176 lines
13 KiB
HTML
176 lines
13 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
||
<html>
|
||
<head>
|
||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
|
||
<title>FAQ</title>
|
||
<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css">
|
||
<meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
|
||
<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset">
|
||
<link rel="up" href="../boost_units.html" title="Chapter 44. Boost.Units 1.1.0">
|
||
<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 class="toc">
|
||
<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">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>
|
||
</pre>
|
||
<p>
|
||
What should this print? If only heterogeneous systems are available it would
|
||
print 1.5708 rad Why? Well, <code class="computeroutput"><span class="identifier">sin</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<br>Copyright © 2007-2010 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>
|