How to Use Chrono in C++

The C++ chrono library provides powerful tools for working with time in C++ applications. Here is a comprehensive, expert-level guide on utilizing all capabilities of chrono to measure and manipulate time.

Overview of Chrono

The chrono library was introduced in C++11 to provide improved support for date and time functionality. It allows for high-precision timing and makes it easy to measure and calculate durations. Key capabilities include:

• High resolution clocks for precision timing
• Timepoints to represent instants in time
• Durations to represent time intervals
• Time manipulation functions (add, subtract, etc.)

Using chrono can simplify many time-related programming tasks in areas like benchmarks, simulations, games, real-time systems, and more.

Including and Using Chrono

To use chrono, include the <chrono> header and bring the std::chrono namespace into scope:

#include <chrono>
using namespace std::chrono;  

This provides access to the full chrono library.

As a best practice, limit the chrono namespace usage to code files where it is actually needed, rather than globally, to avoid namespace pollution.

Clocks

Chrono provides several clock types to measure time:

• System clock – Wall clock time from the system clock. Tracks real world local time with daylight savings changes.
• Steady clock – Monotonic clock that never goes backwards. Useful for benchmarking or elapsed times.
• High resolution clock – The clock with the shortest tick period available on a particular system. Provides maximum precision.

For example:

// Get current system time 
auto now = system_clock::now(); 

// Get most precise time reading supported
auto precise = high_resolution_clock::now();

Use the most suitable clock for your specific timing needs. Favor steady_clock for benchmarking, system_clock when tracking actual dates/times.

Time Points

A time point represents an absolute point in time as measured by a specific clock. It is defined relative to that clock‘s epoch (reference point).

Create time points using the time_point class:

system_clock::time_point start = system_clock::now();

Time points can be compared to calculate durations between them. They are the primary tool for recording and assessing times.

Durations

Durations represent spans of time, independent of the current time or clocks. They only store a time length.

Define durations by specifying the unit, like seconds:

// 5 second duration
seconds dur(5);

Common duration types include:

• nanoseconds
• microseconds
• milliseconds
• seconds
• minutes
• hours

Durations support various arithmetic operations like add, subtract, multiply, compare, etc. This allows easy time math.

For example:

auto hour = hours(1);
auto half_hour = hour / 2; // 0.5 hours

Calculating Elapsed Time

A very common chrono usage is to calculate the duration elapsed between two time points. This measures how much time passes between two events.

auto start = steady_clock::now(); 

// Code to time

auto end = steady_clock::now();

// Calculate elapsed time
auto elapsed = end - start;

Now elapsed contains a duration representing the span between start and end.

UseCare should be taken that any code to time does not significantly impact the precision of the timing measurements. Techniques like repeating timings or isolating threads can help increase accuracy.

Displaying Durations

To display or print durations, use the count() member function. This returns the duration value in the units it represents:

auto elapsed = /* Duration */

// Print elapsed milliseconds 
cout << "Elapsed: " << elapsed.count() << " ms";

For more complex formatting, chrono also provides the duration_cast and round() functions.

For example, properly rounding the milliseconds:

// Round to nearest hundredths  
duration<double, std::milli> elapsed = /* Duration */; 

cout << duration_cast<milliseconds>(round(elapsed)).count() << " ms";

The chrono libraries provide many options for efficiently converting and formatting durations.

Using Chrono with Threads and Asynchrony

When using chrono timing alongside asynchronous threads or parallel code execution, extra care must be taken to ensure accuracy.

If a chrono start point is recorded on one thread, but the end point occurs on another asynchronous thread, the resulting duration measurement can be wildly inaccurate or nonsensical.

// Inaccurate threaded timing
thread_local auto start = steady_clock::now(); 

thread_1.run(); // Records start
thread_2.run(); // Records end   

// Duration includes thread coordination time
auto elapsed = end - start;

To mitigate this, structure threaded code to ensure matching start and end points occur on the same logical thread:

// Accurate threaded timing

auto thread_1_start = steady_clock::now();
thread_1.run([&thread_1_start] {

    auto thread1_end = steady_clock::now(); 

    auto thread1_elapsed = thread1_end - thread1_start; // accurate
});


auto thread_2_start = steady_clock::now(); 
thread_2.run([&thread_2_start] {

    auto thread2_end = steady_clock::now();

    auto thread2_elapsed = thread2_end - thread2_start; // accurate           
});

This captures a distinct start and end on each thread, giving an accurate duration even with asynchrony.

Best Practices for Benchmarking

Chrono is extremely useful for benchmarking the performance of code snippets. But care must be taken to properly design benchmarks for accurate, relevant statistics.

Some chrono-based benchmarking best practices include:

• Isolate the code under test from application side-effects
• Account for JIT, caches, pipelines and other optimizations
• Repeat timings to capture variance and remove outliers
• Scale benchmark data sizes far above expected uses
• Profile low level callstacks to verify measurements

Also be careful to select suitable clocks and durations:

• Favor steady_clock over system or high-res, no drift
• Use nanoseconds or microseconds precision if able
• Round or average values intelligently

Following these practices will improve accuracy immensely compared to naive chrono benchmarks.

For ultimate C++ benchmarking capability, consider benchmarking frameworks like Google Benchmark. But chrono itself remains a simple, zero-dependency option.

Advanced duration Formatters

While count() suffices for basic duration printing, the chrono library supports advanced formatting functionality via the duration type:

auto d = milliseconds(5400); 

cout << format("%H:%M:%S\n", d); // prints 00:01:54

Formats like %H:%M:%S print hours, minutes and seconds as desired.

Other usable duration formats include:

• %d – full days
• %h – hours (00 to 23)
• %m – minutes (00 to 59)
• %s – integer seconds
• %f – decimal fraction seconds
• %F – decimal fraction seconds without trailing zeros

So a duration could be printed for example as 01d 05h 47m 32.81234s.

Consult documentation for full details on chrono‘s rich duration formatting.

Working with Time Points and Calendars

While durations measure relative time spans, time points mark absolute moments in time. Chrono supports defining time points linked to the Gregorian calendar.

auto d = system_clock::now(); 

auto dp = floor<days>(d); // midnight today 

year_month_day ymd(dp); // ymd contains date components

This breaks a time point down into meaningful calendar parts like year, month, day.

Additional calendar-related time point functions allow easy conversion from C++ chrono types into other date/time representations like POSIX time or ISO 8601 strings.

For example:

time_t t = system_clock::to_time_t(system_clock::now()); // to POSIX time

string iso = system_clock::to_iso_extended_string(tp); // to ISO 8601 string

Working with dates via the chrono system makes many timestamp and calendar tasks much simpler.

Handling Time Zones

When using chrono time points representing real world clock times, time zones can complicate matters.

By default, the chrono system clock operates in UTC. So a system clock time point may differ substantially from a local wall clock reading.

To mitigate this, opt to store times in UTC whenever possible. Convert to local times only when absolutely necessary for display purposes:

system_clock::time_point tp; // stored in UTC

auto utc = tp.time_since_epoch(); // duration since UTC epoch

auto local_secs = utc.count() + timezone_offset; // convert

Operating primarily in UTC prevents many time zone bugs like values incorrectly shifting during daylight savings transitions.

Even better, use a dedicated date/time library like Howard Hinnant‘s date library to shift the burden of handling time zones.

Portability Considerations

While C++ aims for excellent cross-platform support, chrono can still pose portability challenges in some areas:

• Clock implementations may differ across operating systems
• Tick precisions vary significantly by hardware
• Old C++11 compilers lack some newer features
• 32 vs 64-bit sizes impact range/precision

Test rigorously when writing performance sensitive or portable code:

• Verify clocks maintain expected precision within margins
• Check counts and values match expectations
• Consider alternate clocks or fallbacks if precision varies

Unit test suites provide an excellent way to catch platform specific chrono issues.

Comparisons to Other Languages

Most modern languages provide date/time libraries, but C++ chrono remains a standout implementation.

Java – The Java datetime API is notoriously troublesome – lacking thread safety, null pointer issues and poor immutability. Chrono avoids these shortcomings.

.NET – Similar capabilities to C++ chrono, but with more constrained method names. Also faces greater portability concerns on Mono.

Python – Native datetime module is convenient but less featured. Popular add-on module Arrow improves functionality. Overall, Python datetime capabilities still lag behind C++ chrono.

JavaScript – The ES6 Date API improves greatly on the original JavaScript Date object but still lacks the range, precision and durability of chrono.

So while competent timestamps can be achieved in any language, C++ chrono remains a uniquely powerful and robust solution.

Philosophical Implications

Advanced timekeeping capabilities raise interesting philosophical questions. If durations can be arbitrarily precise, what effect does this have on reality and human perception?

Can moments truly be experienced if time points are little more than infinitesimal slices of a temporal continuum? Do we lose the present to excessive precision surrounding past and future?

Perhaps chrono highlights the artificiality of clock time overall – revealing it as a mathematical construct indifferent to the subjective now within which we dwell.

Or instead does ultra-fine timing reinforce an eternal present, overflowing with significance at every picosecond?

Such inquiries slip away as quickly as a nanosecond duration itself. Yet chrono‘s implications lurk within.

Interstellar Spacecraft

Let‘s imagine chrono helping control timing systems on an interstellar spacecraft required to operate autonomously for decades.

Special relativistic effects come into play as the craft‘s high velocity causes significant time dilation relative to observers on Earth.

Chrono‘s steady_clock provides a crucial consistent baseline against which the apparent rate of external system-clock advancement can be compared.

By factoring out relativity impacts, chrono enables appropriately adjusting scheduled maintenance, data logging and other shipboard events to account for time dilation as the years pass for Earthling controllers.

Fortunately, even relativistic effects barely dent chrono‘s ability to represent time durations up to 292 years with nanosecond precision!

So while alien vistas warp past, C++ chrono steadily ticks on – ensuring smooth function of spacecraft systems over very extended voyages.

Geological Timescales

At the other extreme from nanoseconds, chrono can also manage insane time spans like millions of years for uses like advanced geological simulations.

// Scale chrono for megayears  

using megapyear = duration<int, ratio<3155760000>>;

megapyear dura(4); // Four million year duration

This maps a year unit to ~3.15576e13 nanoseconds toprovide megapyear precision until 292 million CE – enough for most geological apps!

Of course, practical timeouts may be difficult to implement on these timelines – but the concepts remain valid.

Even continental drift can be measured as a duration between time points. The precision of chrono thus comfortably stretches from human time frames to epochs matching the Earth itself.

Conclusion

From nanoseconds to eons, C++ chrono delivers an unmatched set of timing tools. It enables not just duration management, but a complete mastery over measurement, calibration, conversion and calculation of time itself.

Robust, thoughtful use of chrono can thus streamline the modeling of systems and enhance understanding across nearly any application domain.

Whether plotting stars or optimizing code, tapping chrono elevates programming to shift time itself into a flexible medium controlled through C++ alone.

In short – this most temporal of libraries offers the power to finally tame time. Use it wisely!