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Bug #9506

open

Cannot move callbacks

Added by Egor Pugin about 3 years ago. Updated about 3 years ago.

Status:
New
Priority:
Normal
Assignee:
-
Target version:
-
Start date:
12/15/2021
Due date:
% Done:

0%

Estimated time:

Description

Hi,

I'm posting some events to server's io loop (io_context).
But I can't move non-copyable data inside those arguments (lambdas with unique ptrs) because they are all
'const Function& ' (ApplicationEvent) or
'const std::function &' (WServer::post/schedule).

So I do ugly workarounds like this

void process_response(auto response) {
        struct handler {
            decltype(response) o;
        };
        auto h = std::make_shared<handler>(); // here make some copyable ptr
        h->o = std::move(response);
                                                                   vvv not it can be copied ok
        environment().server()->post(session()->sessionId(), [this, h]() {
            process_td_response2(std::move(h->o));
            session()->setTriggerUpdate(true); // not always?
        });
}

Wt is written in some old days conservative C and classes style.
But C++20 is out and C++23 is near, they bring great benefits to code effectiveness and code culture.
Template metaprogramming is very improved since C++98/11/14.

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 #1

Updated by Egor Pugin about 3 years ago

Possible solution is to use arguments like

std::function<void(void)> f // user will copy or std::move() his callback
or
std::function<void(void)> &&f // user will std::move() his callback
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 #2

Updated by Egor Pugin about 3 years ago

Also don't be scared of templates and pass everything directly to boost::asio::post() in case if you do not store these callbacks.

void post(auto &&f) {
    schedule(std::forward<decltype(f)>(f));
    ...
    io_context.post(std::forward<decltype(f)>(f));
}
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