C++ thread 01

Table of contents

The std::thread has a obvious drawback: it is simply a thread, it can not return any value. So if we want to get something from a thread, we can only use some global variable to exchange the result. Even we use global variable to pass result, we still don’t know when the result is ready, then we need one more variable as signal variable to record the status, that’s not acceptable.

void thread_function()
{
  fmt::print( "Thread start\n" );
  sleep( 2 );
  fmt::print( "Thread finish\n" );
  // I can't return a value :(
}

int async_function()
{
  fmt::print( "Async thread start\n" );
  sleep( 3 );
  fmt::print( "Async thread finish\n" );

  return 42;
}

int main( [[maybe_unused]] int argc, [[maybe_unused]] char **argv )
{
  fmt::print( "main start\n" );
  std::thread t1( thread_function ); // now it's running
  std::future<int> fut = std::async( std::launch::async, async_function );

  // do something else ....

  // Block main and wait for t1 finish
  fmt::print( "joining t1\n" );
  t1.join(); // before the ending, t1 should join
  fmt::print( "t1 joined\n" );

  int ret = fut.get(); // wait for function finishes
  fmt::print( "Async ret = {}\n", ret );

  fmt::print( "main finish\n" );
  return 0;
}

Possible output

main start
joining t1
Async thread start
Thread start
Thread finish
t1 joined
Async thread finish
Async ret = 42
main finish

References

• Effective Modern C++:multi-threading