[jira] [Commented] (AVRO-3710) C++ - don't take ownership of I/OStream in DataFile

["Pietro Cerutti (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/AVRO-3710?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17870951#comment-17870951 ] 

Pietro Cerutti commented on AVRO-3710:
--------------------------------------

Thanks, [~gerrit0]!

> C++ - don't take ownership of I/OStream in DataFile
> ---------------------------------------------------
>
>                 Key: AVRO-3710
>                 URL: https://issues.apache.org/jira/browse/AVRO-3710
>             Project: Apache Avro
>          Issue Type: Improvement
>          Components: c++
>            Reporter: Pietro Cerutti
>            Priority: Minor
>              Labels: pull-request-available
>          Time Spent: 40m
>  Remaining Estimate: 0h
>
> In AVRO-2014, I raised concerns regarding how DataFile(Reader|Writer)[Base] take their streams by unique_ptr.
> Here, I would like to propose a fix.
> The problem is that, because streams are taken by unique_ptr, caller code doesn't have access to the streams after the construction of the DataFile* object. This makes it impossible to use custom streams, or even the built-in memory streams:
> {code:cpp}
> auto schema{ giveMeMySchema(); }
> auto os{ avro::memoryOutputStream() };
> DataFileWriter w{ os, schema };
> auto is{ memoryInputStream(*os) }; // ouch: os is gone
> doSomethingWithTheAvroStream(is); 
> {code}
>  
> I am proposing an almost backwards-compatible change, which is to change the DataFile classes to take and hold the streams by shared_ptr.
> The semantics for client code don't change: you can still move a unique_ptr<Stream> into the DataFile constructor, and in that case the DataFile will be the only owner.
> But this enable a client from doing something like this:
> {code:cpp}
> auto schema{ giveMeMySchema(); }
> auto os{ avro::memoryOutputStream() };
> std::shared_ptr<OutputStream> shared_os{ os.get(), boost::null_deleter{} };
> DataFileWriter w{ shared_os, schema };
> auto is{ memoryInputStream(*os) }; // good: os is alive
> doSomethingWithTheAvroStream(is); 
> {code}
> Alternatively / additionally, DataFile could also take the stream by ref, and build a shared_ptr with a null_deleter internally. This would make the API in line with the Encoder.
> I will submit a PR if this is accepted.



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