Chapter 6. Boost.Chrono 2.0.8

Howard Hinnant

Beman Dawes

Vicente J. Botet Escriba

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

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Overview

Motivation
Description

“What is time, then? If nobody asks me, I know; if I have to explain it to someone who has asked me, I do not know."”

-- Augustine

How to Use This Documentation

This documentation makes use of the following naming and formatting conventions.

Code is in fixed width font and is syntax-highlighted.
Replaceable text that you will need to supply is in italics.
Free functions are rendered in the code font followed by (), as in free_function().
If a name refers to a class template, it is specified like this: class_template<>; that is, it is in code font and its name is followed by <> to indicate that it is a class template.
If a name refers to a function-like macro, it is specified like this: MACRO(); that is, it is uppercase in code font and its name is followed by () to indicate that it is a function-like macro. Object-like macros appear without the trailing ().
Names that refer to concepts in the generic programming sense are specified in CamelCase.

	Note
	In addition, notes such as this one specify non-essential information that provides additional background or rationale.

Finally, you can mentally add the following to any code fragments in this document:

// Include all of Chrono files
#include <boost/chrono.hpp>

We all deal with time every day of our lives. We've intuitively known it since birth. Thus we are all very familiar with it and believe it to be a simple matter. The modeling of time in computer programs should be similarly simple. The unfortunate truth is that this perceived simplicity is only skin deep. Fortunately, we do not need a terribly complicated solution to meet the bulk of our needs. However, overly simplistic solutions can be dangerous and inefficient, and won't adapt as the computer industry evolves.

Boost.Chrono implements the new time facilities in C++11, as proposed in N2661 - A Foundation to Sleep On. That document provides background and motivation for key design decisions and is the source of a good deal of information in this documentation.

In addition to the clocks provided by the standard proposal, Boost.Chrono provides specific process and thread clocks.

Wall clock versus system and user time

To make the timing facilities of Boost.Chrono more generally useful, the library provides a number of clocks that are thin wrappers around the operating system's process time API, thereby allowing the extraction of wall clock time, user CPU time, and system CPU time of the process. Wall clock time is the sum of CPU time and system CPU time. (On POSIX-like systems, this relies on times(). On Windows, it relies on GetProcessTimes().)

Description

The Boost.Chrono library provides:

Standard

A means to represent time durations: managed by the generic duration class . Examples of time durations include days, minutes, seconds and nanoseconds, which can be represented with a fixed number of clock ticks per unit. All of these units of time duration are united with a generic interface by the duration facility.
A type for representing points in time: time_point. A time_point represents an epoch plus or minus a duration. The library leaves epochs unspecified. A time_point is associated with a clock.
Several clocks, some of which may not be available on a particular platform: system_clock, steady_clock and high_resolution_clock. A clock is a pairing of a time_point and duration, and a function which returns a time_point representing now.

Other clocks

To make the timing facilities more generally useful, Boost.Chrono provides a number of clocks that are thin wrappers around the operating system's time APIs, thereby allowing the extraction of wall clock time, user CPU time, system CPU time spent by the process,

process_real_cpu_clock, captures wall clock CPU time spent by the current process.
process_user_cpu_clock, captures user-CPU time spent by the current process.
process_system_cpu_clock, captures system-CPU time spent by the current process.
A tuple-like class process_cpu_clock, that captures real, user-CPU, and system-CPU process times together.
A thread_clock thread steady clock giving the time spent by the current thread (when supported by a platform).

Lastly, Boost.Chrono includes typeof registration for duration and time_point to permit using emulated auto with C++03 compilers.

I/O

It provides I/O for duration and time_point. This I/O makes use of these types much more convenient. In following the "you only pay for what you use" philosophy, this extra functionality is located in a header separate from <boost/chrono/chrono.hpp>, namely <boost/chrono/chrono_io.hpp>.

It builds on <boost/ratio/ratio_io.hpp> to provide readable and flexible formatting and parsing for types in <boost/chrono.hpp>. This textural representation uses SI prefixes whenever possible. This makes it easy for boost::milliseconds to be represented by the text "milliseconds", or a hypothetical meter class to print out "millimeter". The duration and the time_point i/o can be customized through the new facets: duration_units and time_point_units. The user can specialize these facets so that the chrono i/o could be localizable. However Boost.Chrono doesn't provides a complete locale solution.

system_clock::time_point I/O is proposed in terms of UTC timepoints, strongly guided by ISO 9899:1999, Programming languages - C, ISO 9945:2003, Information Technology - Portable Operating System Interface (POSIX) and ISO 8601:2004, Data elements and interchange formats - Information interchange - Representation of dates and times.

Rounding utilities

A few simple rounding utility functions for working with durations.

Caveat Emptor

The underlying clocks provided by operating systems are subject to many seemingly arbitrary policies and implementation irregularities. That's a polite way of saying they tend to be flakey, and each operating system or even each clock has its own cruel and unusual forms of flakiness. Don't bet the farm on their accuracy, unless you have become deeply familiar with exactly what the specific operating system is guaranteeing, which is often very little.