Using reflect, how to create struct "type" dynamically?

Question

Consider the following struct :

type Foo struct{}

func (f *Foo) foo() {
    fmt.Println("Hello")
}

Using reflect I would like to generate a custom struct type that overrides a set of methods. When doing the work manually what I want to do is :

type Edit struct{
    Foo
}

func (e *Edit) foo() {
    e.Foo.foo()
    fmt.Println("World")
}

But instead of writing this manually, I would like to make a function that does such override automatically for any given struct.

I know that I can instantiate a struct using :

res := reflect.New(reflect.TypeOf(origin))

but in this situation, we can't edit methods.

So how with reflect can I create a struct type that wraps an existing type like explained above?

Doing things like this at runtime is not particularly compatible with Go. This might be a good use case for a generator though, see `go generate`. — Adrian, Jul 19 '18 at 16:36
@Adrian: thanks, though if that's not the case, I'm not sure what other answer there could be. — JimB, Jul 19 '18 at 16:40
Indeed theses questions aren't related to what I want. And ideally I'd want to avoid code generation — Rémi Rousselet, Jul 19 '18 at 16:40
Why do you want to avoid code generation? Go does not support creating methods at runtime. Your options are limited. — Adrian, Jul 19 '18 at 16:41
Because this is for a library. So I'd like to avoid forcing the library users to use code-gen. This may be impossible though — Rémi Rousselet, Jul 19 '18 at 16:44
Are the methods signatures known, or are they determined at runtime? — Charlie Tumahai, Jul 19 '18 at 17:57
@ThunderCat Methods signature is known. But they don't have a predifined name. I already managed to extract a `[]reflect.Method` list of methods I want to override — Rémi Rousselet, Jul 19 '18 at 18:29
In the end, I used `go generated` combined with `go/parser` and `dave/jennifer/jen` — Rémi Rousselet, Jul 24 '18 at 10:44

Geoherna · Answer 1 · 2018-07-19T18:27:57.450

You can achieve something extremely similar to what you are asking for using Method Values. You could do something like the following:

package main

import (
    "fmt"
)

type Foo struct {
    sayHelloAgain func()
}

func (f *Foo) SayHello() { 
    fmt.Print("Hello,")
    f.sayHelloAgain() 
}

func main() {
    edit := struct{Foo}{}
    edit.sayHelloAgain = func(){ fmt.Println(" World!") }
    edit.SayHello()

}

Go Playground

As you can see, you can create an anonymous struct which will be able to call the SayHello function, which then calls the receiver's dynamically set sayHelloAgain method.

I think this will be the closest you will get to your goal, and works as you requested (without relection)

Thanks for the thought, but this doesn't really fit with what I want to achieve — Rémi Rousselet, Jul 19 '18 at 18:31

Using reflect, how to create struct "type" dynamically?

1 Answers1