C++中的线程安全数据结构

Question

Like to make a thread safe C++ data structure as follows.

struct infos{
  int crowdinfos[10][horGridNums*verGridNums];
  int cameraSourceID;
  static int idx;
  std::mutex mutex;
  
};

mutex.lock and unlock will be used to be thread safe.

EDIT: In my header file, I'll have a vector of infos.

std::vector<infos> c_infos;

When I build with g++, I have errors as

/usr/include/c++/7/bits/stl_construct.h:75:7: error: use of deleted function ‘infos::infos(const infos&)’
     { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from gstdsexample.cpp:29:0:
gstdsexample.h:66:8: note: ‘infos::infos(const infos&)’ is implicitly deleted because the default definition would be ill-formed:
 struct infos{
        ^~~~~
gstdsexample.h:66:8: error: use of deleted function ‘std::mutex::mutex(const std::mutex&)’

The whole log infos in build are as follows.

-fPIC -DDS_VERSION="5.0.0" -I /usr/local/cuda-10.2/include -I ../../includes -pthread -I/usr/include/gstreamer-1.0 -I/usr/include/orc-0.4 -I/usr/include/gstreamer-1.0 -I/usr/include/glib-2.0 -I/usr/lib/aarch64-linux-gnu/glib-2.0/include -I/usr/include/opencv4/opencv -I/usr/include/opencv4
g++ -c -o gstdsexample.o -fPIC -DDS_VERSION=\"5.0.0\" -I /usr/local/cuda-10.2/include -I ../../includes -pthread -I/usr/include/gstreamer-1.0 -I/usr/include/orc-0.4 -I/usr/include/gstreamer-1.0 -I/usr/include/glib-2.0 -I/usr/lib/aarch64-linux-gnu/glib-2.0/include -I/usr/include/opencv4/opencv -I/usr/include/opencv4 gstdsexample.cpp
In file included from /usr/include/c++/7/bits/stl_tempbuf.h:60:0,
                 from /usr/include/c++/7/bits/stl_algo.h:62,
                 from /usr/include/c++/7/algorithm:62,
                 from /usr/include/opencv4/opencv2/core/base.hpp:55,
                 from /usr/include/opencv4/opencv2/core.hpp:54,
                 from /usr/include/opencv4/opencv2/imgproc.hpp:46,
                 from /usr/include/opencv4/opencv2/imgproc/imgproc.hpp:48,
                 from gstdsexample.h:30,
                 from gstdsexample.cpp:29:
/usr/include/c++/7/bits/stl_construct.h: In instantiation of ‘void std::_Construct(_T1*, _Args&& ...) [with _T1 = infos; _Args = {const infos&}]’:
/usr/include/c++/7/bits/stl_uninitialized.h:83:18:   required from ‘static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const infos*, std::vector<infos> >; _ForwardIterator = infos*; bool _TrivialValueTypes = false]’
/usr/include/c++/7/bits/stl_uninitialized.h:134:15:   required from ‘_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const infos*, std::vector<infos> >; _ForwardIterator = infos*]’
/usr/include/c++/7/bits/stl_uninitialized.h:289:37:   required from ‘_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = __gnu_cxx::__normal_iterator<const infos*, std::vector<infos> >; _ForwardIterator = infos*; _Tp = infos]’
/usr/include/c++/7/bits/stl_vector.h:331:31:   required from ‘std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&) [with _Tp = infos; _Alloc = std::allocator<infos>]’
gstdsexample.cpp:444:40:   required from here
/usr/include/c++/7/bits/stl_construct.h:75:7: error: use of deleted function ‘infos::infos(const infos&)’
     { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from gstdsexample.cpp:29:0:
gstdsexample.h:66:8: note: ‘infos::infos(const infos&)’ is implicitly deleted because the default definition would be ill-formed:
 struct infos{
        ^~~~~
gstdsexample.h:66:8: error: use of deleted function ‘std::mutex::mutex(const std::mutex&)’
In file included from /usr/include/c++/7/mutex:43:0,
                 from /usr/include/opencv4/opencv2/core/utility.hpp:62,
                 from /usr/include/opencv4/opencv2/core.hpp:3291,
                 from /usr/include/opencv4/opencv2/imgproc.hpp:46,
                 from /usr/include/opencv4/opencv2/imgproc/imgproc.hpp:48,
                 from gstdsexample.h:30,
                 from gstdsexample.cpp:29:
/usr/include/c++/7/bits/std_mutex.h:97:5: note: declared here
     mutex(const mutex&) = delete;

What could be the correct way of using inside Struct?

Answer 1

As you haven't posted your code entirely, I'm not sure what your problem is. So please look at my example below instead.

Here is how I use a mutex for a queue. By looking at it you can adapt your struct for whatever it is you are trying to accomplish.

class Queue {
public:
    Queue() = default;

    /**
     * Push a message to the queue.
     */
    void push(const std::array<int, 4>& message) {
        while (true) {
            std::unique_lock<std::mutex> locker(mu);
            cond.wait(locker, [this](){ return buffer_.size() < size_; });
            buffer_.push_back(message);
            locker.unlock();
            cond.notify_all();
            return;
        }
    }

    /**
     * Pop a message off the queue.
     */
    bool pop(std::array<int, 4>& value) {
        while (true) {
            std::unique_lock<std::mutex> locker(mu);
            if (buffer_.size() == 0) {
                return false;
            }
            value = buffer_.front();
            buffer_.pop_front();
            locker.unlock();
            cond.notify_all();
            return true;
        }
    }

private:
    std::mutex mu;
    std::condition_variable cond;

    std::deque<std::array<int, 4>> buffer_;
    const unsigned int size_ = 200;
    
};

In a nutshell, wherever you want thread safe access to some object do something like:

std::unique_lock<std::mutex> locker(mu);
            
//Add Your code on the object you want thread safe access to.

            locker.unlock();
            cond.notify_all();

The important thing is, that you only access the object you want thread safe access to within the locker / unlock as above, and that you notify anyone waiting on the object as described above.

Please see your error message:

‘infos::infos(const infos&)’ is implicitly deleted because the default definition would be ill-formed:  struct infos{