reflect: MakeStruct

[sbinet]

Feature request for MakeStruct, to go along with the newly-arrived MakeFunc ( issue #1765
).

It would be greatly useful for interacting with libraries expecting a certain binary
layout, and also to have a programmatic interface to code against.

[cznic]

Comment 1:

Would you mind to somehow describe what MakeStruct is/does?

[sbinet]

Comment 2:

sure (actually, for consistency with other functions in 'reflect', let's call it
StructOf):
func StructOf(name string, fields []StructField) Type
this would generate a reflect.structType with the correct field names, offsets and
alignment informations.
each field in []StructField would not need to be completely initialized ie: only
StructField.Name and StructField.Type would be mandatory
or perhaps introduce a new struct Field with only those mandatory informations and use
that instead of StructField ?
-s

[cznic]

Comment 3:

Thanks. IIUC, one would not be able to directly reference any field of the run-time
defined struct, like in 'InstanceName.FieldName', because the type is not known at
compile time. The fields would be gettable/settable only through reflect again.
I don't think Go needs such functionality. If eg., as you wrote, precise binary layout
is required by a non Go library, I think some C glue should be written instead. That's
fast and accessible in both of the worlds. Abusing reflection for run-time type-unsafe
bypassing of tasks which can be type-safely handled at compile time seems like a bad
idea to me.

[sbinet]

Comment 4:

well, another use-case would be a reflect-based go interpreter.
but I am not sure that would convince you either :)
-s

[ianlancetaylor]

Comment 5:

Labels changed: added priority-later, removed priority-triage.

Owner changed to @rsc.

[robpike]

Comment 6:

Status changed to Accepted.

[rsc]

Comment 7:

Labels changed: added go1.3maybe.

[rsc]

Comment 8:

Labels changed: added release-none, removed go1.3maybe.

[rsc]

Comment 9:

Labels changed: added repo-main.

[glycerine]

Comment 10:

I'd like (it would be terrific) to be able to create structs from data loaded at runtime.

[gopherbot]

Comment 11 by bprosnitz@google.com:

It would be very useful for my current project to be able to create structs type objects
from a type description at runtime.

[gopherbot]

Comment 12 by carlchatfield:

Here is a patch for this. I'm new to compiler foo so It would be good if someone could
if someone could check this over. Especially the field alignment and GC.

Attachments:

1. structof.patch (10651 bytes)

[glycerine]

Comment 13:

woot! excellent!! I'll try it out shortly.

[ianlancetaylor]

Comment 14:

Thanks for working on this.  Please submit patches through the process described at
http://golang.org/doc/contribute.html.  Thanks.
I think that the comparison algorithm isn't quite right.  I don't think it will work for
struct { a, b string }.

[adonovan]

Comment 15:

I agree this would be a useful feature for programs that create a graph of compact
objects based on a dynamically loaded schema, e.g. a .proto file or XML schema, or the
stack frames of an interpreter.
0xjnml: it needn't be type-unsafe.  So long as MakeStruct rejects structs whose fields
overlap, you can't create pointer/uintptr union types.
MakeStruct should probably also reject anonymous fields, since the resulting struct
could have a non-empty method-set but would have compiler-generated wrapper methods for
them.