<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>FAQ</title> <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.2"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../boost_units.html" title="Chapter 26. Boost.Units 1.0.0"> <link rel="prev" href="Installation.html" title="Installation"> <link rel="next" href="Acknowledgements.html" title="Acknowledgements"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> <td align="center"><a href="../../../index.html">Home</a></td> <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="Installation.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../boost_units.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="Acknowledgements.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="boost_units.FAQ"></a><a class="link" href="FAQ.html" title="FAQ"> FAQ</a> </h2></div></div></div> <div class="toc"><dl> <dt><span class="section"><a href="FAQ.html#boost_units.FAQ.Distinguishing_Quantities_With_Same_Units"> How does one distinguish between quantities that are physically different but have the same units (such as energy and torque)?</a></span></dt> <dt><span class="section"><a href="FAQ.html#boost_units.FAQ.Angle_Are_Units"> Angles are treated as units</a></span></dt> <dt><span class="section"><a href="FAQ.html#boost_units.FAQ.Why_Homogeneous_Systems"> Why are there homogeneous systems? Aren't heterogeneous systems sufficient?</a></span></dt> <dt><span class="section"><a href="FAQ.html#boost_units.FAQ.NoConstructorFromValueType"> Why can't I construct a quantity directly from the value type?</a></span></dt> <dt><span class="section"><a href="FAQ.html#boost_units.FAQ.ExplicitConversions"> Why are conversions explicit by default?</a></span></dt> </dl></div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="boost_units.FAQ.Distinguishing_Quantities_With_Same_Units"></a><a class="link" href="FAQ.html#boost_units.FAQ.Distinguishing_Quantities_With_Same_Units" title="How does one distinguish between quantities that are physically different but have the same units (such as energy and torque)?"> How does one distinguish between quantities that are physically different but have the same units (such as energy and torque)?</a> </h3></div></div></div> <p> Because Boost.Units includes plane and solid angle units in the SI system, torque and energy are, in fact, distinguishable (see <a href="http://en.wikipedia.org/wiki/SI_units" target="_top">torque</a>). In addition, energy is a true <a href="http://mathworld.wolfram.com/Scalar.html" target="_top">scalar</a> quantity, while torque, despite having the same units as energy if plane angle is not included, is in fact a <a href="http://mathworld.wolfram.com/Pseudovector.html" target="_top">pseudovector</a>. Thus, a value type representing pseudovectors and encapsulating their algebra could also be implemented. </p> <p> There are, however, a few SI units that are dimensionally indistinguishable within the SI system. These include the <a href="http://en.wikipedia.org/wiki/Becquerel" target="_top">becquerel</a>, which has units identical to frequency (Hz), and the <a href="http://en.wikipedia.org/wiki/Sievert" target="_top">sievert</a>, which is degenerate with the <a href="http://en.wikipedia.org/wiki/Gray_%28unit%29" target="_top">gray</a>. In cases such as this, the proper way to treat this difference is to recognize that expanding the set of base dimensions can provide disambiguation. For example, adding a base dimension for radioactive decays would allow the becquerel to be written as decays/second, differentiating it from the signature of hertz, which is simply 1/second. </p> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="boost_units.FAQ.Angle_Are_Units"></a><a class="link" href="FAQ.html#boost_units.FAQ.Angle_Are_Units" title="Angles are treated as units"> Angles are treated as units</a> </h3></div></div></div> <p> If you don't like this, you can just ignore the angle units and go on your merry way (periodically screwing up when a routine wants degrees and you give it radians instead...) </p> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="boost_units.FAQ.Why_Homogeneous_Systems"></a><a class="link" href="FAQ.html#boost_units.FAQ.Why_Homogeneous_Systems" title="Why are there homogeneous systems? Aren't heterogeneous systems sufficient?"> Why are there homogeneous systems? Aren't heterogeneous systems sufficient?</a> </h3></div></div></div> <p> Consider the following code: </p> <pre class="programlisting"><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">sin</span><span class="special">(</span><span class="identifier">asin</span><span class="special">(</span><span class="number">180.0</span> <span class="special">*</span> <span class="identifier">degrees</span><span class="special">));</span> </pre> <p> What should this print? If only heterogeneous systems are available it would print 3.14159+ rad Why? Well, <code class="computeroutput"><span class="identifier">asin</span></code> would return a <code class="computeroutput"><span class="identifier">quantity</span><span class="special"><</span><span class="identifier">dimensionless</span><span class="special">></span></code> effectively losing the information that degrees are being used. In order to propogate this extra information we need homogeneous systems. </p> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="boost_units.FAQ.NoConstructorFromValueType"></a><a class="link" href="FAQ.html#boost_units.FAQ.NoConstructorFromValueType" title="Why can't I construct a quantity directly from the value type?"> Why can't I construct a quantity directly from the value type?</a> </h3></div></div></div> <p> This only breaks generic code--which ought to break anyway. The only literal value that ought to be converted to a quantity by generic code is zero, which should be handled by the default constructor. In addition, consider the search and replace problem allowing this poses: </p> <pre class="programlisting"><span class="identifier">quantity</span><span class="special"><</span><span class="identifier">si</span><span class="special">::</span><span class="identifier">length</span><span class="special">></span> <span class="identifier">q</span><span class="special">(</span><span class="number">1.0</span><span class="special">);</span> </pre> <p> Here, the intent is clear - we want a length of one in the SI system, which is one meter. However, imagine some well-intentioned coder attempting to reuse this code, but to have it perform the calculations in the CGS unit system instead. After searching for <code class="computeroutput"><span class="identifier">si</span><span class="special">::</span></code> and replacing it with <code class="computeroutput"><span class="identifier">cgs</span><span class="special">::</span></code> , we have: </p> <pre class="programlisting"><span class="identifier">quantity</span><span class="special"><</span><span class="identifier">cgs</span><span class="special">::</span><span class="identifier">length</span><span class="special">></span> <span class="identifier">q</span><span class="special">(</span><span class="number">1.0</span><span class="special">);</span> </pre> <p> Unfortunately, the meaning of this statement has suddenly changed from one meter to one centimeter. In contrast, as implemented, we begin with: </p> <pre class="programlisting"><span class="identifier">quantity</span><span class="special"><</span><span class="identifier">si</span><span class="special">::</span><span class="identifier">length</span><span class="special">></span> <span class="identifier">q</span><span class="special">(</span><span class="number">1.0</span><span class="special">*</span><span class="identifier">si</span><span class="special">::</span><span class="identifier">meter</span><span class="special">);</span> </pre> <p> and, after search and replace: </p> <pre class="programlisting"><span class="identifier">quantity</span><span class="special"><</span><span class="identifier">cgs</span><span class="special">::</span><span class="identifier">length</span><span class="special">></span> <span class="identifier">q</span><span class="special">(</span><span class="number">1.0</span><span class="special">*</span><span class="identifier">cgs</span><span class="special">::</span><span class="identifier">meter</span><span class="special">);</span> </pre> <p> which gives us an error. Even if the code has a @using namespace boost::units::si; declaration, the latter is still safe, with: </p> <pre class="programlisting"><span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">units</span><span class="special">::</span><span class="identifier">si</span><span class="special">;</span> <span class="identifier">quantity</span><span class="special"><</span><span class="identifier">length</span><span class="special">></span> <span class="identifier">q</span><span class="special">(</span><span class="number">1.0</span><span class="special">*</span><span class="identifier">meter</span><span class="special">);</span> </pre> <p> going to </p> <pre class="programlisting"><span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">units</span><span class="special">::</span><span class="identifier">cgs</span><span class="special">;</span> <span class="identifier">quantity</span><span class="special"><</span><span class="identifier">length</span><span class="special">></span> <span class="identifier">q</span><span class="special">(</span><span class="number">1.0</span><span class="special">*</span><span class="identifier">meter</span><span class="special">);</span> </pre> <p> The latter will involve an explicit conversion from meters to centimeters, but the value remains correct. </p> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="boost_units.FAQ.ExplicitConversions"></a><a class="link" href="FAQ.html#boost_units.FAQ.ExplicitConversions" title="Why are conversions explicit by default?"> Why are conversions explicit by default?</a> </h3></div></div></div> <p> Safety and the potential for unintended conversions leading to precision loss and hidden performance costs. Options are provided for forcing implicit conversions between specific units to be allowed. </p> </div> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <td align="right"><div class="copyright-footer">Copyright © 2003 -2008 Matthias Christian Schabel, 2007-2008 Steven Watanabe<p> Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="Installation.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../boost_units.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="Acknowledgements.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>