I am trying to reflect a slice of pointers on a struct stored in an interface{}
I think I am doing ok until it's time to introspect the content on the pointed struct.
See the below example
package main
import (
"fmt"
"reflect"
)
type teststruct struct {
prop1 string
prop2 string
}
func main() {
test := teststruct{"test", "12"}
var container interface{}
var testcontainer []*teststruct
testcontainer = append(testcontainer, &test)
container = testcontainer
rcontainer := reflect.ValueOf(container)
fmt.Println(rcontainer.Kind())
rtest := rcontainer.Index(0).Elem()
fmt.Println(rtest)
rteststruct := reflect.ValueOf(rtest)
fmt.Println(rteststruct.Kind())
typeOfT := rteststruct.Type()
for i := 0; i < rteststruct.NumField(); i++ {
f := rteststruct.Field(i)
fmt.Printf("%d: %s %s = %v\n", i, typeOfT.Field(i).Name, f.Type(), f.String())
}
}
Which results
slice
{test 12}
struct
0: typ *reflect.rtype = <*reflect.rtype Value>
1: ptr unsafe.Pointer = <unsafe.Pointer Value>
2: flag reflect.flag = <reflect.flag Value>
I am definitely missing something here, someone would be able to explain me what ?
rtest := rcontainer.Index(0).Elem() is already the value, so when you do this: rteststruct := reflect.ValueOf(rtest), you are actually getting a reflect.Value which is of course a struct.
Replace the end of your code with this:
typeOfT := rtest.Type()
for i := 0; i < rtest.NumField(); i++ {
f := rtest.Field(i)
fmt.Printf("%d: %s %s = %v\n", i, typeOfT.Field(i).Name, f.Type(), f.String())
}
Playground
Related
I have this func:
func getStringRepresentation(v interface{}, size int, brk bool, depth int) string {
val := reflect.ValueOf(v)
if val.Kind() == reflect.Ptr {
v = *v.(types.Pointer) // does not work
val = reflect.ValueOf(v)
}
// ...
}
how can I dereference the pointer to get the value of it? When I use:
v = *v.(types.Pointer)
The error says:
Invalid indirect of 'v.(types.Pointer)' (type 'types.Pointer')
I tried this:
val := reflect.ValueOf(v)
if val.Kind() == reflect.Ptr {
v = v.(types.Pointer).Underlying()
val = reflect.ValueOf(v)
}
and I tried this too:
val := reflect.ValueOf(v)
if val.Kind() == reflect.Ptr {
v = val.Elem()
val = reflect.ValueOf(v)
}
I need to get the value of the interface{} from the pointer.
You can dereference pointers using reflect using Elem:
val := reflect.ValueOf(v)
if val.Kind() == reflect.Ptr {
val = val.Elem()
}
After the if statement, val is a reflect.Value representing the value passed in by the interface. If the value passed in is a pointer, val now has the dereferenced value of that pointer.
You have to use 'Elem' for access value referenced by Pointer.
package main
import (
"fmt"
"reflect"
)
func main() {
p := &struct{Hello string}{"world"}
v := reflect.ValueOf(p)
ps := v.Interface().(*struct{Hello string})
fmt.Println(ps)
s := v.Elem().Interface().(struct{Hello string})
fmt.Println(s)
}
Looks like this accomplished what I was trying to do:
func getStringRepresentation(v interface{}, size int, brk bool, depth int) string {
val := reflect.ValueOf(v)
if val.Kind() == reflect.Ptr {
v = val.Elem().Interface() // gets the actual interface from the pointer
val = reflect.ValueOf(v) // gets the value of the actual interface
}
// ...
}
thanks to the other answers which I stole the eventual solution from.
I am trying to create a function that could accept following
*struct
[]*struct
map[string]*struct
Here struct could be any struct not just a specific one.
Converting interface to *struct or []*struct is working fine.
But giving error for map.
After reflect it shows it is map[] but giving error when try to iterate over range.
Here is code
package main
import (
"fmt"
"reflect"
)
type Book struct {
ID int
Title string
Year int
}
func process(in interface{}, isSlice bool, isMap bool) {
v := reflect.ValueOf(in)
if isSlice {
for i := 0; i < v.Len(); i++ {
strct := v.Index(i).Interface()
//... proccess struct
}
return
}
if isMap {
fmt.Printf("Type: %v\n", v) // map[]
for _, s := range v { // Error: cannot range over v (type reflect.Value)
fmt.Printf("Value: %v\n", s.Interface())
}
}
}
func main() {
b := Book{}
b.Title = "Learn Go Language"
b.Year = 2014
m := make(map[string]*Book)
m["1"] = &b
process(m, false, true)
}
Is there any way to convert interface{} to map and iterate or get it's elements.
If the map value can be any type, then use reflect to iterate through the map:
if v.Kind() == reflect.Map {
for _, key := range v.MapKeys() {
strct := v.MapIndex(key)
fmt.Println(key.Interface(), strct.Interface())
}
}
playground example
If there's a small and known set of struct types, then a type switch can be used:
func process(in interface{}) {
switch v := in.(type) {
case map[string]*Book:
for s, b := range v {
// b has type *Book
fmt.Printf("%s: book=%v\n" s, b)
}
case map[string]*Author:
for s, a := range v {
// a has type *Author
fmt.Printf("%s: author=%v\n" s, a)
}
case []*Book:
for i, b := range v {
fmt.Printf("%d: book=%v\n" i, b)
}
case []*Author:
for i, a := range v {
fmt.Printf("%d: author=%v\n" i, a)
}
case *Book:
fmt.Ptintf("book=%v\n", v)
case *Author:
fmt.Printf("author=%v\n", v)
default:
// handle unknown type
}
}
You don't need reflect here. Try:
v, ok := in.(map[string]*Book)
if !ok {
// Can't assert, handle error.
}
for _, s := range v {
fmt.Printf("Value: %v\n", s)
}
Same goes for the rest of your function. It looks like you're using reflection when you would be better served by a type switch.
Alternatively, if you insist on using reflection here (which doesn't make a lot of sense) you can also use Value.MapKeys with the result from your ValueOf (see the answer https://stackoverflow.com/a/38186057/714501)
This may help:
b := []byte(`{"keyw":"value"}`)
var f interface{}
json.Unmarshal(b, &f)
myMap := f.(map[string]interface{})
fmt.Println(myMap)
Another way to convert an interface{} into a map with the package reflect is with MapRange.
I quote:
MapRange returns a range iterator for a map. It panics if v's Kind is
not Map.
Call Next to advance the iterator, and Key/Value to access each entry.
Next returns false when the iterator is exhausted. MapRange follows
the same iteration semantics as a range statement.
Example:
iter := reflect.ValueOf(m).MapRange()
for iter.Next() {
key := iter.Key().Interface()
value := iter.Value().Interface()
...
}
I am trying to parse a yaml file dynamically (Therefore no struct).
package main
import (
"fmt"
"gopkg.in/yaml.v2"
"log"
)
func main() {
var out = `
a: First!
f: Second
b:
c:
f: Third
`
m := make(map[interface{}]interface{})
err := yaml.Unmarshal([]byte(out), &m)
if err != nil {
log.Fatal(err)
}
fmt.Println(m["b"].(map[interface{}]interface{})["c"].(map[interface{}]interface{})["f"])
}
Everytime I have to access a subkey, I am forced to convert map variable in question to (map[interface{}]interface{}). This is causing bit of a hassle for me as I have to iterate through the map.
Is there any easier method for parsing YAML file in Go?
Another approach is to flatten the yaml data structure into a key,value map in which keys and values are strings. Then if you need the actual type (5 being an int) you can do the conversion yourself. Example:
"a" = "First!"
"f" = "Second"
"b.c.f" = "Third"
"b.c.g.size" = "2"
"b.c.g.0 = "zero"
"b.c.g.1 = "one"
In Go:
func main() {
any := map[string]interface{}{}
err := yaml.Unmarshal([]byte(out), &any)
if err != nil {
log.Fatal(err)
}
flatmap := map[string]string{}
for k, v := range any {
flatten(k, v, flatmap)
}
for k, v := range flatmap {
fmt.Println(k, "=", v)
}
}
func flatten(prefix string, value interface{}, flatmap map[string]string) {
submap, ok := value.(map[interface{}]interface{})
if ok {
for k, v := range submap {
flatten(prefix+"."+k.(string), v, flatmap)
}
return
}
stringlist, ok := value.([]interface{})
if ok {
flatten(fmt.Sprintf("%s.size", prefix), len(stringlist), flatmap)
for i, v := range stringlist {
flatten(fmt.Sprintf("%s.%d", prefix, i), v, flatmap)
}
return
}
flatmap[prefix] = fmt.Sprintf("%v", value)
}
I have a struct:
type mystruct struct {
Foo string
Bar int
}
I'd like to create SQL insert statements from the struct which have the following form:
m := mystruct{ "Hello" , 1 }
query := "INSERT INTO mytbl ( foo, bar ) VALUES ( ?,? )"
res,err := db.Exec(query, m.Foo, m.Bar)
Now my question is: how can I make the last line dynamically from the struct (or m) itself? I am able to get the struct names using reflect, but I don't know how to create the []interface{} slice for the db.Exec() call. This is what I have tried: (http://play.golang.org/p/GR1Bb61NFH)
package main
import (
"fmt"
"reflect"
)
type mystruct struct {
Foo string
Bar int
}
func main() {
m := mystruct{"Foo", 1}
fmt.Println(readNames(m))
x := unpackStruct(m)
fmt.Printf("%#v\n", x)
}
func unpackStruct(a interface{}) []interface{} {
// "convert" a to m t
// doesn't work, from 'laws of reflection'
s := reflect.ValueOf(&t).Elem()
typeOfT := s.Type()
for i := 0; i < s.NumField(); i++ {
f := s.Field(i)
fmt.Printf("%d: %s %s = %v\n", i,
typeOfT.Field(i).Name, f.Type(), f.Interface())
}
// this is in principle what I want:
m := mystruct{"Hello", 2}
var ret []interface{}
ret = make([]interface{}, s.NumField())
ret[0] = m.Foo
ret[1] = m.Bar
return ret
}
// works fine:
func readNames(a interface{}) []string {
s := reflect.TypeOf(a)
lenStruct := s.NumField()
ret := make([]string, lenStruct)
for i := 0; i < lenStruct; i++ {
ret[i] = s.Field(i).Name
}
return ret
}
If getting the values of the fields is your issue, this code snippet should help:
s := reflect.ValueOf(a)
ret := make([]interface{}, s.NumField())
for i := 0; i < s.NumField(); i++ {
ret[i] = s.Field(i).Interface()
}
If creating the []interface{} value is your problem, using reflect's slice creation mechanisms should work nicely:
slc := reflect.MakeSlice(InterfaceType, len, cap) // See the link below for creating InterfaceType
slc.Index(0).Set(TargetValue)
return slc.Interface()
(Here's the above-mentioned link).
Modifying the above code to loop over the values in the struct instead of just the 0th index shouldn't be too bad.
How to cast reflect.Value to its type?
type Cat struct {
Age int
}
cat := reflect.ValueOf(obj)
fmt.Println(cat.Type()) // Cat
fmt.Println(Cat(cat).Age) // doesn't compile
fmt.Println((cat.(Cat)).Age) // same
Thanks!
concreteCat,_ := reflect.ValueOf(cat).Interface().(Cat)
see http://golang.org/doc/articles/laws_of_reflection.html
fox example
type MyInt int
var x MyInt = 7
v := reflect.ValueOf(x)
y := v.Interface().(float64) // y will have type float64.
fmt.Println(y)
Ok, I found it
reflect.Value has a function Interface() that converts it to interface{}
This func auto-converts types as needed. It loads a config file values into a simple struct based on struct name and fields:
import (
"fmt"
toml "github.com/pelletier/go-toml"
"log"
"os"
"reflect"
)
func LoadConfig(configFileName string, configStruct interface{}) {
defer func() {
if r := recover(); r != nil {
fmt.Println("LoadConfig.Recovered: ", r)
}
}()
conf, err := toml.LoadFile(configFileName)
if err == nil {
v := reflect.ValueOf(configStruct)
typeOfS := v.Elem().Type()
sectionName := getTypeName(configStruct)
for i := 0; i < v.Elem().NumField(); i++ {
if v.Elem().Field(i).CanInterface() {
kName := conf.Get(sectionName + "." + typeOfS.Field(i).Name)
kValue := reflect.ValueOf(kName)
if (kValue.IsValid()) {
v.Elem().Field(i).Set(kValue.Convert(typeOfS.Field(i).Type))
}
}
}
} else {
fmt.Println("LoadConfig.Error: " + err.Error())
}
}
Seems the only way would be to do a switch statement similar to (code below) (also, something like the commented line would've-been nice though doesn't work (:()):
func valuesFromStruct (rawV interface{}) []interface{} {
v := reflect.ValueOf(rawV)
out := make([]interface{}, 0)
for i := 0; i < v.NumField(); i += 1 {
field := v.Field(i)
fieldType := field.Type()
// out = append(out, field.Interface().(reflect.PtrTo(fieldType)))
switch (fieldType.Name()) {
case "int64":
out = append(out, field.Interface().(int64))
break`enter code here`
case "float64":
out = append(out, field.Interface().(float64))
break
case "string":
out = append(out, field.Interface().(string))
break
// And all your other types (here) ...
default:
out = append(out, field.Interface())
break
}
}
return out
}
Cheers!