std::try_lock

// std::lock example
#include <iostream>       // std::cout
#include <thread>         // std::thread
#include <mutex>          // std::mutex, std::try_lock
std::mutex foo,bar;
void task_a () {
  foo.lock();
  std::cout << "task a\n";
  bar.lock();
  // ...
  foo.unlock();
  bar.unlock();
}
void task_b () {
  int x = try_lock(bar,foo);
  if (x==-1) {
    std::cout << "task b\n";
    // ...
    bar.unlock();
    foo.unlock();
  }
  else {
    std::cout << "[task b failed: mutex " << (x?"foo":"bar") << " locked]\n";
  }
}
int main ()
{
  std::thread th1 (task_a);
  std::thread th2 (task_b);
  th1.join();
  th2.join();
  return 0;
}