How can I tell if my interface{} is a pointer? - go

If I have an interface being passed into a function, is there a way to tell if the item passed in is a struct or a pointer to a struct? I wrote this silly test to illustrate what I need to figure out.
type MyStruct struct {
Value string
}
func TestInterfaceIsOrIsntPointer(t *testing.T) {
var myStruct interface{} = MyStruct{Value: "hello1"}
var myPointerToStruct interface{} = &MyStruct{Value: "hello1"}
// the IsPointer method doesn't exist on interface, but I need something like this
if myStruct.IsPointer() || !myPointerToStruct.IsPointer() {
t.Fatal("expected myStruct to not be pointer and myPointerToStruct to be a pointer")
}
}

func isStruct(i interface{}) bool {
return reflect.ValueOf(i).Type().Kind() == reflect.Struct
}
You can test via changing type according to your needs such as reflect.Ptr. You can even get pointed value with reflect.Indirect(reflect.ValueOf(i)) after you ensured it's a pointer.
Addition:
It seems reflect.Value has a Kind method so reflect.ValueOf(i).Kind() is enough.

You could use the reflect package:
i := 42
j := &i
kindOfJ := reflect.ValueOf(j).Kind()
fmt.Print(kindOfJ == reflect.Ptr)

If you know the "real" type of the interface, you can simply use a type switch:
type MyStruct struct {
Value string
}
func TestInterfaceIsOrIsntPointer(t *testing.T) {
var myStruct interface{} = MyStruct{Value: "hello1"}
var myPointerToStruct interface{} = &MyStruct{Value: "hello1"}
// the IsPointer method doesn't exist on interface, but I need something like this
switch myStruct.(type) {
case MyStruct:
// ok
break
case *MyStruct:
// error here
break
}
switch myPointerToStruct.(type) {
case MyStruct:
// error here
break
case *MyStruct:
// ok
break
}
}
The code is longer but at least you don't need to use the reflect package.

Related

How to use "reflect" to set interface value inside a struct of struct

Had a rough time trying to set the interface value by using "reflect" package. The interface value is actually inside a struct of a struct. See my code in Go Playground
Basically, inside initProc, I want to assign dummyAFunc function to DummyA field in Box struct
package main
import (
"fmt"
"reflect"
)
type Box struct {
Name string
DummyA interface{}
}
type SmartBox struct {
Box
}
func dummyAFunc(i int) {
fmt.Println("dummyAFunc() is here!")
}
func initProc(inout interface{}) {
// Using "inout interface{}", I can take any struct that contains Box struct
// And my goal is assign dummyAFunc to dummyA in Box struct
iType:=reflect.TypeOf(inout)
iValue:=reflect.ValueOf(inout)
fmt.Println("Type & value:", iType.Elem(), iValue.Elem()) // Type & value: *main.SmartBox &{{ <nil>}}
e := reflect.ValueOf(inout).Elem()
fmt.Println("Can set?", e.CanSet()). // true
fmt.Println("NumField", e.NumField()) // panic: reflect: call of reflect.Value.NumField on ptr Value ?????
fmt.Println("NumMethod", e.NumMethod()) // NumMethod = 0
}
func main() {
smartbox := new (SmartBox)
initProc(&smartbox)
}
I'm new to Go and I've read the The laws of Reflection but still can't figure it out. Please help. Thanks!
You are passing a **SmartBix to initProc. So when you dereference once with reflect's Elem() you are still getting a pointer (*Smart box).
Since new already returns a pointer, just use:
smartbox := new (SmartBox)
// InitProc(smartbox) // **SmartBox
InitProc(smartbox) // *SmartBox
https://play.golang.org/p/j4q6aq6QL_4
EDIT
To update the input struct's DummyA field, you can do something like this:
func initProc2(v interface{}) error {
if reflect.TypeOf(v).Kind() != reflect.Ptr {
return fmt.Errorf("value must be a pointer")
}
dv := reflect.ValueOf(v).Elem()
if dv.Kind() != reflect.Struct {
return fmt.Errorf("value must be a pointer to a struct/interface")
}
const fname = "DummyA" // lookup field name
f := dv.FieldByName(fname)
if !f.CanSet() {
return fmt.Errorf("value has no field %q or cannot be set", fname)
}
nv := reflect.ValueOf(dummyAFunc)
f.Set(nv)
return nil
}
Working example: https://play.golang.org/p/VE751GtSGEw

Creating array of struct dynamically in golang

I try to create a generic function that accepts any struct value and create a array of that struct type. Here is the code I tried. But I get the error "t is not a type". How can I implement this.
type RegAppDB struct {
nm string
data []interface{}
}
func CreateRegTable(tbl string, rec interface{}) RegAppDB {
t := reflect.TypeOf(rec)
fmt.Println(t)
return RegAppDB{"log", []t}
}
Go does not support generics, and any attempt to do something like that is not going to work out well. In your specific case, there are a couple of key problems:
You cannot use a variable as a type. Go is compile-time static typed, so anything that gets type information at runtime (i.e. reflect.TypeOf) happens too late to use the way you're trying to do it.
Equally important, your struct's field is of type []interface{}, which means the only type you can use for that field is []interface{}. []string, for example, is a different type, and cannot be assigned to that field.
I took another route. Need some beautification. But this works. So now data is an array of interface and from calling function I pass pointer to structure variables to Save function.
type RegAppDB struct {
nm string
data []interface{}
cnt int
}
// CreateRegTable creates a data structure to hold the regression result
func CreateRegTable(tbl string) *RegAppDB {
return &RegAppDB{tbl, make([]interface{}, 20), 0}
}
// Save implements saving a record Regression application DB
func (rga *RegAppDB) Save(rec interface{}) error {
rga.data[rga.cnt] = rec
rga.cnt++
return nil
}
// Show implements showing the regression table
func (rga *RegAppDB) Show() error {
fmt.Println(rga.cnt)
for i := 0; i <= rga.cnt; i++ {
fmt.Println(rga.data[i])
}
return nil
}
// Compare compares two regression table for equality
func (rga *RegAppDB) Compare(rgt *RegAppDB) bool {
return reflect.DeepEqual(rga, rgt)
}
It cannot be done for a generic type. If there are a fixed number of possible types, then you can do something like the following:
type RegAppDB struct {
nm string
data interface{}
}
func CreateRegTable(rec interface{}) RegAppDB {
switch rec.(type) {
case int:
return &RegAppDB{"log", []int{}}
case string:
return &RegAppDB{"log", []string{}}
}
return nil
}
Note: Your data in RegAppDB should be of type interface{} since []int implements interface{} but not []interface{}

Assigning type / creating struct via switch statement

I'm having a bit of trouble figuring out how to either create a struct in a switch statement or assign a type to it in a switch statement.
Here's some non-working code illustrating what I'm trying to do:
var result
switch structPickingString {
case "struct1":
result = new(struct1)
case "struct2":
result = new(struct2)
}
//unmarshall some json into the appropriate struct type
if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
log.Println(err)
}
//print out that json with a function specific to that type of struct
result.Print()
I think something involving an empty interface{} might be related to solving this issue but unfortunately I'm still a bit ignorant with golang and I'm not seeing how to make it work.
Here's a link to a slightly modified version of the code for more context: https://play.golang.org/p/Rb1oaMuvmU2
The issue is not defining the print function, it's assigning a specific type of struct to the result variable based on using the individual Print function that the struct implements.
Let me know if there's any more info I could provide.
Since you are calling .Print, you need to use an interface which has that method. I think you are looking for something like
type Printer interface {
Print()
}
func (s *struct1) Print() {
// ...
}
func (s *struct2) Print() {
// ...
}
var result Printer
switch structPickingString {
case "struct1":
result = new(struct1)
case "struct2":
result = new(struct2)
}
https://play.golang.org/p/W9r6UfeQSCz
You needs match "generical interface" with structs. See this:
//my struct1
type MyStruct1 struct {
ID int
}
//my struct2
type MyStruct2 struct {
ID int
}
//my interface
type MyStructGenerical interface {
Print()
}
//method (match with MyStructGenerical)
func (m1 MyStruct1) Print() {
println(m1.ID)
}
//method (match with MyStructGenerical)
func (m2 MyStruct2) Print() {
println(m2.ID)
}
In this way you can does assertions between structs and generical interface. See this:
//here result is an generical interface
var result MyStructGenerical = nil
//checkin
switch structPickingString {
case "struct1":
tmp := new(MyStruct1)
result = tmp
case "struct2":
tmp := new(MyStruct2)
result = tmp
}
The result is:
//unmarshall some json into the appropriate struct type
if err := json.NewDecoder(body()).Decode(&result); err != nil {
log.Println(err)
}
//print out that json with a function specific to that type of struct
tmp := result.(*MyStruct1)
tmp.Print()
// or
result.Print()
Run in: https://play.golang.org/p/nHrJnImsqNN
I think you want to use an interface like
type ResultIface interface {
Print()
}
Then you can do
var result ResultIface
switch structPickingString {
case "struct1":
result = new(struct1)
case "struct2":
result = new(struct2)
}
As long as your struct1 and struct2 fulfills the ResultIface by having a Print function it will print

cannot use type interface {} as type person in assignment: need type assertion

I try to convert interface{} to struct person...
package main
import (
"encoding/json"
"fmt"
)
func FromJson(jsonSrc string) interface{} {
var obj interface{}
json.Unmarshal([]byte(jsonSrc), &obj)
return obj
}
func main() {
type person struct {
Name string
Age int
}
json := `{"Name": "James", "Age": 22}`
actualInterface := FromJson(json)
fmt.Println("actualInterface")
fmt.Println(actualInterface)
var actual person
actual = actualInterface // error fires here -------------------------------
// -------------- type assertion always gives me 'not ok'
// actual, ok := actualInterface.(person)
// if ok {
// fmt.Println("actual")
// fmt.Println(actual)
// } else {
// fmt.Println("not ok")
// fmt.Println(actual)
// }
}
... But got error:
cannot use type interface {} as type person in assignment: need type assertion
To solve this error I tried to use type assertion actual, ok := actualInterface.(person) but always got not ok.
Playground link
The usual way to handle this is to pass a pointer to the output value to your decoding helper function. This avoids type assertions in your application code.
package main
import (
"encoding/json"
"fmt"
)
func FromJson(jsonSrc string, v interface{}) error {
return json.Unmarshal([]byte(jsonSrc), v)
}
func main() {
type person struct {
Name string
Age int
}
json := `{"Name": "James", "Age": 22}`
var p person
err := FromJson(json, &p)
fmt.Println(err)
fmt.Println(p)
}
Your problem is that you're creating an empty interface to begin with, and telling json.Unmarshal to unmarshal into it. While you've defined a person type, json.Unmarshal has no way of knowing that that's what you intend the type of the JSON to be. To fix this, move the definition of person to the top level (that is, move it out of the body of main), and changeFromJson` to this:
func FromJson(jsonSrc string) interface{} {
var obj person{}
json.Unmarshal([]byte(jsonSrc), &obj)
return obj
}
Now, when you return obj, the interface{} that's returned has person as its underlying type. You can run this code on the Go Playground.
By the way, your code is a bit un-idiomatic. I left the original Playground link unmodified except for my corrections so that it wouldn't be needlessly confusing. If you're curious, here's a version that's cleaned up to be more idiomatic (including comments on why I made the changes I did).

Go: Am I creating too many values?

If I have a struct like this
type myStruct struct {
mystring string
myint int
}
and if I have a function that returns a new myStruct like this
func New() myStruct {
s := myStruct{}
s.mystring = "string"
s.myint = 1
return s
}
Because I first store it in the "s" variable before returning it, is my function actually making 2 myStruct values instead of one?
And if so, is it then a better practice to make sure I don't first store it in the variable?
The return statement will return a copy of the myStruct object value. If it is a small object then this is fine.
If you intend for the caller to be able to modify this object, and the struct will have methods that use a pointer as the receiver, then it makes more sense to return a pointer to your struct instead:
func New() *myStruct {
s := myStruct{}
s.mystring = "string"
s.myint = 1
return &s
}
You can see the copy happening when you compare the memory address of value vs pointer return types: http://play.golang.org/p/sj6mivYSHg
package main
import (
"fmt"
)
type myStruct struct {
mystring string
myint int
}
func NewObj() myStruct {
s := myStruct{}
fmt.Printf("%p\n", &s)
return s
}
func NewPtr() *myStruct {
s := &myStruct{}
fmt.Printf("%p\n",s)
return s
}
func main() {
o := NewObj()
fmt.Printf("%p\n",&o)
p := NewPtr()
fmt.Printf("%p\n",p)
}
0xf8400235a0 // obj inside NewObj()
0xf840023580 // obj returned to caller
0xf840023640 // ptr inside of NewPtr()
0xf840023640 // ptr returned to caller
I'm definitely not a Go expert (or even novice :) ), but as #max.haredoom mentioned, you can allocate variables in the function signature itself. In that way, you can also omit the s in the return:
package main
import "fmt"
type myStruct struct {
mystring string
myint int
}
func New() (s myStruct) {
s.mystring = "string"
s.myint = 1
return
}
func main() {
r := New()
fmt.Println(r)
}
// Outputs {string 1}
In the examples that I have come across in Effective Go, it does seem to be the most common way of doing things of this nature, but again, I am definitely not an authority on the subject (and will look for additional info on the actual performance).
I think I found the answer by using defer.
I updated the function so that there's a deferred modification to the myStruct value. This means it will happen after the return, but before it is received on the other end.
When I do this, the struct that is received by the caller does not show the updated value, so it appears as though I am indeed returning a copy.
func New() myStruct {
s := myStruct{}
defer func() {
s.mystring = "new value" // defer an update to the value
}()
s.mystring = "string"
s.myint = 1
return s
}
func main() {
b := New()
fmt.Println(b) // still shows the original value
}
http://play.golang.org/p/WWQi8HpDny

Resources