std::hash<std::chrono::time_point>

#include <chrono>
#include <cstddef>
#include <iostream>
#include <string>
#include <thread>
#include <unordered_map>

struct my_system_clock : std::chrono::system_clock
{
    using time_point = std::chrono::time_point<my_system_clock>;
    
    static time_point now() noexcept
    {
        return time_point{std::chrono::system_clock::now().time_since_epoch()};
    }

    template<class CharT, class Traits>
    friend auto operator<<(std::basic_ostream<CharT, Traits>& os, const time_point& tp)
        -> decltype(os)
    {
        return os << std::chrono::system_clock::time_point{tp.time_since_epoch()};
    }
};

using my_system_clock_tp = std::chrono::time_point<my_system_clock>;

#if __cpp_lib_chrono < 202306L
// custom specialization of std::hash can be injected in namespace std
template<>
struct std::hash<my_system_clock_tp>
{
    std::size_t operator()(const my_system_clock_tp& d) const noexcept
    {
        return d.time_since_epoch().count();
    }
};
#endif

int main()
{
    using namespace std::chrono_literals;
    
    std::unordered_map<my_system_clock_tp, std::string> log;
    
    for (int i{}; i != 4; ++i)
    {
        std::this_thread::sleep_for(100ms);
        log[my_system_clock::now()] = "event #" + std::to_string(i);
    }
    
    for (auto const& [time, message] : log)
        std::cout << '[' << time << "], message: " << message << '\n';
}

Возможный вывод:

[2024-03-22 10:47:14.966238436], message: event #3
[2024-03-22 10:47:14.866096194], message: event #2
[2024-03-22 10:47:14.765965786], message: event #1
[2024-03-22 10:47:14.665817365], message: event #0