C++ std::vector of independent std::threads

Question

I´m building a real time software where I have a main infinite loops on main() and threads used to read and process data.

One of the issues is keeping a

Answer 1

/usr/local/include/c++/5.1.0/ext/new_allocator.h: In instantiation of 'void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::thread; _Args = {const std::thread&}; _Tp = std::thread]':
/usr/local/include/c++/5.1.0/bits/alloc_traits.h:256:4:   required from 'static std::_Require::__has_construct<_Tp, _Args ...> > std::allocator_traits<_Alloc>::_S_construct(_Alloc&, _Tp*, _Args&& ...) [with _Tp = std::thread; _Args = {const std::thread&}; _Alloc = std::allocator; std::_Require::__has_construct<_Tp, _Args ...> > = void]'
/usr/local/include/c++/5.1.0/bits/alloc_traits.h:402:16:   required from 'static decltype (_S_construct(__a, __p, (forward<_Args>)(std::allocator_traits::construct::__args)...)) std::allocator_traits<_Alloc>::construct(_Alloc&, _Tp*, _Args&& ...) [with _Tp = std::thread; _Args = {const std::thread&}; _Alloc = std::allocator; decltype (_S_construct(__a, __p, (forward<_Args>)(std::allocator_traits::construct::__args)...)) = ]'
/usr/local/include/c++/5.1.0/bits/stl_vector.h:917:30:   required from 'void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::thread; _Alloc = std::allocator; std::vector<_Tp, _Alloc>::value_type = std::thread]'
main.cpp:37:30:   required from here
/usr/local/include/c++/5.1.0/ext/new_allocator.h:120:4: error: use of deleted function 'std::thread::thread(const std::thread&)'
  { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }

Lets peel this back a bit.

error: use of deleted function 'std::thread::thread(const std::thread&)'

Your code is doing something that attempts to introduce a std::thread.

required from 'void std::vector<_Tp, _Alloc>::push_back(const value_type&)

push_back is the culprit.

std::thread is not copyable - what would it mean to copy a thread?

std::thread t1([](){});
std::thread t2 = t1;

So instances of std::thread objects are intended to be unique owners. Aside from the simple confusion, much pain would ensue.

They are, however, movable.

std::thread t1([](){});
std::thread t2 = std::move(t1);

t1 is no-longer a valid thread descriptor, the thread it was describing is now owned by t2.

For placing such things into a container you can either use std::move or std::emplace/std::emplace_back.

std::vector threads;
threads.push_back(std::move(std::thread([](){})));
threads.emplace_back([](){});

While your code is focusing on this particular issue, let me point out that the C++ standard declares it as an error for a thread destructor to be invoked while a thread is still attached and not joined.

int main() {
    std::thread t1([](){ while (true) { std::this_thread::yield(); } };
}

When main terminates, t1.~thread() is called, which detects that the thread is still attached and not joined, this raises an exception causing a shutdown crash.

You either need to join() the thread, waiting for it to terminate running, or detach() it. You'll need some way to tell the thread to stop if you want to use join() and if you detach() the program may exit in the middle of the thread doing something like writing data etc, you may introduce a serious bug.

#include 
#include 
#include 

int main () {
  std::promise cnx_promise;
  std::shared_future cnx_future;

  std::thread t1([cnx_future]() {
      while (cnx_future.valid()) {
        std::this_thread::yield();
      }
  });

  std::this_thread::sleep_for(std::chrono::seconds(1));

  cnx_promise.set_value();

  t1.join();
}

Here we use a promise to let the thread know when it's time to stop running, but you could use condition variables, signals, etc, or even just a simple std::atomic ok_to_run { true }; that you test for false.