使用TBB重置类成员

Question

我使用TBB并行地在队列中加载数据。在下面的类中，load_data方法工作得很好。我正在编写方法cancel_load，它应该取消加载过程，并重置arena和task，以便从一开始就重新启动加载过程。在加载过程停止后，我将使用clear_queue方法清除队列。

在方法cancel_load中，task.cancel工作得很好。但是，当我试图将成员arena和task重置为新实例时，在编译时会看到error: use of deleted function。

有人能解释一下为什么会发生这种事吗？我没有删除task对象。我只是尝试用一个新的实例来重置它。是否有更好的方法来重置这些类成员？

#include <iostream>
#include <tbb/concurrent_queue.h>
#include <tbb/task_group.h>
#include <tbb/tbb_thread.h>

class ParallelDataset {
    public:
        tbb::concurrent_bounded_queue<record_data::Record> MyQueue;
        tbb::task_arena arena;
        tbb::task_group task;

    ~ParallelDataset()
    {
        task.wait();
    }

    void load_() {
        // start loading data into MyQueue
    }

    void load_data() {
        arena.execute([&]() {
            task.run([&]() { 
                load_();
                    });       
        });
    }

    void cancel_load() {
        task.cancel();
        tbb::task_arena new_arena;
        tbb::task_group new_task;
        arena = new_arena;
        task = new_task;
        std::cout << "Data loading is cancelled. Arena and Task have been reset" << std::endl;
    }

    void clear_queue() {
        MyQueue.clear();
    }
}

误差

/home/src/dataset.cpp: In member function 'void ParallelDataset::cancel_load()':
/home/src/dataset.cpp:275:16: error: use of deleted function 'tbb::task_group& tbb::task_group::operator=(const tbb::task_group&)'
  275 |         task = new_group;
      |                ^~~~~~~~~
In file included from /home/src/dataset.cpp:6:
/usr/include/tbb/task_group.h:192:7: note: 'tbb::task_group& tbb::task_group::operator=(const tbb::task_group&)' is implicitly deleted because the default definition would be ill-formed:
  192 | class task_group : public internal::task_group_base {
      |       ^~~~~~~~~~
/usr/include/tbb/task_group.h:192:7: error: use of deleted function 'tbb::internal::task_group_base& tbb::internal::task_group_base::operator=(const tbb::internal::task_group_base&)'
/usr/include/tbb/task_group.h:92:7: note: 'tbb::internal::task_group_base& tbb::internal::task_group_base::operator=(const tbb::internal::task_group_base&)' is implicitly deleted because the default definition would be ill-formed:
   92 | class task_group_base : internal::no_copy {
      |       ^~~~~~~~~~~~~~~
/usr/include/tbb/task_group.h:92:7: error: use of deleted function 'tbb::internal::no_copy& tbb::internal::no_copy::operator=(const tbb::internal::no_copy&)'
In file included from /usr/include/tbb/task.h:23,
                 from /usr/include/tbb/parallel_for.h:24,
                 from /home/src/dataset.cpp:4:
/usr/include/tbb/tbb_stddef.h:330:7: note: 'tbb::internal::no_copy& tbb::internal::no_copy::operator=(const tbb::internal::no_copy&)' is implicitly deleted because the default definition would be ill-formed:
  330 | class no_copy: no_assign {
      |       ^~~~~~~
/usr/include/tbb/tbb_stddef.h:330:7: error: use of deleted function 'void tbb::internal::no_assign::operator=(const tbb::internal::no_assign&)'
/usr/include/tbb/tbb_stddef.h:324:10: note: declared here
  324 |     void operator=( const no_assign& ) = delete;
      |          ^~~~~~~~
In file included from /home/src/dataset.cpp:6:
/usr/include/tbb/task_group.h:92:7: error: use of deleted function 'tbb::task_group_context& tbb::task_group_context::operator=(const tbb::task_group_context&)'
   92 | class task_group_base : internal::no_copy {
      |       ^~~~~~~~~~~~~~~
In file included from /usr/include/tbb/parallel_for.h:24,
                 from /home/src/dataset.cpp:4:
/usr/include/tbb/task.h:347:7: note: 'tbb::task_group_context& tbb::task_group_context::operator=(const tbb::task_group_context&)' is implicitly deleted because the default definition would be ill-formed:
  347 | class task_group_context : internal::no_copy {
      |       ^~~~~~~~~~~~~~~~~~
/usr/include/tbb/task.h:347:7: error: use of deleted function 'tbb::internal::no_copy& tbb::internal::no_copy::operator=(const tbb::internal::no_copy&)'

Answer 1

tbb::task_arena和tbb::task_group对象是不可复制的，就像您可能从std::unique_ptr那里知道的那样。这是通过已删除的operator=实现的，它解释了编译器的错误消息。

您必须以另一种方式重新初始化您的成员，或者使用指针来实例化它们的就地替换。

在您的场景中，我假设task.cancel()和arena.terminate()应该完成这项工作。

Answer 2

基于@ypnos关于使用唯一指针的注释，我想出了下面的解决方案。

#include <iostream>
#include <tbb/concurrent_queue.h>
#include <tbb/task_group.h>
#include <tbb/tbb_thread.h>

class ParallelDataset {
public:
    tbb::concurrent_bounded_queue<record_data::Record> MyQueue;
    std::unique_ptr<tbb::task_arena> arena;
    std::unique_ptr<tbb::task_group> task;

    ParallelDataset () : arena(std::make_unique<tbb::task_arena>()),
                         task(std::make_unique<tbb::task_group>()) {}

    ~ParallelDataset()
    {
        task->wait();
    }

    void load_() {
        // start loading data into MyQueue
    }

    void load_data() {
        arena->execute([&]() {
            task->run([&]() { 
                load_();
                    });       
        });
    }

    void cancel_load() {
        task->cancel();
        arena = std::move(std::make_unique<tbb::task_arena>());
        task = std::move(std::make_unique<tbb::task_group>());
        std::cout << "Data loading is cancelled. Arena and Task have been reset" << std::endl;
    }

    void clear_queue() {
        MyQueue.clear();
    }
}