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https://issues.apache.org/jira/browse/THRIFT-4060?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15861701#comment-15861701
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James E. King, III commented on THRIFT-4060:
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One use case of this is if you stream these objects to storage on the server
side, you likely need a more structured format string than provided, and if
your structure has a container of another class, you don't have the opportunity
to change the ostream << behavior (in this case, MyHigher's
set<detail::lower>). Another use case is that the generated behavior is
sufficient for some logging implementations however there may be auditing /
formatting requirements that demand more structured logging of these items as
well.
By modifying the mechanism with the annotation or compiler flag, one can say "I
am going to provide a definition for operator << of this structure" and disable
the one thrift generates. One can say {{os << higher_;}} and know that the
consuming applications output operators will be used for everything.
You made the case that due to slicing that:
{quote}there is no way for MyHigher to print out MyLower via lowers{quote}
I believe this solution makes it unnecessary to subclass Lower or Higher at
all, or to put them into a detail namespace; instead you can define how
operator << works instead of having thrift define it. This is more efficient.
> Thrift printTo ostream overload mechanism breaks down when types are nested
> ---------------------------------------------------------------------------
>
> Key: THRIFT-4060
> URL: https://issues.apache.org/jira/browse/THRIFT-4060
> Project: Thrift
> Issue Type: Bug
> Components: C++ - Compiler
> Affects Versions: 0.9.3, 0.10.0
> Environment: Ubuntu 14.04 LTS
> Reporter: James E. King, III
> Assignee: James E. King, III
>
> I'm in the middle of converting a project that provides some ostream
> operators (for logging purposes) for a number of thrift structures. The
> project was using 0.8.0 and in 0.9.3 some ostream operator overloads were
> added with THRIFT-3336. The solution that was provided here runs into
> complications if a thrift structure is contained within another one. Take
> this simple example:
> h4. Thrift
> {noformat}
> namespace cpp example.detail
> struct Lower {
> 1: binary lowerBinary
> }
> struct Higher {
> 1: set<Lower> lowers
> }
> {noformat}
> h4. C++
> {noformat}
> namespace example {
> class MyLower : public detail::Lower
> {
> virtual void printTo(std::ostream& os) const override;
> }
> class MyHigher : public detail::Higher
> {
> virtual void printTo(std::ostream& os) const override
> {
> os << lowers;
> // here's the problem, lowers is a set<detail::Lower> when it
> serializes in,
> // not a set<MyLower>, so I cannot override it...
> }
> }
> }
> {noformat}
> I'm considering adding an annotation to the thrift IDL that the compiler will
> recognize that allows someone to say, "I am going to provide my own
> operator<< for this structure, don't generate one". This would replace the
> printTo mechanism that was added in THRIFT-3336.
> Here is an example:
> {noformat}
> namespace cpp example.detail
> struct MyLower {
> 1: binary lowerBinary
> } (cpp.customostream)
> struct MyHigher {
> 1: set<Lower> lowers
> }
> {noformat}
> The annotation {{cpp.customostream}} (or the compiler option {{--gen
> cpp:no_ostream_operators}} for global effect) tells the compiler to emit only
> the declaration of the {{operator <<}} but does not emit a definition
> (implementation). It would be up to the implementation building against the
> generated code to provide an operator << if such an operator is needed for
> conversion to a stream (cout, lexical_cast to string, etc..).
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