[jira] [Commented] (THRIFT-4484) C++ codegen invalid for optional self-membership

["Craig Ringer (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/THRIFT-4484?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16348660#comment-16348660 ] 

Craig Ringer commented on THRIFT-4484:
--------------------------------------

[~davejwatson] Brilliant! Apparently it's now spelled "&", e.g.

 
{code}
-    6: optional Downstream downstream
+    6: optional Downstream &downstream
{code}

but that does the job.

It'd be nice if Thrift detected a struct that self-referenced by value, bailed out with a compile error and offered this as a hint. But hopefully now others will find this bug if they search for the error messages, and as I've spent less than a day looking at thrift IDL (working on this Jaeger issue) let alone the backend, it's sure beyond me.

Thanks so much for the pointer. Heh.

> 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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