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https://issues.apache.org/jira/browse/KUDU-2028?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16033762#comment-16033762
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Henry Robinson commented on KUDU-2028:
--------------------------------------
Ugh, this is harder than I thought: {{std::function}} and {{boost::function}}
both require copyable types, so it's impossible to construct a
{{ResponseCallback}} from a move-only lambda. All solutions I've seen fake it
out with a {{shared_ptr}} somewhere in the mix.
> Async Proxy callbacks cannot capture non-copyable types
> -------------------------------------------------------
>
> Key: KUDU-2028
> URL: https://issues.apache.org/jira/browse/KUDU-2028
> Project: Kudu
> Issue Type: Improvement
> Reporter: Henry Robinson
> Assignee: Henry Robinson
> Priority: Minor
>
> Callbacks that are passed to {{Proxy::AsyncRequest()}} are passed by
> reference. They are then passed by value into {{OutboundCall()}}, where that
> copy is saved.
> This means that you can't have a callback that captures a non-copyable type:
> {code}
> unique_ptr<Foo> my_foo = make_unique<Foo>();
> auto cb = [f(move(my_foo))]() {
> };
> proxy->AsyncRequest("method", req, response, controller, cb); // <-- error,
> missing copy c'tor for unique_ptr<Foo>
> {code}
> This is a sometimes useful idiom for passing ownership of something into the
> callback, where it can get destroyed some time in the future, after the
> caller has ceased to exist.
> A solution would be for {{AsyncRequest()}} (and its generated wrappers) to
> take a {{ResponseCallback}} by value, and to {{move()}} it until it was
> copied. This kind of transformation has its drawbacks (if for some reason the
> parameter doesn't get copied, it is more expensive than pass-by-reference),
> but seems a reasonable tradeoff here.
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