Re: [go-nuts] panic: interface conversion: interface is nil, not encoding.BinaryUnmarshaler

[Robert Engels]

I think you will need to encode a struct that contains the type name and the struct. Then you decode using reflection.

-----Original Message-----
From: 'Axel Wagner' via golang-nuts
Sent: Aug 15, 2019 3:12 AM
To: Jochen Voss
Cc: golang-nuts
Subject: Re: [go-nuts] panic: interface conversion: interface is nil, not encoding.BinaryUnmarshaler

I haven't really used gob much, so unfortunately I can't *really* help you. But one thing to note is that you can dump the content of the buffer and see that it doesn't actually contain the name of the type you are encoding: https://play.golang.org/p/R8HB6RP8kS0

I agree that from the documentation, it seems to me that it *should* and I can't really see what this is doing wrong. And I agree that this probably shouldn't panic, but instead return an error. But given that the encoded bytes don't contain the name of the concrete struct, there is no way the decoder can know where you want to put it, without giving it the correct type.

On Thu, Aug 15, 2019 at 9:22 AM Jochen Voss <jochen.v...@gmail.com> wrote:

Dear Marcin,

My aim is to unmarshal into an interface variable, without having to know in advance which concrete type I'm receiving (the actual interface has several possible implementations, and gob data comes in over a network connection). So, while your code avoids the panic, it does not solve my problem.

Many thanks,

Jochen

On Wednesday, 14 August 2019 22:12:54 UTC+1, Marcin Romaszewicz wrote:

Here you go:

https://play.golang.org/p/_cPmaSxRatC

You want to unmarshal into &A, not into &Duck

This means:

var duck2 A

not:

var duck2 Duck

On Wed, Aug 14, 2019 at 8:46 AM Jochen Voss <joche...@gmail.com> wrote:

Hello,

I'm trying to read gob-encoded data into a variable of interface type.  In simple cases, this works for me, but when I use a custom encoding via MarshalBinary() and UnmarshalBinary() methods, my code keeps crashing, with the error message

    panic: interface conversion: interface is nil, not encoding.BinaryUnmarshaler

Example:

- The code at https://play.golang.org/p/y8nWNhObUwb, letting gob do the en-/decoding works.

- The very similar code at https://play.golang.org/p/-zS7QjEJg9x, which uses MarshalBinary() and UnmarshalBinary() crashes with the panic shown above.

What am I doing wrong?

[I asked the same question at https://stackoverflow.com/questions/57485698/, with no answers so far]

Many thanks,

Jochen

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