forked from mpusz/mp-units
Mateusz Pusz
2116 lines
66 KiB
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<h1 id="interface-introduction">Interface Introduction<a class="headerlink" href="#interface-introduction" title="Permanent link">¶</a></h1>
|
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<h2 id="new-style-of-definitions">New style of definitions<a class="headerlink" href="#new-style-of-definitions" title="Permanent link">¶</a></h2>
|
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<p>The <strong>mp-units</strong> library decided to use a rather unusual pattern to define entities.
|
|
Here is how we define <code>metre</code> and <code>second</code> <a href="../../../appendix/glossary/#si">SI</a> base units:</p>
|
|
<div class="language-cpp highlight"><pre><span></span><code><span id="__span-0-1"><a id="__codelineno-0-1" name="__codelineno-0-1" href="#__codelineno-0-1"></a><span class="kr">inline</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">metre</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">named_unit</span><span class="o"><</span><span class="s">"m"</span><span class="p">,</span><span class="w"> </span><span class="n">kind_of</span><span class="o"><</span><span class="n">isq</span><span class="o">::</span><span class="n">length</span><span class="o">>></span><span class="w"> </span><span class="p">{}</span><span class="w"> </span><span class="n">metre</span><span class="p">;</span>
|
|
</span><span id="__span-0-2"><a id="__codelineno-0-2" name="__codelineno-0-2" href="#__codelineno-0-2"></a><span class="kr">inline</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">second</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">named_unit</span><span class="o"><</span><span class="s">"s"</span><span class="p">,</span><span class="w"> </span><span class="n">kind_of</span><span class="o"><</span><span class="n">isq</span><span class="o">::</span><span class="n">time</span><span class="o">>></span><span class="w"> </span><span class="p">{}</span><span class="w"> </span><span class="n">second</span><span class="p">;</span>
|
|
</span></code></pre></div>
|
|
<p>Please note that the above reuses the same identifier for a type and its object. The rationale
|
|
behind this is that:</p>
|
|
<ul>
|
|
<li>Users always work with objects and never have to spell such a type name.</li>
|
|
<li>The types appear in the compilation errors and during debugging.</li>
|
|
</ul>
|
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<div class="admonition important">
|
|
<p class="admonition-title">Important</p>
|
|
<p>To improve compiler errors' readability and make it easier to correlate them with
|
|
a user's written code, a new idiom in the library is to use the same identifier for
|
|
a type and its instance.</p>
|
|
</div>
|
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<h2 id="strong-types-instead-of-aliases">Strong types instead of aliases<a class="headerlink" href="#strong-types-instead-of-aliases" title="Permanent link">¶</a></h2>
|
|
<p>Let's look again at the above units definitions. Another important point to notice is that
|
|
all the types describing entities in the library are short, nicely named identifiers
|
|
that derive from longer, more verbose class template instantiations. This is really important
|
|
to improve the user experience while debugging the program or analyzing the compilation error.</p>
|
|
<div class="admonition note">
|
|
<p class="admonition-title">Note</p>
|
|
<p>Such a practice is rare in the industry. Some popular C++ physical units libraries
|
|
generate enormously long error messages where even only the first line failed o fit
|
|
on a slide with a tiny font.</p>
|
|
</div>
|
|
<h2 id="entities-composability">Entities composability<a class="headerlink" href="#entities-composability" title="Permanent link">¶</a></h2>
|
|
<p>Many physical units libraries (in C++ or any other programming language) assign strong types
|
|
to library entities (i.e. derived units). While <code>metre_per_second</code> as a type may not look too
|
|
scary, consider, for example, units of angular momentum. If we followed this path, its
|
|
<a href="../../../appendix/glossary/#coherent-derived-unit">coherent unit</a> would look like
|
|
<code>kilogram_metre_sq_per_second</code>. Now, consider how many scaled versions of this unit would you
|
|
predefine in the library to ensure that all users are happy with your choice?
|
|
How expensive would it be from the implementation point of view?
|
|
What about potential future standardization efforts?</p>
|
|
<p>This is why in <strong>mp-units</strong>, we put a strong requirement to make everything as composable as
|
|
possible. For example, to create a quantity with a unit of speed, one may write:</p>
|
|
<div class="language-cpp highlight"><pre><span></span><code><span id="__span-1-1"><a id="__codelineno-1-1" name="__codelineno-1-1" href="#__codelineno-1-1"></a><span class="n">quantity</span><span class="o"><</span><span class="n">si</span><span class="o">::</span><span class="n">metre</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="n">si</span><span class="o">::</span><span class="n">second</span><span class="o">></span><span class="w"> </span><span class="n">q</span><span class="p">;</span>
|
|
</span></code></pre></div>
|
|
<p>In case you use such an unit often and would prefer to have a handy helper for it, you can
|
|
always do something like this:</p>
|
|
<div class="language-cpp highlight"><pre><span></span><code><span id="__span-2-1"><a id="__codelineno-2-1" name="__codelineno-2-1" href="#__codelineno-2-1"></a><span class="k">constexpr</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">metre_per_second</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">si</span><span class="o">::</span><span class="n">metre</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="n">si</span><span class="o">::</span><span class="n">second</span><span class="p">;</span>
|
|
</span><span id="__span-2-2"><a id="__codelineno-2-2" name="__codelineno-2-2" href="#__codelineno-2-2"></a><span class="n">quantity</span><span class="o"><</span><span class="n">metre_per_second</span><span class="o">></span><span class="w"> </span><span class="n">q</span><span class="p">;</span>
|
|
</span></code></pre></div>
|
|
<p>or choose any shorter identifier of your choice.</p>
|
|
<p>Coming back to the angular momentum case, thanks to the composability of units, a user can
|
|
create such a quantity in the following way:</p>
|
|
<div class="language-cpp highlight"><pre><span></span><code><span id="__span-3-1"><a id="__codelineno-3-1" name="__codelineno-3-1" href="#__codelineno-3-1"></a><span class="k">using</span><span class="w"> </span><span class="k">namespace</span><span class="w"> </span><span class="nn">mp_units</span><span class="o">::</span><span class="nn">si</span><span class="o">::</span><span class="nn">unit_symbols</span><span class="p">;</span>
|
|
</span><span id="__span-3-2"><a id="__codelineno-3-2" name="__codelineno-3-2" href="#__codelineno-3-2"></a><span class="k">auto</span><span class="w"> </span><span class="n">q</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">la_vector</span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">}</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">isq</span><span class="o">::</span><span class="n">angular_momentum</span><span class="p">[</span><span class="n">kg</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">m2</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="n">s</span><span class="p">];</span>
|
|
</span></code></pre></div>
|
|
<p>It is a much better solution. It is terse and easy to understand. Please also notice how
|
|
easy it is to obtain any scaled version of such a unit (i.e. <code>mg * square(mm) / min</code>)
|
|
without having to introduce hundreds of types to predefine them.</p>
|
|
<h2 id="value-based-equations">Value-based equations<a class="headerlink" href="#value-based-equations" title="Permanent link">¶</a></h2>
|
|
<p>The <strong>mp-units</strong> library is based on C++20, which greatly improves a user's experience. One of
|
|
such improvements are value-based equations.</p>
|
|
<p>As we have learned above, the entities are being used as values in the code, and they compose.
|
|
Moreover, derived entities can be defined in the library using such value-based equations.
|
|
This is a huge improvement compared to what we can find in other physical units libraries or
|
|
what we have to deal with when we want to write some equations for <code>std::ratio</code>.</p>
|
|
<p>For example, below are a few definitions of the SI derived units showing the power of C++20
|
|
extensions to Non-Type Template Parameters, which allows us to directly pass a result of
|
|
the value-based <a href="../../../appendix/glossary/#unit-equation">unit equation</a> to a class template
|
|
definition:</p>
|
|
<div class="language-cpp highlight"><pre><span></span><code><span id="__span-4-1"><a id="__codelineno-4-1" name="__codelineno-4-1" href="#__codelineno-4-1"></a><span class="kr">inline</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">newton</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">named_unit</span><span class="o"><</span><span class="s">"N"</span><span class="p">,</span><span class="w"> </span><span class="n">kilogram</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">metre</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="n">square</span><span class="p">(</span><span class="n">second</span><span class="p">)</span><span class="o">></span><span class="w"> </span><span class="p">{}</span><span class="w"> </span><span class="n">newton</span><span class="p">;</span>
|
|
</span><span id="__span-4-2"><a id="__codelineno-4-2" name="__codelineno-4-2" href="#__codelineno-4-2"></a><span class="kr">inline</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">pascal</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">named_unit</span><span class="o"><</span><span class="s">"Pa"</span><span class="p">,</span><span class="w"> </span><span class="n">newton</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="n">square</span><span class="p">(</span><span class="n">metre</span><span class="p">)</span><span class="o">></span><span class="w"> </span><span class="p">{}</span><span class="w"> </span><span class="n">pascal</span><span class="p">;</span>
|
|
</span><span id="__span-4-3"><a id="__codelineno-4-3" name="__codelineno-4-3" href="#__codelineno-4-3"></a><span class="kr">inline</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">joule</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">named_unit</span><span class="o"><</span><span class="s">"J"</span><span class="p">,</span><span class="w"> </span><span class="n">newton</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">metre</span><span class="o">></span><span class="w"> </span><span class="p">{}</span><span class="w"> </span><span class="n">joule</span><span class="p">;</span>
|
|
</span></code></pre></div>
|
|
<h2 id="expression-templates">Expression templates<a class="headerlink" href="#expression-templates" title="Permanent link">¶</a></h2>
|
|
<p>The previous chapter provided a rationale for not having predefined types for derived entities.
|
|
In many libraries, such an approach results in long and unreadable compilation errors, as
|
|
framework-generated types are typically far from being easy to read and understand.</p>
|
|
<p>The <strong>mp-units</strong> library greatly improves the user experience by extensively using
|
|
expression templates. Such expressions are used consistently throughout the entire library
|
|
to describe the results of:</p>
|
|
<ul>
|
|
<li><a href="../../../appendix/glossary/#dimension-equation">dimension equation</a> - the result is put into
|
|
the <code>derived_dimension<></code> class template</li>
|
|
<li><a href="../../../appendix/glossary/#quantity-equation">quantity equation</a> - the result is put into
|
|
the <code>derived_quantity_spec<></code> class template</li>
|
|
<li><a href="../../../appendix/glossary/#unit-equation">unit equation</a> - the result is put into the
|
|
<code>derived_unit<></code> class template</li>
|
|
</ul>
|
|
<p>For example, if we take the above-defined base units and put the results of their division into
|
|
the quantity class template like this:</p>
|
|
<div class="language-cpp highlight"><pre><span></span><code><span id="__span-5-1"><a id="__codelineno-5-1" name="__codelineno-5-1" href="#__codelineno-5-1"></a><span class="n">quantity</span><span class="o"><</span><span class="n">metre</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="n">second</span><span class="o">></span><span class="w"> </span><span class="n">q</span><span class="p">;</span>
|
|
</span></code></pre></div>
|
|
<p>we will observe the following type in the debugger</p>
|
|
<div class="language-text highlight"><pre><span></span><code><span id="__span-6-1"><a id="__codelineno-6-1" name="__codelineno-6-1" href="#__codelineno-6-1"></a>(gdb) ptype q
|
|
</span><span id="__span-6-2"><a id="__codelineno-6-2" name="__codelineno-6-2" href="#__codelineno-6-2"></a>type = class mp_units::quantity<mp_units::derived_unit<metre, mp_units::per<second>>(), double> [with Rep = double] {
|
|
</span></code></pre></div>
|
|
<p>The same type identifier will be visible in the compilation error (in case it happens).</p>
|
|
<div class="admonition important">
|
|
<p class="admonition-title">Important</p>
|
|
<p>Expressions templates are extensively used throughout the library to improve the readability
|
|
of the resulting types.</p>
|
|
</div>
|
|
<h3 id="identities">Identities<a class="headerlink" href="#identities" title="Permanent link">¶</a></h3>
|
|
<p>As mentioned above, equations can be done on dimensions, quantities, and units. Each such domain must
|
|
introduce an identity object that can be used in the resulting expressions. Here is the list of
|
|
identities used in the library:</p>
|
|
<table>
|
|
<thead>
|
|
<tr>
|
|
<th>Domain Concept</th>
|
|
<th style="text-align: center;">Identity</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td><code>Dimension</code></td>
|
|
<td style="text-align: center;"><code>dimension_one</code></td>
|
|
</tr>
|
|
<tr>
|
|
<td><code>QuantitySpec</code></td>
|
|
<td style="text-align: center;"><code>dimensionless</code></td>
|
|
</tr>
|
|
<tr>
|
|
<td><code>Unit</code></td>
|
|
<td style="text-align: center;"><code>one</code></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<p>In the equations, a user can refer to an identity object either explicitly:</p>
|
|
<div class="language-cpp highlight"><pre><span></span><code><span id="__span-7-1"><a id="__codelineno-7-1" name="__codelineno-7-1" href="#__codelineno-7-1"></a><span class="k">constexpr</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">my_unit</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">one</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="n">second</span><span class="p">;</span>
|
|
</span></code></pre></div>
|
|
<p>or implicitly:</p>
|
|
<div class="language-cpp highlight"><pre><span></span><code><span id="__span-8-1"><a id="__codelineno-8-1" name="__codelineno-8-1" href="#__codelineno-8-1"></a><span class="k">constexpr</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">my_unit</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="n">second</span><span class="p">;</span>
|
|
</span></code></pre></div>
|
|
<p>Both cases with result in the same expression template being generated and put into the wrapper
|
|
class template.</p>
|
|
<h3 id="supported-operations-and-their-results">Supported operations and their results<a class="headerlink" href="#supported-operations-and-their-results" title="Permanent link">¶</a></h3>
|
|
<p>There are only a few operations that one can do on such entities and the result of each of them has
|
|
its unique representation in the library:</p>
|
|
<table>
|
|
<thead>
|
|
<tr>
|
|
<th style="text-align: center;">Operation</th>
|
|
<th style="text-align: center;">Resulting template expression arguments</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td style="text-align: center;"><code>A * B</code></td>
|
|
<td style="text-align: center;"><code>A, B</code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>B * A</code></td>
|
|
<td style="text-align: center;"><code>A, B</code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>A * A</code></td>
|
|
<td style="text-align: center;"><code>power<A, 2></code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>{identity} * A</code></td>
|
|
<td style="text-align: center;"><code>A</code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>A * {identity}</code></td>
|
|
<td style="text-align: center;"><code>A</code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>A / B</code></td>
|
|
<td style="text-align: center;"><code>A, per<B></code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>A / A</code></td>
|
|
<td style="text-align: center;"><code>{identity}</code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>A / {identity}</code></td>
|
|
<td style="text-align: center;"><code>A</code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>{identity} / A</code></td>
|
|
<td style="text-align: center;"><code>{identity}, per<A></code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>pow<2>(A)</code></td>
|
|
<td style="text-align: center;"><code>power<A, 2></code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>pow<2>({identity})</code></td>
|
|
<td style="text-align: center;"><code>{identity}</code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>sqrt(A)</code> or <code>pow<1, 2>(A)</code></td>
|
|
<td style="text-align: center;"><code>power<A, 1, 2></code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>sqrt({identity})</code> or <code>pow<1, 2>({identity})</code></td>
|
|
<td style="text-align: center;"><code>{identity}</code></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<h3 id="simplifying-the-resulting-expression-templates">Simplifying the resulting expression templates<a class="headerlink" href="#simplifying-the-resulting-expression-templates" title="Permanent link">¶</a></h3>
|
|
<p>To limit the length and improve the readability of generated types, there are many rules to simplify
|
|
the resulting expression template.</p>
|
|
<ol>
|
|
<li>
|
|
<p><strong>Ordering</strong></p>
|
|
<p>The resulting comma-separated arguments of multiplication are always sorted according to
|
|
a specific predicate. This is why:</p>
|
|
<div class="language-cpp highlight"><pre><span></span><code><span id="__span-9-1"><a id="__codelineno-9-1" name="__codelineno-9-1" href="#__codelineno-9-1"></a><span class="k">static_assert</span><span class="p">(</span><span class="n">A</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">B</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">B</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">A</span><span class="p">);</span>
|
|
</span><span id="__span-9-2"><a id="__codelineno-9-2" name="__codelineno-9-2" href="#__codelineno-9-2"></a><span class="k">static_assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_same_v</span><span class="o"><</span><span class="k">decltype</span><span class="p">(</span><span class="n">A</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">B</span><span class="p">),</span><span class="w"> </span><span class="k">decltype</span><span class="p">(</span><span class="n">B</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">A</span><span class="p">)</span><span class="o">></span><span class="p">);</span>
|
|
</span></code></pre></div>
|
|
<p>This is probably the most important of all steps, as it allows comparing types and enables the rest of
|
|
simplification rules.</p>
|
|
</li>
|
|
<li>
|
|
<p><strong>Aggregation</strong></p>
|
|
<p>In case two of the same identifiers are found next to each other on the argument list they
|
|
will be aggregated in one entry:</p>
|
|
<table>
|
|
<thead>
|
|
<tr>
|
|
<th style="text-align: center;">Before</th>
|
|
<th style="text-align: center;">After</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td style="text-align: center;"><code>A, A</code></td>
|
|
<td style="text-align: center;"><code>power<A, 2></code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>A, power<A, 2></code></td>
|
|
<td style="text-align: center;"><code>power<A, 3></code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>power<A, 1, 2>, power<A, 2></code></td>
|
|
<td style="text-align: center;"><code>power<A, 5, 2></code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>power<A, 1, 2>, power<A, 1, 2></code></td>
|
|
<td style="text-align: center;"><code>A</code></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</li>
|
|
<li>
|
|
<p><strong>Simplification</strong></p>
|
|
<p>In case two of the same identifiers are found in the numerator and denominator argument lists;
|
|
they are being simplified into one entry:</p>
|
|
<table>
|
|
<thead>
|
|
<tr>
|
|
<th style="text-align: center;">Before</th>
|
|
<th style="text-align: center;">After</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td style="text-align: center;"><code>A, per<A></code></td>
|
|
<td style="text-align: center;"><code>{identity}</code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>power<A, 2>, per<A></code></td>
|
|
<td style="text-align: center;"><code>A</code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>power<A, 3>, per<A></code></td>
|
|
<td style="text-align: center;"><code>power<A, 2></code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>A, per<power<A, 2>></code></td>
|
|
<td style="text-align: center;"><code>{identity}, per<A></code></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</li>
|
|
<li>
|
|
<p><strong>Repacking</strong></p>
|
|
<p>In case an expression uses two results of other operations, the components of its arguments are repacked
|
|
into one resulting type and simplified there.</p>
|
|
<p>For example, assuming:</p>
|
|
<div class="language-cpp highlight"><pre><span></span><code><span id="__span-10-1"><a id="__codelineno-10-1" name="__codelineno-10-1" href="#__codelineno-10-1"></a><span class="k">constexpr</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">X</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">A</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="n">B</span><span class="p">;</span>
|
|
</span></code></pre></div>
|
|
<p>then:</p>
|
|
<table>
|
|
<thead>
|
|
<tr>
|
|
<th style="text-align: center;">Operation</th>
|
|
<th style="text-align: center;">Resulting template expression arguments</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td style="text-align: center;"><code>X * B</code></td>
|
|
<td style="text-align: center;"><code>A</code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>X * A</code></td>
|
|
<td style="text-align: center;"><code>power<A, 2>, per<B></code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>X * X</code></td>
|
|
<td style="text-align: center;"><code>power<A, 2>, per<power<B, 2>></code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>X / X</code></td>
|
|
<td style="text-align: center;"><code>{identity}</code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>X / A</code></td>
|
|
<td style="text-align: center;"><code>{identity}, per<B></code></td>
|
|
</tr>
|
|
<tr>
|
|
<td style="text-align: center;"><code>X / B</code></td>
|
|
<td style="text-align: center;"><code>A, per<power<B, 2>></code></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</li>
|
|
</ol>
|
|
<h2 id="example">Example<a class="headerlink" href="#example" title="Permanent link">¶</a></h2>
|
|
<p>Thanks to all of the features described above, a user may write the code like this one:</p>
|
|
<div class="language-cpp highlight"><pre><span></span><code><span id="__span-11-1"><a id="__codelineno-11-1" name="__codelineno-11-1" href="#__codelineno-11-1"></a><span class="k">using</span><span class="w"> </span><span class="k">namespace</span><span class="w"> </span><span class="nn">mp_units</span><span class="o">::</span><span class="nn">si</span><span class="o">::</span><span class="nn">unit_symbols</span><span class="p">;</span>
|
|
</span><span id="__span-11-2"><a id="__codelineno-11-2" name="__codelineno-11-2" href="#__codelineno-11-2"></a><span class="k">auto</span><span class="w"> </span><span class="n">speed</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mf">60.</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">isq</span><span class="o">::</span><span class="n">speed</span><span class="p">[</span><span class="n">km</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="n">h</span><span class="p">];</span>
|
|
</span><span id="__span-11-3"><a id="__codelineno-11-3" name="__codelineno-11-3" href="#__codelineno-11-3"></a><span class="k">auto</span><span class="w"> </span><span class="n">duration</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">8</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">s</span><span class="p">;</span>
|
|
</span><span id="__span-11-4"><a id="__codelineno-11-4" name="__codelineno-11-4" href="#__codelineno-11-4"></a><span class="k">auto</span><span class="w"> </span><span class="n">acceleration</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">speed</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="n">duration</span><span class="p">;</span>
|
|
</span><span id="__span-11-5"><a id="__codelineno-11-5" name="__codelineno-11-5" href="#__codelineno-11-5"></a><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"acceleration: "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">acceleration</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">" ("</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">acceleration</span><span class="p">.</span><span class="n">in</span><span class="p">(</span><span class="n">m</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="n">s2</span><span class="p">)</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">")</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span>
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</span></code></pre></div>
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<p>The <code>acceleration</code>, being the result of the above code, has the following type
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(after stripping the <code>mp_units</code> namespace for brevity):</p>
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<div class="language-text highlight"><pre><span></span><code><span id="__span-12-1"><a id="__codelineno-12-1" name="__codelineno-12-1" href="#__codelineno-12-1"></a>quantity<reference<derived_quantity_spec<isq::speed, per<isq::time>>{}, derived_unit<si::kilo_<si::metre{}>, per<non_si::hour, si::second>>{}>{}, int>
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</span></code></pre></div>
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<p>and the text output presents:</p>
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<div class="language-text highlight"><pre><span></span><code><span id="__span-13-1"><a id="__codelineno-13-1" name="__codelineno-13-1" href="#__codelineno-13-1"></a>acceleration: 7.5 km h⁻¹ s⁻¹ (2.08333 m/s²)
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</span></code></pre></div>
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