Gob cannot work with pointers in a slice

[Eric Z]

Hello Everyone,

I've encountered a problem when I was trying to serialize pointers in a slice using gob package in golang. I tried to follow the example in http://blog.golang.org/gobs-of-data

The original code defines two struts

type P struct {
    X, Y, Z int
    Name    string
}

type Q struct {
    X, Y *int32
    Name string
}

Encode P and Decode Q works great for me. However, when I try to encode []P and decode []Q. It won't work unless I revise the two pointers X, Y *int32 as X, Y int32.

How can I solve this problem? I have put the error code here http://play.golang.org/p/MTmMb-1DdJ

If you change X, Y *int32 in the above code to X, Y int32. It will work but from my understanding of reflection, both should work with Gob.

Thank you very much for help.

Cheers,
Eric

[Rob Pike]

It's working as intended. You're possibly confused for two reasons.

1) You're printing pointers using %d, which will print the pointer address as a decimal number.

2) The first entry in the array has a zero, which gob will not send, so the corresponding pointer on the receiver is never allocated.

Point 2 has caused confusion for others. It's a perhaps unfortunate consequence of the way zero values are handled, but it's there.

This version works without trouble: http://play.golang.org/p/lwA7ejTYmp

So does this: http://play.golang.org/p/oqlfrct9tV

-rob

[Eric Z]

Hello Rob,

Thank you very much for the help. It solves my problem. It seems like I have check the zero values for serialization format upgrades.

Cheers,
Eric

[Dan Kortschak]

On Tue, 2013-10-22 at 10:36 -0700, Rob Pike wrote:
> Point 2 has caused confusion for others. It's a perhaps unfortunate
> consequence of the way zero values are handled, but it's there.
>

What's the recommended approach to serialising pointer fields where the
nil state versus the zero state of the value pointed to is significant?
For bool I've used Gob{Encoder,Decoder}s to map the states to ints
[0,2], but this is less nice for string, float or int (maybe map to
struct{bool; string}, struct{bool; int} or struct{bool; float}?).

Dan

[Rob Pike]

There is no recommended solution. It's an unintended consequence of the design that would be nice to address if it can be done well.

[Dan Kortschak]

Yes, I understand the reasoning behind it - I've had this discussion
with Nigel Tao[1]. I was wondering what a reasonable work around is for
the current state of encoding/gob.

[1] thread starting at
https://groups.google.com/d/msg/golang-nuts/Q5sn8851sc4/BmqdcCE-cdEJ

[Rob Pike]

The safest is to write your own transcoder for the relevant type. Otherwise, check for nil.

I do not claim the situation is satisfactory.

-rob