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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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| <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, 2007-2010 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>) |
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