Why does std::chrono::time_point not behave as expected?

Question

#include <chrono>

int main()
{
    using clock = std::chrono::system_clock;
    using time_point = std::chrono::time_point<clock>;

    auto tp_now = clock::now();
    auto tp_min = time_point::min();

    bool b1 = tp_now > tp_min;
    bool b2 = (tp_now - tp_min) > std::chrono::seconds{ 0 };
    cout << boolalpha << b1 << endl << b2 << endl;
}

The expected output is:

true

true

But the actual output is:

true

false

Why does std::chrono::time_point not behave as expected?

Answer 1

With:

using clock = std::chrono::system_clock;
using time_point = std::chrono::time_point<clock>;

time_point is implemented as if it stores a value of type Duration indicating the time interval from the start of the Clock's epoch. (See std::chrono::time_point)

The duration member type of clock (and of time_point) is capable of representing negative durations.

Thus duration in your implementation may be implemented with a back-end signed integer, (it can be implemented with unsigned integer but with a complicated comparison).

In that particular implementation,

time_point::min();
time_point t(clock::duration::min());
time_point t(clock::duration(std::numeric_limits<Rep>::lowest()));

and tp_now is greater than zero, thus when you subtract them, you get an integer overflow because the result is larger than std::numeric_limits<Rep>::max(). In implementation with signed back-end, it's undefined behavior, in implementation with unsigned back-end, I don't know about it, but I guess its special comparison will make its false.

In this example, tp_min is -9223372036854775808 ticks from its epoch, that number is the same with std::numeric_limits<duration::rep>::lowest()

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TL;DR; It's integer overflow. Don't use

(tp1 - tp2) > std::chrono::duration<whatever_rep>::zero

Instead, use

tp1 > tp2