Initialize std::chrono::time_point variable with zero

Question

I have a variable called last_timestamp_ which is declared as follows:

using TimePoint = std::chrono::time_point<std::chrono::system_clock, std::chrono::duration<double>> TimePoint last_timestamp_

and I want to initialize it with zero, how do I do that? Thanks

Answer 1

using TimePoint = std::chrono::time_point<std::chrono::system_clock, 
    std::chrono::duration<double>>;
TimePoint last_timestamp_{};

It would actually work without the {} too, but imho it is better style to explicitly zero-initialize it.

You can read this two ways:

1. It internally sets the value of last_timestamp_ to 0.0s, or to its epoch.
2. It sets last_timestamp_ to 1970-01-01 00:00:00 UTC.

The reason you can say the second part is because it is a de-facto standard that all implementations track Unix Time with system_clock in C++11/14/17. For C++20 this industry practice has been officially specified.

Important note: This specification is strictly for time_points based on system_clock. One can not assume this epoch for all chrono time_points.

Presuming from the comments that your use looks like:

while (...)
{
    if (std::chrono::system_clock::now() > last_timestamp_ + threshold)
    {
        // now() - last_timestamp_ > threshold
        ...
        last_timestamp_ = std::chrono::system_clock::now();
    }
};

Your first iteration will be true assuming threshold isn't larger than 51 years.

Just to future-proof your code, it's worth a comment:

TimePoint last_timestamp_{};  // 1970-01-01

Now if you change clocks for any reason, you know that you may have to revisit this initial value. For example on some platforms the epoch of file_clock will be far in the future, breaking this code if it switched to file_clock. C++20 offers syntax to make this initialization more explicit:

using namespace std::chrono_literals;
TimePoint last_timestamp_ = std::chrono::sys_days{1970y/1/1};

Now the comment is unnecessary.