std::min of std::chrono::duration of different types

Question

Consider following code:

// durations are from std::chrono
auto a = get_duration_1(); // milliseconds, will vary in future versions
auto b = get_duration_2(); // seconds, will vary in future versions
auto c = std::min(a, b);

It doesn't compile because compiler cannot instantiate correct version of std::min because of different argument types.

Of course, now it's possible to specify type explicitly using std::min<milliseconds>. In future versions of this code, types will vary. What is the generic way of doing that without knowing exact duration types?

Answer 1

Given two durations, D1 d1 and D2 d2 ...

You can convert both durations to their common type, std::common_type_t<D1, D2>, and then find the minimum of those values.

Or just call std::min<std::common_type_t<D1, D2>>(d1, d2) and let them be converted to that type as needed.

This works because std::common_type is specialized to do the right thing for duration types, see [time.traits.specializations] in the C++ standard.

Answer 2

You can use the following function:

#include <chrono>

template <typename T1, typename T2>
auto generic_min(const T1& duration1, const T2& duration2)
{
    using CommonType = typename std::common_type<T1,T2>::type;
    const auto d1 = std::chrono::duration_cast<CommonType>(duration1);
    const auto d2 = std::chrono::duration_cast<CommonType>(duration2);
    return std::min(d1,d2);
}