A type alias gives an existing type another name. In C++, using provides a more general alternative to typedef, making lengthy types easier to read and reuse. section 4.2.2
Naming a custom duration
A century can be represented with a named chrono duration: using centuries = std::chrono::duration<long long, std::ratio_multiply<std::chrono::years::period, std::hecto>>;. Afterward, centuries{2} can be converted to seconds or hours directly, while conversion to days requires duration_cast because the result may contain a fractional day. section 4.2.2
Listing 4.5 Defining a duration
void defining_a_duration()
{
using namespace std::chrono;
using centuries = duration<long long,
std::ratio_multiply<years::period, std::hecto>>; #1
centuries two_hundred_years = centuries(2); #1
seconds sec = two_hundred_years; #2
hours hrs = two_hundred_years; #2
days day_count = duration_cast<days>(two_hundred_years); #3
std::cout << "Two centuries is approximately " << day_count << '\n';
}
See the complete custom-duration example, including the alias, duration construction, implicit conversions, and explicit conversion to days. section 4.2.2
