[jira] [Assigned] (MESOS-1008) Reduce copying in stout / libprocess primitives {Try, Option, Result, Future}.

["Dominic Hamon (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/MESOS-1008?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Dominic Hamon reassigned MESOS-1008:
------------------------------------

    Assignee: Dominic Hamon  (was: Benjamin Mahler)

> Reduce copying in stout / libprocess primitives {Try, Option, Result, Future}.
> ------------------------------------------------------------------------------
>
>                 Key: MESOS-1008
>                 URL: https://issues.apache.org/jira/browse/MESOS-1008
>             Project: Mesos
>          Issue Type: Improvement
>            Reporter: Benjamin Mahler
>            Assignee: Dominic Hamon
>
> The following will discuss {{Try}}, but the same applies to {{Option}} and {{Result}}.
> Currently retrieving the value from a {{Try}} requires a copy:
> {code}
>   T get() const { ... return *t; }
> {code}
> Instead, we can return a const&:
> {code}
>   const T& get() const { ... return *t; }
> {code}
> For existing callers, this should be fairly seamless:
> {code}
>   const T& t = try.get(); // No change needed.
>   T t = try.get(); // No change needed, T is already required to be copyable.
>   try.get().mutator(); // Will no longer compile, but we should not allow this anyway!
> {code}
> {code}
>   const T& t = try.get();
>   try = T(); // t is now garbage!
>   t.foo(); // No longer works.
> {code}
> The last case is the most concerning as this mistake cannot be caught at compile-time. We could remove the assignment operators, but this seems overly restrictive. We could also guard (via {{CHECK}}) the assignment operator after a {{get()}} operation, but of course, we're also preventing valid {{Try}} usage if we go this route. The best path may simply be to leave the assignment operators as is (many callers already make copies).
> We can also add C++11 support for move to pilfer the {{Try}}, to avoid copies in the caller entirely:
> {code}
>   T&& move() const { ... } // After a move(), we must guard Try operations.
> {code}



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