how to convert "std::chrono::system_clock::now()" to double

Question

auto current_time = std::chrono::system_clock::now();

Using std::chrono in C++ I get current time like above.
How can I use it to get the number of seconds since the clock's epoch as a double?

Answer 1

time_since_epoch can give you a duration since the clock's epoch (whatever the epoch may be):

auto current_time = std::chrono::system_clock::now();
auto duration_in_seconds = std::chrono::duration<double>(current_time.time_since_epoch());

double num_seconds = duration_in_seconds.count();

Demo