Defined in header <chrono>
template <class Rep1, class Period1, class Rep2, class Period2>
struct common_type<std::chrono::duration<Rep1, Period1>,
                   std::chrono::duration<Rep2, Period2>> {
    typedef std::chrono::duration<
        typename std::common_type<Rep1, Rep2>::type, /*see note*/> type;
(since C++11)

Exposes the type named type, which is the common type of two std::chrono::durations, whose period is the greatest common divisor of Period1 and Period2.


The period of the resulting duration can be computed by forming a ratio of the greatest common divisor of Period1::num and Period2::num and the least common multiple of Period1::den and Period2::den.


#include <type_traits>
#include <iostream>
#include <chrono>
// std::chrono already finds the greatest common divisor,
// likely using std::common_type<>. We make the type
// deduction externally. 
template <typename T,typename S>
constexpr auto durationDiff(const T& t, const S& s)
    -> typename std::common_type<T,S>::type
    typedef typename std::common_type<T,S>::type Common;
    return Common(t) - Common(s);
int main() 
    using namespace std::literals;
    constexpr auto ms = 30ms;
    constexpr auto us = 1100us;
    constexpr auto diff = durationDiff(ms, us);
    std::cout << ms.count() << "ms - " << us.count() << "μs = " << diff.count()
              << (std::is_same<decltype(diff), decltype(ms)>() ? "ms" : "μs")
              << '\n';


30ms - 1100μs = 28900μs

See also

specializes the std::common_type trait
(class template specialization)
determines the common type of a group of types
(class template)

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