How to new an object with string in Golang

Question

How can I create an object when I am only having its type in string? I am looking for something like:

type someStruct struct {}

resultObject := new "someStruct"

It will be really helpful and convenience with this when using different ORM library like GORP and GORM.

Is it possible to do it in Golang?

possible duplicate of [is there a way to create an instance of a struct from a string?](http://stackoverflow.com/questions/23030884/is-there-a-way-to-create-an-instance-of-a-struct-from-a-string) — Linear, Jun 09 '14 at 10:35

score 7 · Accepted Answer · answered Jun 09 '14 at 08:53

No...

Well, the answer is "yes, but" and it's a big but. There's no central registry of struct names in Go. You're not going to get a nice, clean standard library function called StructFromName(string) which is probably what you were hoping for.

Instead, you have to write that mapping yourself, something like

func StringToStruct(name string) (interface{}, error) {
    switch name {
    case "SomeStruct":
        return SomeStruct{}, nil
    case "SomeOtherStruct":
        return SomeOtherStruct{}, nil
    case "subpackage.Struct":
        return subpackage.Struct{}, nil
    default:
        return nil, fmt.Errorf("%s is not a known struct name", name)
    }
}

Na...this kind of factory is not what I want. But still thx for answering. This solution maybe to final one if there is not better. — Anthony Tsang, Jun 09 '14 at 09:39

Not_a_Golfer · Answer 2 · 2014-06-09T08:58:32.607

6

You can't directly do that in Go. The only thing close to that Go supports is reflect.New() in the reflection library, which accepts a reflect.Type object and creates a new value of its type.

Now, if you'd like to initialize a type instance by name, you can build a registry of types by name, and have a function that looks up the type by name and creates an instance. It's really ugly though and not very idiomatic to Go, but it can be done.

[EDIT] here's a working example of this. You still need to do type conversion manually:

package main

import (
    "fmt"
    "reflect"
)

//just a couple of structs
type Foo struct {
    Lol string
}

type Bar struct {
    Wut string
}


//this is the registry of types by name
var registry = map[string]reflect.Type{}

// add a type to the registry
func registerType(t reflect.Type) {
    name := t.Name()
    registry[name] = t
}


// create a new object by name, returning it as interface{}
func newByName(name string) interface{} {

    t, found := registry[name]
    if !found {
        panic("name not found!")
    }

    return reflect.New(t).Elem().Interface()
}

func main() {

    //register foo and bar
    registerType(reflect.TypeOf(Foo{}))
    registerType(reflect.TypeOf(Bar{}))

    //create new instances
    foo := newByName("Foo").(Foo)
    bar := newByName("Bar").(Bar)

    fmt.Println(reflect.TypeOf(foo), reflect.TypeOf(bar))
}

And again, I wouldn't advise you to do this, it's ugly and slow and non idiomatic

edited Jun 09 '14 at 08:58

answered Jun 09 '14 at 08:51

Not_a_Golfer

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Which part exactly you mean by slow? For me this is good enough to solve my case, really think you. I want to apply this keep of technique on ORM so that the objects will be more "related" and speed up the development. Or, is there a so-called "normal" way to duel with database entity? – Anthony Tsang Jun 09 '14 at 10:17
@AnthonyTsang just, you know, doing these dynamic things with strings, etc. It feels like Python, not Go :) It will work, but it's just no very go-ish, that's all. – Not_a_Golfer Jun 09 '14 at 11:26
1

Reflection in general has a huge overhead, you're thinking in Python mentality right now, sit down and read http://golang.org/doc/effective_go.html. – OneOfOne Jun 09 '14 at 13:06
1

@OneOfOne right, just worth noting that that that's how the json and xml libraries do this, more or less :) they use reflection to create instances, but they're not using a strinly typed registry. – Not_a_Golfer Jun 09 '14 at 13:17
This is a bit old but, @OneOfOne generally reflection in library code is acceptable, but in application code it is considered not considered idiomatic go – robbert229 Aug 23 '16 at 01:02
@robbert229 it's more complicated than that, there are things that you *have* to use reflection for, for example json/gob marshalling, but it doesn't change the fact it's a lot slower than not using it. Compare encoding/json to ffjson (which uses an external tool to generate structs) for example. – OneOfOne Aug 23 '16 at 21:39
@OneOfOne I completely agree. json/gob marshalling uses reflection heavily but in general it should be avoided. Hence why I said that the use of reflection in libraries is good, and in application code it is not... Rob Pike on reflection - It's a powerful tool that should be used with care and avoided unless strictly necessary. - https://blog.golang.org/laws-of-reflection – robbert229 Aug 24 '16 at 20:52
@robbert229 I misread what you said, I thought you were asking a question. – OneOfOne Aug 24 '16 at 21:02
1

@OneOfOne, Its cool. Half the time I write a comment it doesn't make any sense. – robbert229 Aug 24 '16 at 22:40

How to new an object with string in Golang

2 Answers2