The following works fine:
type MyStruct struct {
MyField int32
}
func SetReflectConcrete(obj *MyStruct, fieldName string, newValue interface{}) {
objElem := reflect.ValueOf(obj).Elem()
field := objElem.FieldByName(fieldName)
field.Set(reflect.ValueOf(newValue))
}
func main() {
myStruct := MyStruct{123}
SetReflectConcrete(myStruct, "MyField", int32{1234})
}
How can I make a variant of the SetReflect function that works on any struct? All my attempts so far have failed. The signature would be something like this:
func SetReflectInterface(obj interface{}, fieldName string, newValue interface{})
And is this even possible, when calling it like
SetReflectInterface(myStruct, "MyField", int32{1234})
or would it have to be called like
SetReflectInterface(&myStruct, "MyField", int32{1234})
(After all, interface{} has a pointer to the struct.)
Declare the argument as type interface{} as you noted. Pass a pointer to the struct as in the last code snippet.
func SetReflectConcrete(obj interface{}, fieldName string, newValue interface{}) {
objElem := reflect.ValueOf(obj).Elem()
field := objElem.FieldByName(fieldName)
field.Set(reflect.ValueOf(newValue))
}
myStruct := MyStruct{123}
SetReflectConcrete(&myStruct, "MyField", int32(1234))
Run it on the playground.
The reflect value must be addressable to set a field. The value will not be addressable if created directly from a struct.
Related
Im trying to define a couple of interface to refactor some code and I'm getting a problem where Go is not letting me assign functions to variables.
This is the set up
main.go
type Gettable interface {
Get() int64
}
type MyFunction func(int64) (Gettable, error)
func main() {
var f MyFunction
f = sub.TestFn2
a, _ := f(1)
fmt.Println(a)
}
main/sub
package sub
type MyStruct struct {
Val int64
}
func (v MyStruct) Get() int64 {
return v.Val
}
func TestFn2(a int64) (MyStruct, error) {
return MyStruct{a}, nil
}
I'm trying to define a generic function type, and in the sub package create the concrete functions
and ideally i want to store the functions in a map and call them with something like
FnMap["fnName"]()
I'm not there yet,
im getting an error saying
/topics.go:27:4: cannot use sub.TestFn2 (type func(int64) (sub.MyStruct, error)) as type MyFunction in assignment
but MyStruct clearly implements the interface Gettable
This error occurs because of signature don't match.
You code shared is:
//shared code
//---------------------------------------------
type Gettable interface {
Get() int64
}
type MyFunction func(int64) (Gettable, error)
So you need replace MyStruct by Gettable.
//main/sub
//---------------------------------------------
type MyStruct struct {
Val int64
}
func (v MyStruct) Get() int64 {
return v.Val
}
//this signature of TestFn2 is different of MyStruct
//-[TestFn2] func (a int64) (MyStruct, error)
//-[MyFunction] func(int64) (Gettable, error)
func TestFn2(a int64) (Gettable, error) {//<-- replace by Gettable here
return MyStruct{a}, nil
}
Running your code:
//main.go
//---------------------------------------------
func main() {
var f MyFunction
f = TestFn2
a, _ := f(1)
fmt.Println(a)
}
Result is:
{1}
See in playground: https://play.golang.org/p/sRsXix8E_83
As per the Go's assignability rules a function f is assignable to a variable v only if the variable's type T exactly matches the f's signature.
The ability to assign a more specific type in some other languages called "covariance", and Go's type system does not have it.
I got one variable's address which type is converted to unsafe.Pointer, and I got its type name, such as "int64" or "struct main.Person" or others.
see the code:
package main
type Person struct {
Name string
Age int
}
p := Person{"john", 25}
addr := unsafe.Pointer(&p)
typName := "struct main.Person"
typ := module.TypeOfByAddr(addr, typName)
// now, I got variable addr, typName in func module.TypeOfByAddr
// how can I get reflect.Type of variable p without using package.Person to convert?
package module
func TypeOfByAddr(addr unsafe.Pointer, typName string) reflect.Type {
// how to convert addr to reflect.Type?
}
Why would you need the pointer to get the Type? All you need is the type name.
In Go, it is not possible to get the reflect.Type from a string directly. The type must be referenced somewhere in order for the compiler to know it is to be included. However, you can get the same effect using map[string]reflect.Type.
If your types are static, just declare:
var types = map[string]reflect.Type{
"int64": reflect.TypeOf(int64(0)),
"string": reflect.TypeOf(string("")),
// ... and so on
}
If your types are registered dynamically from different packages, you can register your types from init functions like this:
var types = make(map[string]reflect.Type)
func AddType(name string, i interface{}) {
if _, ok := types[name]; ok {
panic("Type " + name + " registered multiple times")
}
types[name] = reflect.TypeOf(i)
}
// Called somewhere else
func init() {
AddType("struct main.Person", Person{})
}
If you want to get the reflect.Value from a pointer and a type name, you can do the following:
func ValueOfByAddr(addr unsafe.Pointer, typName string) reflect.Value {
t, ok := types[typName]
if !ok {
panic("Unknown type name")
}
return reflect.NewAt(t, addr)
}
I have a function that has a parameter with the type interface{}, something like:
func LoadTemplate(templateData interface{}) {
In my case, templateData is a struct, but each time it has a different structure. I used the type "interface{}" because it allows me to send all kind of data.
I'm using this templateData to send the data to the template:
err := tmpl.ExecuteTemplate(w, baseTemplateName, templateData)
But now I want to append some new data and I don't know how to do it because the "interface" type doesn't allow me to add/append anything.
I tried to convert the interface to a struct, but I don't know how to append data to a struct with an unknown structure.
If I use the following function I can see the interface's data:
templateData = appendAssetsToTemplateData(templateData)
func appendAssetsToTemplateData(t interface{}) interface{} {
switch reflect.TypeOf(t).Kind() {
case reflect.Struct:
fmt.Println("struct")
s := reflect.ValueOf(t)
fmt.Println(s)
//create a new struct based on current interface data
}
return t
}
Any idea how can I append a child to the initial interface parameter (templateData)? Or how can I transform it to a struct or something else in order to append the new child/data?
Adrian is correct. To take it a step further, you can only do anything with interfaces if you know the type that implements that interface. The empty interface, interface{} isn't really an "anything" value like is commonly misunderstood; it is just an interface that is immediately satisfied by all types.
Therefore, you can only get values from it or create a new "interface" with added values by knowing the type satisfying the empty interface before and after the addition.
The closest you can come to doing what you want, given the static typing, is by embedding the before type in the after type, so that everything can still be accessed at the root of the after type. The following illustrates this.
https://play.golang.org/p/JdF7Uevlqp
package main
import (
"fmt"
)
type Before struct {
m map[string]string
}
type After struct {
Before
s []string
}
func contrivedAfter(b interface{}) interface{} {
return After{b.(Before), []string{"new value"}}
}
func main() {
b := Before{map[string]string{"some": "value"}}
a := contrivedAfter(b).(After)
fmt.Println(a.m)
fmt.Println(a.s)
}
Additionally, since the data you are passing to the template does not require you to specify the type, you could use an anonymous struct to accomplish something very similar.
https://play.golang.org/p/3KUfHULR84
package main
import (
"fmt"
)
type Before struct {
m map[string]string
}
func contrivedAfter(b interface{}) interface{} {
return struct{
Before
s []string
}{b.(Before), []string{"new value"}}
}
func main() {
b := Before{map[string]string{"some": "value"}}
a := contrivedAfter(b)
fmt.Println(a)
}
You can't append data arbitrarily to a struct; they're statically typed. You can only assign values to the fields defined for that specific struct type. Your best bet is probably to use a map instead of structs for this.
Not recommended, but you can create structs dynamically using the reflect package.
Here is an example:
package main
import (
"encoding/json"
"os"
"reflect"
)
type S struct {
Name string
}
type D struct {
Pants bool
}
func main() {
a := Combine(&S{"Bob"}, &D{true})
json.NewEncoder(os.Stderr).Encode(a)
}
func Combine(v ...interface{}) interface{} {
f := make([]reflect.StructField, len(v))
for i, u := range v {
f[i].Type = reflect.TypeOf(u)
f[i].Anonymous = true
}
r := reflect.New(reflect.StructOf(f)).Elem()
for i, u := range v {
r.Field(i).Set(reflect.ValueOf(u))
}
return r.Addr().Interface()
}
You could use something like the Combine function above to shmush any number of structs together. Unfortunately, from the documentation:
StructOf currently does not generate wrapper methods for embedded fields. This limitation may be lifted in a future version.
So your created struct won't inherit methods from the embedded types. Still, maybe it does what you need.
If you are just looking to convert your interface to struct, use this method.
type Customer struct {
Name string `json:"name"`
}
func main() {
// create a customer, add it to DTO object and marshal it
receivedData := somefunc() //returns interface
//Attempt to unmarshall our customer
receivedCustomer := getCustomerFromDTO(receivedData)
fmt.Println(receivedCustomer)
}
func getCustomerFromDTO(data interface{}) Customer {
m := data.(map[string]interface{})
customer := Customer{}
if name, ok := m["name"].(string); ok {
customer.Name = name
}
return customer
}
Basic situation:
you can do this in golang:
func addrOf(foo interface{}) interface{} {
return &foo
}
but you cannot do this:
func addrOf(foo interface{}) interface{} {
fooValue := reflect.ValueOf(foo)
return fooValue.Addr().Interface()
}
Am I doing something wrong? why is foo addressable to the language but not to reflect?
I'm trying to dynamically call functions returning different types of struct.
For example, let's take the following code.
struct A {
Name string
Value int
}
struct B {
Name1 string
Name2 string
Value float
}
func doA() (A) {
// some code returning A
}
func doB() (B) {
// some code returning B
}
I would like to pass either the function doA or doB as an argument to a generic function that would execute the function and JSON-encode the result. Like the following:
func Generic(w io.Writer, fn func() (interface {}) {
result := fn()
json.NewEncoder(w).Encode(result)
}
But when I do:
Generic(w, doA)
I get the following error:
cannot use doA (type func() (A)) as type func() (interface {})
Is there a way to achieve this dynamic call?
First, let me remark that func() (interface{}) means the same thing as func() interface{}, so I'll use the shorter form.
Passing a function of type func() interface{}
You can write a generic function that takes a func() interface{} argument as long as the function that you pass to it has type func() interface{}, like this:
type A struct {
Name string
Value int
}
type B struct {
Name1 string
Name2 string
Value float64
}
func doA() interface{} {
return &A{"Cats", 10}
}
func doB() interface{} {
return &B{"Cats", "Dogs", 10.0}
}
func Generic(w io.Writer, fn func() interface{}) {
result := fn()
json.NewEncoder(w).Encode(result)
}
You can try out this code in a live playground:
http://play.golang.org/p/JJeww9zNhE
Passing a function as an argument of type interface{}
If you want to write functions doA and doB that return concretely typed values, you can pass the chosen function as an argument of type interface{}. Then you can use the reflect package to make a func() interface{} at run-time:
func Generic(w io.Writer, f interface{}) {
fnValue := reflect.ValueOf(f) // Make a concrete value.
arguments := []reflect.Value{} // Make an empty argument list.
fnResults := fnValue.Call(arguments) // Assume we have a function. Call it.
result := fnResults[0].Interface() // Get the first result as interface{}.
json.NewEncoder(w).Encode(result) // JSON-encode the result.
}
More concisely:
func Generic(w io.Writer, fn interface{}) {
result := reflect.ValueOf(fn).Call([]reflect.Value{})[0].Interface()
json.NewEncoder(w).Encode(result)
}
Complete program:
package main
import (
"encoding/json"
"io"
"os"
"reflect"
)
type A struct {
Name string
Value int
}
type B struct {
Name1 string
Name2 string
Value float64
}
func doA() *A {
return &A{"Cats", 10}
}
func doB() *B {
return &B{"Cats", "Dogs", 10.0}
}
func Generic(w io.Writer, fn interface{}) {
result := reflect.ValueOf(fn).Call([]reflect.Value{})[0].Interface()
json.NewEncoder(w).Encode(result)
}
func main() {
Generic(os.Stdout, doA)
Generic(os.Stdout, doB)
}
Live playground:
http://play.golang.org/p/9M5Gr2HDRN
Your return signature is different for these functions:
fn func() (interface {}) vs. func doA() (A) and func doB() (B)
You are getting a compiler error because you are passing a function with a different signature into your Generic function. To address this issue you can change your functions to return interface{}.
This is an example of how to do that, I am using anonymous structs and printing the return value out rather than serializing them but this applies just the same to your example:
package main
import "fmt"
func doA() interface{} {
return struct {
Name string
Value int
}{
"something",
5,
}
}
func doB() interface{} {
return struct {
Name1 string
Name2 string
Value float64
}{
"something",
"or other",
5.3,
}
}
func main() {
fmt.Println("Hello, playground", doA(), doB())
}
Experiment with this in the Go Playground: http://play.golang.org/p/orrJw2XMW8