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I want to defer the execution of a packaged task in a loop.

class ILoop { public: virtual void Deffer(std::function<void()>&& task) = 0; void Deffer(std::function<void()>& task) = delete; static void set_loop(ILoop*); static ILoop& loop(); private: static ILoop* loop_; }; template <typename T> struct LoopExecutor { std::future<T> Commit(std::function<T()>&& task) { std::packaged_task<T()> wrapper([t = std::move(task)] { return t(); }); std::future<T> future = wrapper.get_future(); ILoop::loop().Deffer([wrapper = std::move(wrapper)] { wrapper(); }); return future; } };

when I try to compile this

auto t = executor.Commit([] { return 100; });

I have compilation error:

[1/2] Building CXX object unit_tests/CMakeFiles/reduct_tests.dir/reduct/async/awaiters_test.cc.o FAILED: unit_tests/CMakeFiles/reduct_tests.dir/reduct/async/awaiters_test.cc.o /usr/bin/g++-11 -I/home/atimin/.conan/data/fmt/8.0.1/_/_/package/20da98908a15523bbe9f086a5c57a4bde424a9c0/include -I/home/atimin/.conan/data/uwebsockets/20.8.0/_/_/package/c84c7dca9672f88b95bafb2f5754a22669d1bbe5/include -I/home/atimin/.conan/data/uwebsockets/20.8.0/_/_/package/c84c7dca9672f88b95bafb2f5754a22669d1bbe5/include/uWebSockets -I/home/atimin/.conan/data/nlohmann_json/3.9.1/_/_/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/include -I/home/atimin/.conan/data/gtest/1.11.0/_/_/package/63868df56b76903d4ad40ecbd5b2e8238cee50c9/include -I/home/atimin/.conan/data/catch2/2.13.7/_/_/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/include -I/home/atimin/.conan/data/zlib/1.2.11/_/_/package/be27726f9885116da1158027505be62e913cd585/include -I/home/atimin/.conan/data/usockets/0.8.1/_/_/package/f3766c5a18b2feee4dc2c3a94335008aaaea2543/include -I/home/atimin/Projects/flipback/reduct-storage/src -I/home/atimin/Projects/flipback/reduct-storage/cmake-build-debug-gcc11 -I/home/atimin/Projects/flipback/reduct-storage/unit_tests -g -DLIBUS_NO_SSL -std=gnu++20 -MD -MT unit_tests/CMakeFiles/reduct_tests.dir/reduct/async/awaiters_test.cc.o -MF unit_tests/CMakeFiles/reduct_tests.dir/reduct/async/awaiters_test.cc.o.d -o unit_tests/CMakeFiles/reduct_tests.dir/reduct/async/awaiters_test.cc.o -c /home/atimin/Projects/flipback/reduct-storage/unit_tests/reduct/async/awaiters_test.cc In file included from /home/atimin/Projects/flipback/reduct-storage/src/reduct/async/awaiters.h:10, from /home/atimin/Projects/flipback/reduct-storage/unit_tests/reduct/async/awaiters_test.cc:3: /home/atimin/Projects/flipback/reduct-storage/src/reduct/async/executors.h: In instantiation of ‘std::future<_Res> reduct::async::LoopExecutor<T>::Commit(std::function<T()>&&) [with T = int]’: /home/atimin/Projects/flipback/reduct-storage/src/reduct/async/awaiters.h:40:68: required from ‘reduct::async::Run<T, Executor>::Run(std::function<T()>&&) [with T = int; Executor = reduct::async::LoopExecutor<int>]’ /home/atimin/Projects/flipback/reduct-storage/unit_tests/reduct/async/awaiters_test.cc:52:92: required from here /home/atimin/Projects/flipback/reduct-storage/src/reduct/async/executors.h:19:66: error: no match for call to ‘(const std::packaged_task<int()>) ()’ 19 | ILoop::loop().Deffer([wrapper = std::move(wrapper)] { wrapper(); }); | ~~~~~~~^~ In file included from /home/atimin/Projects/flipback/reduct-storage/src/reduct/async/awaiters.h:8, from /home/atimin/Projects/flipback/reduct-storage/unit_tests/reduct/async/awaiters_test.cc:3: /usr/include/c++/11/future:1577:7: note: candidate: ‘void std::packaged_task<_Res(_ArgTypes ...)>::operator()(_ArgTypes ...) [with _Res = int; _ArgTypes = {}]’ (near match) 1577 | operator()(_ArgTypes... __args) | ^~~~~~~~ /usr/include/c++/11/future:1577:7: note: passing ‘const std::packaged_task<int()>*’ as ‘this’ argument discards qualifiers In file included from /usr/include/c++/11/future:47, from /home/atimin/Projects/flipback/reduct-storage/src/reduct/async/awaiters.h:8, from /home/atimin/Projects/flipback/reduct-storage/unit_tests/reduct/async/awaiters_test.cc:3: /usr/include/c++/11/bits/std_function.h:414:9: error: ‘std::function<_Res(_ArgTypes ...)>::function(_Functor) [with _Functor = reduct::async::LoopExecutor<int>::Commit(std::function<int()>&&)::<lambda()>; <template-parameter-2-2> = void; <template-parameter-2-3> = void; _Res = void; _ArgTypes = {}]’, declared using local type ‘reduct::async::LoopExecutor<int>::Commit(std::function<int()>&&)::<lambda()>’, is used but never defined [-fpermissive] 414 | function(_Functor __f)

Actually, even a simpler code (no Defer function) is also not working:

 auto t = [w = std::move(wrapper)] { w(); }; t();

On the other hand, it is ok:

 std::thread t(std::move(wrapper));

I use GCC 11.2.

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  • You should read syntax for lambda. [](){} declares a lambda. You forgot to write () in your declaration. Commented Dec 29, 2021 at 13:14
  • 1
    @ALX23z The () for an empty parameter list are optional in a lambda. Commented Dec 29, 2021 at 13:16
  • @user17732522 it is ok. I found it in the standard. I think it is the purpose of std::packaged_task to move it into some loop or thread to execute in parallel but be able to get the result in the main thread. Commented Dec 29, 2021 at 13:38

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3

By default a lambda's call operator is const-qualified.

Inside the lambda's body the this pointer to the lambda is therefore also const-qualified and so is the member wrapper. std::packaged_task does not have a const-qualified operator(), so it cannot be called.

You can make the lambda's operator() non-const-qualified by adding the mutable keyword:

[wrapper = std::move(wrapper)]()mutable{ wrapper(); }

Note that currently the syntax for a lambda with a specifier does require the empty parameter list to be given as ().

However, you have a bigger issue: Storing a std::packaged_task inside the lambda makes the lambda non-copyable. But std::function requires the stored type to be copyable. You cannot use it for this use case, see also How to create an std::function from a move-capturing lambda expression?

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3 Comments

Thanks, you're right! Only one question more, how can I declare the mutable lambda explicitly. My Deffer method expects immutable lambda - void Deffer(std::function<void()>&& task) and it is not compiling now.
Unfortunately not, so you can't compile: std::function<void()> f = []() mutable {}; But with auto ofc, it is working. My problem is that i have an interface and I don't know so far the proper signature. But it may be another topic.
@flipback I see now what the issue is. I updated the answer. Unfortunately your approach will not work.

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