1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>FAQ</title> 6<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../boost_units.html" title="Chapter 43. Boost.Units 1.1.0"> 10<link rel="prev" href="Installation.html" title="Installation"> 11<link rel="next" href="Acknowledgements.html" title="Acknowledgements"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 16<td align="center"><a href="../../../index.html">Home</a></td> 17<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<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> 25</div> 26<div class="section"> 27<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 28<a name="boost_units.FAQ"></a><a class="link" href="FAQ.html" title="FAQ">FAQ</a> 29</h2></div></div></div> 30<div class="toc"><dl class="toc"> 31<dt><span class="section"><a href="FAQ.html#boost_units.FAQ.Distinguishing_Quantities_With_Same_Units">How 32 does one distinguish between quantities that are physically different but have 33 the same units (such as energy and torque)?</a></span></dt> 34<dt><span class="section"><a href="FAQ.html#boost_units.FAQ.Angle_Are_Units">Angles are treated as 35 units</a></span></dt> 36<dt><span class="section"><a href="FAQ.html#boost_units.FAQ.Why_Homogeneous_Systems">Why are there 37 homogeneous systems? Aren't heterogeneous systems sufficient?</a></span></dt> 38<dt><span class="section"><a href="FAQ.html#boost_units.FAQ.NoConstructorFromValueType">Why can't 39 I construct a quantity directly from the value type?</a></span></dt> 40<dt><span class="section"><a href="FAQ.html#boost_units.FAQ.ExplicitConversions">Why are conversions 41 explicit by default?</a></span></dt> 42</dl></div> 43<div class="section"> 44<div class="titlepage"><div><div><h3 class="title"> 45<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 46 does one distinguish between quantities that are physically different but have 47 the same units (such as energy and torque)?</a> 48</h3></div></div></div> 49<p> 50 Because Boost.Units includes plane and solid angle units in the SI system, 51 torque and energy are, in fact, distinguishable (see <a href="http://en.wikipedia.org/wiki/SI_units" target="_top">torque</a>). 52 In addition, energy is a true <a href="http://mathworld.wolfram.com/Scalar.html" target="_top">scalar</a> 53 quantity, while torque, despite having the same units as energy if plane 54 angle is not included, is in fact a <a href="http://mathworld.wolfram.com/Pseudovector.html" target="_top">pseudovector</a>. 55 Thus, a value type representing pseudovectors and encapsulating their algebra 56 could also be implemented. 57 </p> 58<p> 59 There are, however, a few SI units that are dimensionally indistinguishable 60 within the SI system. These include the <a href="http://en.wikipedia.org/wiki/Becquerel" target="_top">becquerel</a>, 61 which has units identical to frequency (Hz), and the <a href="http://en.wikipedia.org/wiki/Sievert" target="_top">sievert</a>, 62 which is degenerate with the <a href="http://en.wikipedia.org/wiki/Gray_%28unit%29" target="_top">gray</a>. 63 In cases such as this, the proper way to treat this difference is to recognize 64 that expanding the set of base dimensions can provide disambiguation. For 65 example, adding a base dimension for radioactive decays would allow the becquerel 66 to be written as decays/second, differentiating it from the signature of 67 hertz, which is simply 1/second. 68 </p> 69</div> 70<div class="section"> 71<div class="titlepage"><div><div><h3 class="title"> 72<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 73 units</a> 74</h3></div></div></div> 75<p> 76 If you don't like this, you can just ignore the angle units and go on your 77 merry way (periodically screwing up when a routine wants degrees and you 78 give it radians instead...) 79 </p> 80</div> 81<div class="section"> 82<div class="titlepage"><div><div><h3 class="title"> 83<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 84 homogeneous systems? Aren't heterogeneous systems sufficient?</a> 85</h3></div></div></div> 86<p> 87 Consider the following code: 88 </p> 89<pre class="programlisting"><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">asin</span><span class="special">(</span><span class="identifier">sin</span><span class="special">(</span><span class="number">90.0</span> <span class="special">*</span> <span class="identifier">degrees</span><span class="special">));</span> 90</pre> 91<p> 92 What should this print? If only heterogeneous systems are available it would 93 print 1.5708 rad Why? Well, <code class="computeroutput"><span class="identifier">sin</span></code> 94 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> 95 effectively losing the information that degrees are being used. In order 96 to propogate this extra information we need homogeneous systems. 97 </p> 98</div> 99<div class="section"> 100<div class="titlepage"><div><div><h3 class="title"> 101<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 102 I construct a quantity directly from the value type?</a> 103</h3></div></div></div> 104<p> 105 This only breaks generic code--which ought to break anyway. The only literal 106 value that ought to be converted to a quantity by generic code is zero, which 107 should be handled by the default constructor. In addition, consider the search 108 and replace problem allowing this poses: 109 </p> 110<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> 111</pre> 112<p> 113 Here, the intent is clear - we want a length of one in the SI system, which 114 is one meter. However, imagine some well-intentioned coder attempting to 115 reuse this code, but to have it perform the calculations in the CGS unit 116 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: 117 </p> 118<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> 119</pre> 120<p> 121 Unfortunately, the meaning of this statement has suddenly changed from one 122 meter to one centimeter. In contrast, as implemented, we begin with: 123 </p> 124<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> 125</pre> 126<p> 127 and, after search and replace: 128 </p> 129<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> 130</pre> 131<p> 132 which gives us an error. Even if the code has a @using namespace boost::units::si; 133 declaration, the latter is still safe, with: 134 </p> 135<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> 136<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> 137</pre> 138<p> 139 going to 140 </p> 141<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> 142<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> 143</pre> 144<p> 145 The latter will involve an explicit conversion from meters to centimeters, 146 but the value remains correct. 147 </p> 148</div> 149<div class="section"> 150<div class="titlepage"><div><div><h3 class="title"> 151<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 152 explicit by default?</a> 153</h3></div></div></div> 154<p> 155 Safety and the potential for unintended conversions leading to precision 156 loss and hidden performance costs. Options are provided for forcing implicit 157 conversions between specific units to be allowed. 158 </p> 159</div> 160</div> 161<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 162<td align="left"></td> 163<td align="right"><div class="copyright-footer">Copyright © 2003-2008 Matthias Christian Schabel<br>Copyright © 2007-2010 Steven 164 Watanabe<p> 165 Distributed under the Boost Software License, Version 1.0. (See accompanying 166 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>) 167 </p> 168</div></td> 169</tr></table> 170<hr> 171<div class="spirit-nav"> 172<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> 173</div> 174</body> 175</html> 176
