println(a)
//create a nil pointer of an arbitrary type
dynamicType := reflect.TypeOf(a)
dynamicNil := reflect.Zero(dynamicType).Interface()
a = dynamicNil //is it possible to do this without explicitly casting to A (ie. avoiding `a = dynamicNil.(*A)`)
fmt.Println(a)
}
Your question's prose and code contradict.
In your code, dynamicNil has the type interface{}, not reflect.Value as the prose suggests. As a has concrete type *A you'll have to type-assert dynamicNil to *A. There's no way around that.
Also note that Go doesn't have casts -- only type-conversions and assertions.
Edit: maybe you're looking for reflect.Value.Set? it's unclear to me.
I wrote some odd code, but I'm not sure why it works and what I can learn from it. I have a slice type build from another struct. I made a function on the slice type to modify itself. To do this, I seem to have to throw around *'s a little much.
I'm trying to learn about pointers in Go and would like a little help. Here's an example (http://play.golang.org/p/roU3MEeT3q):
var ClientNames = []string {"Client A", "Client B", "ClientC"}
type InvoiceSummaries []InvoiceSummary
type InvoiceSummary struct {
Client string
Amt int
}
func (summaries *InvoiceSummaries) BuildFromAbove() {
for _, name := range ClientNames {
*summaries = append(*summaries, InvoiceSummary{name, 100})
}
}
My question is: What is the purpose for each of these * and why am I not using any &?
What is the purpose for each of these * ?
By making the method receiver as pointer, you could easily change the property of the object. I think that's one of the benefit. This example below will prove it.
package main
import "fmt"
type someStruct struct {
someVar int
}
func (s someStruct) changeVal1(newVal int) {
s.someVar = newVal
}
func (s *someStruct) changeVal2(newVal int) {
s.someVar = newVal
}
func main() {
s := someStruct{0}
fmt.Println(s) // {0}
s.changeVal1(3)
fmt.Println(s) // {0}
s.changeVal2(4)
fmt.Println(s) // {4}
(&s).changeVal2(5)
fmt.Println(s) // {5}
}
and why am I not using any &?
Pointer method receiver is quite special, it can also be called from non-pointer struct object. Both of s.changeVal2(4) and (&s).changeVal2(5) are valid & will affect the value of someVar.
Example http://play.golang.org/p/sxCnCD2D6d
You have to use a pointer for the receiver - (summaries *InvoiceSummaries) - because otherwise the argument is passed by value, having a pointer means you pass a reference to the value instead. If not for that, then you couldn't modify the collection at all.
Inside of the methods body you have use * because it is the dereferncing operator and returns the value at the address. Ampersand (&) is the opposite, it gives the address of a value.
Nothing wrong with your code but normally addresses to slices aren't used. A slice is a small struct that gophers are normally happy to pass by value. If a method or function is creating a new slice, the gopher is happy to return the new slice, by value again, as the return value.
Of course passing a slice by value doesn't guarantee anything about the backing store remaining unchanged when the method/function returns. So it can't be used as a way of guaranteeing the data elements of the slice haven't mutated.
I am trying to create a function that takes a []byte and an interface{} (standing for the struct) and returns an interface{} as the struct type passed into the func.
Something like this:
package main
import (
"encoding/json"
)
func UnmarshalFromJSONArray(sms []byte,tt string) (interface{}) {
var ts = new(tt)
err := json.Unmarshal(sms,&ts)
if(err != nil) {
fmt.Println(err)
}
return sms
}
So that method would run something like this:
// let's say a struct has the following definition:
type MyStructType struct {
Id int
Name string
Desc string
}
// we can some how get its fully qualified class name (this may require reflection?) or pass it into the UnMarshal method direction some how.
mst := "package.MyStructType",
// and then assume a byte array ba that has JSON format for
ba := []byte(`{"Id":"3","Name":"Jack","Desc":"the man"}`)
stct := UnmarshalFromJSONArray(ba,mst)
MyStructureType out := stct
// leaving "stct" being the unmarshalled byte array which can be used like any other struct of type "MyStructureType"
The key being that I never need to know what the fields of MyStructureType are before unmarshalling. All I need are the name of the struct and some way to instance one and then populate it with JSON byte array data that matches its fields. Hopefully that is possible (it is trivial in java using reflection). So I want to basically unmarshal an anonymous struct type by it's name without needing to know what fields it has.
Any suggestions?
The short answer is that this is impossible. There is no string to type translator in Go. You can make a map of strings to reflect.Type's, but you would need to know the possible options ahead of time or you need to provide the caller with a way to register types (perhaps in init).
Assuming you have found a way to resolve the string to its reflect.Type, you can simply call reflect.New(typ).Interface() to get the pointer you need to pass to json.Unmarshal().
The best answer is to avoid trying this all together. Writing idiomatic Java in Go isn't really possible. If I knew more about your problem, I could give you a more idiomatic Go solution.