How to cast a double into std::chrono::milliseconds

One nice trick is to multiply your values with chrono literals:

using namespace std::chrono_literals;

double time = 82.0;
auto t_82ms = time * 1ms;
std::this_thread::sleep_for(t_82ms);

It also works the other way around:

double time = t_82ms / 1s; // time == 0.082

m_timer.expires_after will accept any duration which is convertible to boost::asio::steady_timer::duration it doesn't need to be std::chrono::milliseconds (and if you don't want to discard the fractional milliseconds from your duration you shouldn't be converting to std::chrono::milliseconds).

You can convert your double into a std::chrono::duration as follows:

double milliseconds = 0.1;
std::chrono::duration<double, std::milli> chrono_milliseconds{ milliseconds };

chrono_milliseconds can't however be passed automatically into expires_after as there is no automatic conversion from floating point durations to integer ones. You can fix this with a std::chrono::duration_cast:

m_timer.expires_after(
  std::chrono::duration_cast<boost::asio::steady_timer::duration>(chrono_milliseconds));