How to convert std::chrono::time_point to string

Question

How to convert std::chrono::time_point to string? For example: \"201601161125\".

Answer 1

Code solution

The following function converts from chrono time_point to string (serialization).

#include 
#include 
#include 

using time_point = std::chrono::system_clock::time_point;
std::string serializeTimePoint( const time_point& time, const std::string& format)
{
    std::time_t tt = std::chrono::system_clock::to_time_t(time);
    std::tm tm = *std::gmtime(&tt); //GMT (UTC)
    //std::tm tm = *std::localtime(&tt); //Locale time-zone, usually UTC by default.
    std::stringstream ss;
    ss << std::put_time( &tm, format.c_str() );
    return ss.str();
}

// example
int main()
{
    time_point input = std::chrono::system_clock::now();
    std::cout << serializeTimePoint(input, "UTC: %Y-%m-%d %H:%M:%S") << std::endl;

}

Time zone

The time_point data-type has no internal representation for the time-zone, in consequence, the time-zone is aggregated by the conversion to std::tm (by the functions gmtime or localtime). It is not recommended to add/substract the time-zone from the input, because you would get an incorrect time-zone displayed with %Z, thus, it is better to set the correct local time (OS dependent) and use localtime().

Technical vs User-friendly serialization

For technical usage, hard-coded time format is a good solution. However, to display to users, one should use a locale to retrieve the user preference and show time-stamp in that format.

C++20

Since C++20, we have nice serialization and parsing functions for time_point and duration.

• std::chrono::to_stream
• std::chrono::format