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<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">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>
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<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>
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