How to cast *interface{} into *T ?

[Denis P]

Hi guys, tried to look everywhere and cannot find an answer.

So literally I have this:

```

var result interface{} = &MyStruct{}

...

return result.(*MyStruct) // Error:  invalid operation: result (variable of type *interface{}) is not an interface

```

Is there any way to get the &MyStruct as a result?

[Axel Wagner]

Hi,

On Thu, Nov 24, 2022 at 11:16 PM Denis P <denis....@gmail.com> wrote:

Hi guys, tried to look everywhere and cannot find an answer.

So literally I have this:

```

var result interface{} = &MyStruct{}

...

return result.(*MyStruct) // Error:  invalid operation: result (variable of type *interface{}) is not an interface

```

No, offense, but you literally do not have that code, because the error message does not fit that code. In your code, `result` has type `interface{}`, whereas in the `return` statement, according to the error message, it has type `*interface{}`. Ergo, you are either shadowing `result` in-between, or the code is not equivalent.

In any case, the immediate solution could be to dereference the pointer:

return (*result).(*MyStruct)

 

Is there any way to get the &MyStruct as a result?

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[Denis P]

Ok, looks like it doesn't work.

I have got the following error:

panic: interface conversion: interface {} is MyStruct, not *MyStruct

So the actual code is this: https://go.dev/play/p/jBiPII7rOxG

Cannot find out what is going on :-(

[Denis P]

Ok, finally got it: https://go.dev/play/p/QFQOpqWsR6S

Thanks everyone, especially Axel

[Denis P]

Still bad solution.

Because I am dereferencing the struct it clones the instance, and it what I am trying to avoid:

Any other solutions?

[Axel Wagner]

On Fri, Nov 25, 2022 at 12:14 AM Denis P <denis....@gmail.com> wrote:

Ok, looks like it doesn't work.

I have got the following error:

panic: interface conversion: interface {} is MyStruct, not *MyStruct

So the actual code is this: https://go.dev/play/p/jBiPII7rOxG

Cannot find out what is going on :-(

The runtime is telling you - the dynamic type of the interface is not `MyStruct`, it is `*MyStruct`:

 w := &MyWrapper{Obj: MyStruct{}}

Here, you are putting a `MyStruct` into `w.Obj`.

 s := GetOf[MyStruct](&w.Obj)

You are calling `GetOf` with the address of `w.Obj`. That is, you are passing an `*interface{}` to `GetOf[MyStruct]`. That `*interface{}` points at an interface containing a `MyStruct` (as you just put that in).

func GetOf[T interface{}](obj *interface{}) *T {
    return (*obj).(*T)
}

After instantiation, you are type-asserting the pointee of the passed pointer to `*MyStruct`. But the value in there is a `MyStruct`. So this panics.

I don't know how to fix your code, because I do not understand what you are trying to do. One way to make the panic go away is to put a `*MyStruct` into `w.Obj`:

 w := &MyWrapper{Obj: &MyStruct{}}

(note the second `&`). Another would be, to not type-assert on a `*T`, but a `T` and return a pointer pointing at the same value:

x := (*obj).(T)

return &x

You also probably do not want to take a `*interface{}`, but just take the interface directly… So maybe you want this? https://go.dev/play/p/5w-sMtVyTE3

The code isn't particularly functional, so I have no idea what the intended fix is. But the compiler and runtime are telling you, what you are doing wrong, in either case.

 

On Thursday, 24 November 2022 at 22:29:21 UTC axel.wa...@googlemail.com wrote:

Hi,

On Thu, Nov 24, 2022 at 11:16 PM Denis P <denis....@gmail.com> wrote:

Hi guys, tried to look everywhere and cannot find an answer.

So literally I have this:

```

var result interface{} = &MyStruct{}

...

return result.(*MyStruct) // Error:  invalid operation: result (variable of type *interface{}) is not an interface

```

No, offense, but you literally do not have that code, because the error message does not fit that code. In your code, `result` has type `interface{}`, whereas in the `return` statement, according to the error message, it has type `*interface{}`. Ergo, you are either shadowing `result` in-between, or the code is not equivalent.

In any case, the immediate solution could be to dereference the pointer:

return (*result).(*MyStruct)

 

Is there any way to get the &MyStruct as a result?

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[Axel Wagner]

To be clear what I mean with "not particularly functional": Your GetOf is just doing a type-assertion, so the right way to write it would be this: https://go.dev/play/p/hTqOCbc07-K

You must have a reason to go through the more complicated dance of your GetOf function, but without knowing that, it's hard to tell what exactly the misunderstanding or the best solution is.

[Nigel Tao]

Possibly relevant:

https://go.dev/doc/faq#pointer_to_interface

[Brian Candler]

To give a real-world example:

https://pkg.go.dev/github.com/prometheus/client_golang/prometheus

type metrics struct {
        cpuTemp    prometheus.Gauge
        hdFailures *prometheus.CounterVec
}

Question: why is prometheus.Gauge not a pointer, but *prometheus.CounterVec is a pointer?

Answer: because prometheus.Gauge is an interface, whereas prometheus.CounterVec is a struct.

An interface essentially already *contains* a pointer to a data value - in fact, a tuple of (type, pointer).  So you should never take a pointer to an interface.  Just pass the interface value, and it will copy the (type, pointer) pair.

However, when you extract a value from an interface, it *will* always copy the internal value.  This avoids aliasing issues: someone who passes an interface value to someone else, won't expect the recipient to be able to change the sender's copy.

e.g.

https://go.dev/play/p/u5y3Kwm9Ydz

Of course, if your interface *contains* a pointer, then the recipient of the interface can modify the thing being pointed to.

https://go.dev/play/p/o_XAJtNuyGF

But they can't modify the pointer itself held within the interface, to make it point to something else.

The short version is: avoid pointers to interfaces, as the FAQ says.  Instead, let a concrete pointer value satisfy an interface.

On Friday, 25 November 2022 at 07:40:59 UTC Nigel Tao wrote:

Possibly relevant:

https://go.dev/doc/faq#pointer_to_interface

[Denis P]

Thank you everyone for your help.

The problem still exists.

I am looking for a solution where s and s2 point to the same instance.

The current example proves they are different instances.

https://go.dev/play/p/_JyfJelhIy4

The main goal is to store any type in a wrapper and then get the reference of the actual struct.

Also it must be the same instance, so the println should print 2, 2, 2

But look like there is no obvious solution. :-(

[Martin Schnabel]

On 11/25/22 10:55, Denis P wrote:
> Thank you everyone for your help.
> The problem still exists.
> I am looking for a solution where s and s2 point to the same instance.
> The current example proves they are different instances.
> https://go.dev/play/p/_JyfJelhIy4
>
> The main goal is to store any type in a wrapper and then get the
> reference of the actual struct.
> Also it must be the same instance, so the println should print 2, 2, 2
>
> But look like there is no obvious solution. :-(

There were only a couple of small mistakes. Hope an example clears it up.

https://go.dev/play/p/VhKIokDPCdq

[Axel Wagner]

If you assign a value to an interface, it will get copied. That's the same as if you assign an int to another int - it will get copied. If you want the interface to contain a reference, you have to assign a pointer to it: https://go.dev/play/p/nLw51pjWh4u

Side note: Go doesn't have "instances". It has "variables", which is probably what you mean.

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[Denis P]

Thanks everyone. :-)

The problem solved. Yay

[Deeptiman Pattnaik]

If you create a wrapper interface type and create generic constraints with the MyWrapper struct then it'll type cast with interface with (T).

https://go.dev/play/p/-VnQxG77sDL

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Thanks & Regards

Deeptiman Pattnaik

[Denis P]

Thanks guys. I have finished my homework thanks to you: https://pkg.go.dev/github.com/fairking/di_plus