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

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< cpp‎ | chrono‎ | duration

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.

### Note

The period of the resulting duration is the greatest common divisor of `Period1` and `Period2`.

### Example

Run this code

    #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>
    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()
    {
        typedef std::chrono::milliseconds milliseconds;
        typedef std::chrono::microseconds microseconds;

        auto ms = milliseconds(30);
        auto us = microseconds(1100);

        std::cout << ms.count() << "ms - " << us.count() << "us = "
                  << durationDiff(ms,us).count() <<  "\n";
    }

Output:

    30ms - 1100us = 28900

### See also

common_type (C++11) | determines the common type of a group of types (class template)

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