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<h1 id="hello_units"><code>hello_units</code><a class="headerlink" href="#hello_units" title="Permanent link">&para;</a></h1>
<div class="admonition example">
<p class="admonition-title"><a href="https://godbolt.org/z/MYn5qjPzh">Try it on Compiler Explorer</a></p>
</div>
<p>This is a really simple example showcasing the features of the <strong>mp-units</strong> library.</p>
<p>First, we either import the <code>mp_units</code> module or include the headers for:</p>
<ul>
<li>an International System of Quantities (ISQ),</li>
<li>an International System of units (SI),</li>
<li>units derived from the International Yard and Pound,</li>
<li>text formatting and stream output support.</li>
</ul>
<div class="language-cpp highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">hello_units.cpp</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-0-1"> 1</a></span>
<span class="normal"><a href="#__codelineno-0-2"> 2</a></span>
<span class="normal"><a href="#__codelineno-0-3"> 3</a></span>
<span class="normal"><a href="#__codelineno-0-4"> 4</a></span>
<span class="normal"><a href="#__codelineno-0-5"> 5</a></span>
<span class="normal"><a href="#__codelineno-0-6"> 6</a></span>
<span class="normal"><a href="#__codelineno-0-7"> 7</a></span>
<span class="normal"><a href="#__codelineno-0-8"> 8</a></span>
<span class="normal"><a href="#__codelineno-0-9"> 9</a></span>
<span class="normal"><a href="#__codelineno-0-10">10</a></span>
<span class="normal"><a href="#__codelineno-0-11">11</a></span>
<span class="normal"><a href="#__codelineno-0-12">12</a></span>
<span class="normal"><a href="#__codelineno-0-13">13</a></span>
<span class="normal"><a href="#__codelineno-0-14">14</a></span>
<span class="normal"><a href="#__codelineno-0-15">15</a></span>
<span class="normal"><a href="#__codelineno-0-16">16</a></span>
<span class="normal"><a href="#__codelineno-0-17">17</a></span></pre></div></td><td class="code"><div><pre><span></span><code><span id="__span-0-1"><a id="__codelineno-0-1" name="__codelineno-0-1"></a><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;mp-units/compat_macros.h&gt;</span>
</span><span id="__span-0-2"><a id="__codelineno-0-2" name="__codelineno-0-2"></a><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;mp-units/ext/format.h&gt;</span>
</span><span id="__span-0-3"><a id="__codelineno-0-3" name="__codelineno-0-3"></a><span class="cp">#ifdef MP_UNITS_IMPORT_STD</span>
</span><span id="__span-0-4"><a id="__codelineno-0-4" name="__codelineno-0-4"></a><span class="k">import</span><span class="w"> </span><span class="n">std</span><span class="p">;</span>
</span><span id="__span-0-5"><a id="__codelineno-0-5" name="__codelineno-0-5"></a><span class="cp">#else</span>
</span><span id="__span-0-6"><a id="__codelineno-0-6" name="__codelineno-0-6"></a><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;iomanip&gt;</span>
</span><span id="__span-0-7"><a id="__codelineno-0-7" name="__codelineno-0-7"></a><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;iostream&gt;</span>
</span><span id="__span-0-8"><a id="__codelineno-0-8" name="__codelineno-0-8"></a><span class="cp">#endif</span>
</span><span id="__span-0-9"><a id="__codelineno-0-9" name="__codelineno-0-9"></a><span class="cp">#ifdef MP_UNITS_MODULES</span>
</span><span id="__span-0-10"><a id="__codelineno-0-10" name="__codelineno-0-10"></a><span class="k">import</span><span class="w"> </span><span class="n">mp_units</span><span class="p">;</span>
</span><span id="__span-0-11"><a id="__codelineno-0-11" name="__codelineno-0-11"></a><span class="cp">#else</span>
</span><span id="__span-0-12"><a id="__codelineno-0-12" name="__codelineno-0-12"></a><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;mp-units/format.h&gt;</span>
</span><span id="__span-0-13"><a id="__codelineno-0-13" name="__codelineno-0-13"></a><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;mp-units/ostream.h&gt;</span>
</span><span id="__span-0-14"><a id="__codelineno-0-14" name="__codelineno-0-14"></a><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;mp-units/systems/international.h&gt;</span>
</span><span id="__span-0-15"><a id="__codelineno-0-15" name="__codelineno-0-15"></a><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;mp-units/systems/isq.h&gt;</span>
</span><span id="__span-0-16"><a id="__codelineno-0-16" name="__codelineno-0-16"></a><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;mp-units/systems/si.h&gt;</span>
</span><span id="__span-0-17"><a id="__codelineno-0-17" name="__codelineno-0-17"></a><span class="cp">#endif</span>
</span></code></pre></div></td></tr></table></div>
<p>Also, to shorten the definitions, we "import" all the symbols from the <code>mp_units</code> namespace.</p>
<div class="language-cpp highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">hello_units.cpp</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-1-18">18</a></span></pre></div></td><td class="code"><div><pre><span></span><code><span id="__span-1-18"><a id="__codelineno-1-18" name="__codelineno-1-18"></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="p">;</span>
</span></code></pre></div></td></tr></table></div>
<p>Next, we define a simple function that calculates the average speed based on the provided
arguments of length and time:</p>
<div class="language-cpp highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">hello_units.cpp</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-2-19">19</a></span>
<span class="normal"><a href="#__codelineno-2-20">20</a></span>
<span class="normal"><a href="#__codelineno-2-21">21</a></span>
<span class="normal"><a href="#__codelineno-2-22">22</a></span></pre></div></td><td class="code"><div><pre><span></span><code><span id="__span-2-19"><a id="__codelineno-2-19" name="__codelineno-2-19"></a><span class="k">constexpr</span><span class="w"> </span><span class="n">QuantityOf</span><span class="o">&lt;</span><span class="n">isq</span><span class="o">::</span><span class="n">speed</span><span class="o">&gt;</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">avg_speed</span><span class="p">(</span><span class="n">QuantityOf</span><span class="o">&lt;</span><span class="n">isq</span><span class="o">::</span><span class="n">length</span><span class="o">&gt;</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">d</span><span class="p">,</span><span class="w"> </span><span class="n">QuantityOf</span><span class="o">&lt;</span><span class="n">isq</span><span class="o">::</span><span class="n">time</span><span class="o">&gt;</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">t</span><span class="p">)</span>
</span><span id="__span-2-20"><a id="__codelineno-2-20" name="__codelineno-2-20"></a><span class="p">{</span>
</span><span id="__span-2-21"><a id="__codelineno-2-21" name="__codelineno-2-21"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">d</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="n">t</span><span class="p">;</span>
</span><span id="__span-2-22"><a id="__codelineno-2-22" name="__codelineno-2-22"></a><span class="p">}</span>
</span></code></pre></div></td></tr></table></div>
<p>The above function template takes any quantities implicitly convertible to <code>isq::length</code>
and <code>isq::time</code>, respectively. Those quantities can use any compatible unit and a
representation type. The function returns a result of a straightforward equation and ensures
that its quantity type is implicitly convertible to <code>isq::speed</code>.</p>
<div class="admonition tip">
<p class="admonition-title">Tip</p>
<p>Besides verifying the type returned from the function, constraining a generic return
type is beneficial for users of such a function as it provides more information
of what to expect from a function than just using <code>auto</code>.</p>
</div>
<div class="language-cpp highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">hello_units.cpp</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-3-23">23</a></span>
<span class="normal"><a href="#__codelineno-3-24">24</a></span>
<span class="normal"><a href="#__codelineno-3-25">25</a></span>
<span class="normal"><a href="#__codelineno-3-26">26</a></span></pre></div></td><td class="code"><div><pre><span></span><code><span id="__span-3-23"><a id="__codelineno-3-23" name="__codelineno-3-23"></a><span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span>
</span><span id="__span-3-24"><a id="__codelineno-3-24" name="__codelineno-3-24"></a><span class="p">{</span>
</span><span id="__span-3-25"><a id="__codelineno-3-25" name="__codelineno-3-25"></a><span class="w"> </span><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-3-26"><a id="__codelineno-3-26" name="__codelineno-3-26"></a><span class="w"> </span><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">international</span><span class="o">::</span><span class="nn">unit_symbols</span><span class="p">;</span>
</span></code></pre></div></td></tr></table></div>
<p>The above lines explicitly opt into using unit symbols from two systems of units.
As this introduces a lot of short identifiers into the current scope, it is not done
implicitly while including a header file.</p>
<div class="language-cpp highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">hello_units.cpp</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-4-27">27</a></span>
<span class="normal"><a href="#__codelineno-4-28">28</a></span>
<span class="normal"><a href="#__codelineno-4-29">29</a></span>
<span class="normal"><a href="#__codelineno-4-30">30</a></span>
<span class="normal"><a href="#__codelineno-4-31">31</a></span>
<span class="normal"><a href="#__codelineno-4-32">32</a></span>
<span class="normal"><a href="#__codelineno-4-33">33</a></span></pre></div></td><td class="code"><div><pre><span></span><code><span id="__span-4-27"><a id="__codelineno-4-27" name="__codelineno-4-27"></a><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">quantity</span><span class="w"> </span><span class="n">v1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">110</span><span class="w"> </span><span class="o">*</span><span class="w"> </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-4-28"><a id="__codelineno-4-28" name="__codelineno-4-28"></a><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">quantity</span><span class="w"> </span><span class="n">v2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">70</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">mph</span><span class="p">;</span>
</span><span id="__span-4-29"><a id="__codelineno-4-29" name="__codelineno-4-29"></a><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">quantity</span><span class="w"> </span><span class="n">v3</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">avg_speed</span><span class="p">(</span><span class="mf">220.</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">km</span><span class="p">,</span><span class="w"> </span><span class="mi">2</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-4-30"><a id="__codelineno-4-30" name="__codelineno-4-30"></a><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">quantity</span><span class="w"> </span><span class="n">v4</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">avg_speed</span><span class="p">(</span><span class="n">isq</span><span class="o">::</span><span class="n">distance</span><span class="p">(</span><span class="mf">140.</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">mi</span><span class="p">),</span><span class="w"> </span><span class="mi">2</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">duration</span><span class="p">[</span><span class="n">h</span><span class="p">]);</span>
</span><span id="__span-4-31"><a id="__codelineno-4-31" name="__codelineno-4-31"></a><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">quantity</span><span class="w"> </span><span class="n">v5</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">v3</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">s</span><span class="p">);</span>
</span><span id="__span-4-32"><a id="__codelineno-4-32" name="__codelineno-4-32"></a><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">quantity</span><span class="w"> </span><span class="n">v6</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">value_cast</span><span class="o">&lt;</span><span class="n">m</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="n">s</span><span class="o">&gt;</span><span class="p">(</span><span class="n">v4</span><span class="p">);</span>
</span><span id="__span-4-33"><a id="__codelineno-4-33" name="__codelineno-4-33"></a><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">quantity</span><span class="w"> </span><span class="n">v7</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">value_cast</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="p">(</span><span class="n">v6</span><span class="p">);</span>
</span></code></pre></div></td></tr></table></div>
<ul>
<li>Lines <code>27</code> &amp; <code>28</code> create a quantity of kind <code>isq::length / isq::time</code> with the numbers
and units provided. Such quantities can be converted or assigned to any other quantity
with a matching kind.</li>
<li>Line <code>29</code> calls our function template with quantities of kind <code>isq::length</code> and
<code>isq::time</code> and number and units provided.</li>
<li>Line <code>30</code> explicitly provides quantity types of the quantities passed to a function template.
This time, those will not be quantity kinds anymore and will have
<a href="../../framework_basics/simple_and_typed_quantities/#quantity_cast-to-force-unsafe-conversions">more restrictive conversion rules</a>.</li>
<li>Line <code>31</code> changes the unit of a quantity <code>v3</code> to <code>m / s</code> in a
<a href="../../framework_basics/value_conversions/#value-preserving-conversions">value-preserving way</a>
(floating-point representations are considered to be value-preserving).</li>
<li>Line <code>32</code> does a similar operation, but this time, it would also succeed for
<a href="../../framework_basics/value_conversions/#value-truncating-conversions">value-truncating cases</a>
(if that was the case).</li>
<li>Line <code>33</code> does a <a href="../../framework_basics/value_conversions/#value-truncating-conversions">value-truncating conversion</a>
of changing the underlying representation type from <code>double</code> to <code>int</code>.</li>
</ul>
<div class="language-cpp highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">hello_units.cpp</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-5-34">34</a></span>
<span class="normal"><a href="#__codelineno-5-35">35</a></span>
<span class="normal"><a href="#__codelineno-5-36">36</a></span>
<span class="normal"><a href="#__codelineno-5-37">37</a></span>
<span class="normal"><a href="#__codelineno-5-38">38</a></span>
<span class="normal"><a href="#__codelineno-5-39">39</a></span>
<span class="normal"><a href="#__codelineno-5-40">40</a></span>
<span class="normal"><a href="#__codelineno-5-41">41</a></span></pre></div></td><td class="code"><div><pre><span></span><code><span id="__span-5-34"><a id="__codelineno-5-34" name="__codelineno-5-34"></a><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">v1</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="sc">&#39;\n&#39;</span><span class="p">;</span><span class="w"> </span><span class="c1">// 110 km/h</span>
</span><span id="__span-5-35"><a id="__codelineno-5-35" name="__codelineno-5-35"></a><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">setw</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">setfill</span><span class="p">(</span><span class="sc">&#39;*&#39;</span><span class="p">)</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">v2</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="sc">&#39;\n&#39;</span><span class="p">;</span><span class="w"> </span><span class="c1">// ***70 mi/h</span>
</span><span id="__span-5-36"><a id="__codelineno-5-36" name="__codelineno-5-36"></a><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">MP_UNITS_STD_FMT</span><span class="o">::</span><span class="n">format</span><span class="p">(</span><span class="s">&quot;{:*^10}</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">,</span><span class="w"> </span><span class="n">v3</span><span class="p">);</span><span class="w"> </span><span class="c1">// *110 km/h*</span>
</span><span id="__span-5-37"><a id="__codelineno-5-37" name="__codelineno-5-37"></a><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">MP_UNITS_STD_FMT</span><span class="o">::</span><span class="n">format</span><span class="p">(</span><span class="s">&quot;{:%N in %U of %D}</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">,</span><span class="w"> </span><span class="n">v4</span><span class="p">);</span><span class="w"> </span><span class="c1">// 70 in mi/h of LT⁻¹</span>
</span><span id="__span-5-38"><a id="__codelineno-5-38" name="__codelineno-5-38"></a><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">MP_UNITS_STD_FMT</span><span class="o">::</span><span class="n">format</span><span class="p">(</span><span class="s">&quot;{::N[.2f]}</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">,</span><span class="w"> </span><span class="n">v5</span><span class="p">);</span><span class="w"> </span><span class="c1">// 30.56 m/s</span>
</span><span id="__span-5-39"><a id="__codelineno-5-39" name="__codelineno-5-39"></a><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">MP_UNITS_STD_FMT</span><span class="o">::</span><span class="n">format</span><span class="p">(</span><span class="s">&quot;{::N[.2f]U[dn]}</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">,</span><span class="w"> </span><span class="n">v6</span><span class="p">);</span><span class="w"> </span><span class="c1">// 31.29 m⋅s⁻¹</span>
</span><span id="__span-5-40"><a id="__codelineno-5-40" name="__codelineno-5-40"></a><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">MP_UNITS_STD_FMT</span><span class="o">::</span><span class="n">format</span><span class="p">(</span><span class="s">&quot;{:%N}</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">,</span><span class="w"> </span><span class="n">v7</span><span class="p">);</span><span class="w"> </span><span class="c1">// 31</span>
</span><span id="__span-5-41"><a id="__codelineno-5-41" name="__codelineno-5-41"></a><span class="p">}</span>
</span></code></pre></div></td></tr></table></div>
<p>The above presents <a href="../../framework_basics/text_output/">various ways to print a quantity</a>.
Both stream insertion operations and <code>std::format</code> facilities are supported.</p>
<div class="admonition tip">
<p class="admonition-title">Tip</p>
<p><code>MP_UNITS_STD_FMT</code> is used for compatibility reasons. If a specific compiler
does not support <code>std::format</code> or a user prefers to use the <code>{fmt}</code> library, this macro
will resolve to <code>fmt</code> namespace. Otherwise, the <code>std</code> namespace will be used.</p>
<p>More about it can be found in the <a href="../../use_cases/wide_compatibility/#mp_units_std_fmt">Wide Compatibility</a>
chapter.</p>
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