# std::common_type(std::chrono::duration)

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< cpp‎ | chrono‎ | duration

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`.

### Note

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.

### Example

Run this code

    #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';
    }

Output:

    30ms - 1100μs = 28900μs

### See also

std::common_type<std::chrono::time_point> (C++11) | specializes the std::common_type trait (class template specialization)
common_type (C++11) | determines the common type of a group of types (class template)

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