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https://issues.apache.org/jira/browse/THRIFT-4484?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16346405#comment-16346405
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Jens Geyer edited comment on THRIFT-4484 at 1/31/18 8:08 AM:
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The more interesting question is whether it makes sense to do that.
Let's say you have an instance A that points to itself and then gets
serialized.
Now you de-serialize the bits and get what ... an A that contains another copy
of A = *two instances*.
And that is a well-known limitation of this feature.
I made another proposal a while ago that essentially introduces a new
{{graph<T>}} container type, THRIFT-2005. That can still be implemented, if
anyone needs it. It does not interfere with the other feature, as it buildson
top of {{list<T>}} but it very likely will need some overhaul to compile.
was (Author: jensg):
The more interesting question is whether it makes sense to do that.
Let's say you have an instance A that points to itself and then gets
serialized.
Now you de-serialize the nbits and get what ... an A that contains another copy
of A = *two instances*.
And that is a well-known limitation of this feature.
I made another proposal a while ago that essentially introduces a new
{{graph<T>}} container type, THRIFT-2005. That can still be implemented, if
anyone needs it. It does not interfere with the other feature, as it buildson
top of {{list<T>}} but it very likely will need some overhaul to compile.
> C++ codegen invalid for optional self-membership
> ------------------------------------------------
>
> Key: THRIFT-4484
> URL: https://issues.apache.org/jira/browse/THRIFT-4484
> Project: Thrift
> Issue Type: Bug
> Components: C++ - Compiler
> Affects Versions: 0.11.0
> Environment: Thrift 0.10.0 tested, but I don't see a change in
> 0.11.0. Fedora 25. gcc 6.4.1. x86_64.
> Reporter: Craig Ringer
> Priority: Minor
>
> Support was added for self-referential objects in in
> [https://github.com/apache/thrift/pull/84] "Tree/Recursive struct support in
> thrift".
>
> The tests cover objects that are co-recursive, objects that have lists of
> themselves, etc. But there's no test for optional self-containment e.g.
> {code}
> struct RecSelf {
> 1: i16 item
> 2: optional RecSelf
> }
> {code}
> This works fine for languages like Java and Go. But in C++ it generates
> nonsensical code that cannot compile because it contains a by-value member of
> its self and a separate {{isset}} member.
> For example, from opentracing jaeger's IDL:
> {code}
> struct Downstream {
> 1: required string serviceName
> 2: required string serverRole
> 3: required string host
> 4: required string port
> 5: required Transport transport
> 6: optional Downstream downstream
> }
> {code}
> code-generation produces
> {code}
> class Downstream {
> public:
>
> /* blah elided blah */
> virtual ~Downstream() throw();
> std::string serviceName;
> std::string serverRole;
> std::string host;
> std::string port;
> Transport::type transport;
> Downstream downstream;
> _Downstream__isset __isset;
> /* blah elided blah */
> };
> {code}
> Compilation fails with
> {code}
> tracetest_types.h:64:14: error: field ‘downstream’ has incomplete type
> ‘jaegertracing::crossdock::thrift::Downstream’
> Downstream downstream;
> ^~~~~~~~~~
> tracetest_types.h:47:7: note: definition of ‘class
> jaegertracing::crossdock::thrift::Downstream’ is not complete until the
> closing brace
> class Downstream {
> ^~~~~~~~~~
> {code}
> I'd argue that the {{__isset}} model is not ideal, and a
> {{std::expected}}-like "optional" or "maybe" construct would be a lot better.
> But presumably there are historical reasons for that.
> The simplest correct solution would be to generate
> {code}
> class Downstream {
> /* ... */
> std::shared_ptr<Downstream> downstream;
> /* ... */
> };
> {code}
> instead.
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