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

Since C++26

Header: <chrono>

The template specialization of std::hash for std::chrono::time_point allows users to obtain hashes of objects of type std::chrono::time_point<Clock, Duration>. This specialization is enabled if and only if std::hash is enabled.

# Declarations

template< class Clock, class Duration >
struct hash<std::chrono::time_point<Clock, Duration>>;

(since C++26)

# Notes

Feature-test macro Value Std Feature __cpp_lib_chrono 202306L (C++26) Hashing support for std::chrono value classes

# Example

#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';
}

# See also

hash