break thread pool async implementation

gistfile1.cpp

#include <iostream>
#include <future>
#include <vector>
#include <cassert>
 
struct S {
  bool on_main_thread = false;
  int tasks_run_on_this_thread = 0;

  S() { std::cerr << "S ctor\n"; }
  ~S() { std::cerr << "S dtor\n"; }
};

thread_local S s;

int main() {
  std::vector<std::future<void> > v;

  const int task_count = 10;
  for (int i = 0; i < task_count; ++i) {
    v.emplace_back(std::async([]{
      assert(s.tasks_run_on_this_thread == 0 || s.on_main_thread);
      ++s.tasks_run_on_this_thread;
    }));
  }

  s.on_main_thread = true;
  for (auto &&t : v) {
    t.get();
  }
  std::cout << "Tasks run asynchronously: "
            << task_count - s.tasks_run_on_this_thread
            << " (Should be this many + 1 ctors/dtors run)\n";
}