forked from mpusz/mp-units
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<h1 id="value-conversions">Value Conversions<a class="headerlink" href="#value-conversions" title="Permanent link">¶</a></h1>
|
|
<h2 id="value-preserving-conversions">Value-preserving conversions<a class="headerlink" href="#value-preserving-conversions" title="Permanent link">¶</a></h2>
|
|
<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="k">auto</span><span class="w"> </span><span class="n">q1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">5</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">km</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="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="n">q1</span><span class="p">.</span><span class="n">in</span><span class="p">(</span><span class="n">m</span><span class="p">)</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="sc">'\n'</span><span class="p">;</span>
|
|
</span><span id="__span-0-3"><a id="__codelineno-0-3" name="__codelineno-0-3" href="#__codelineno-0-3"></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="p">,</span><span class="w"> </span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">q2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">q1</span><span class="p">;</span>
|
|
</span></code></pre></div>
|
|
<p>The second line above converts the current quantity to the one expressed in metres and prints its
|
|
contents. The third line converts the quantity expressed in kilometres into the one measured
|
|
in metres.</p>
|
|
<div class="admonition important">
|
|
<p class="admonition-title">Important</p>
|
|
<p>It is always assumed that one can convert a quantity into another one with a unit of a higher
|
|
resolution. There is no protection against overflow of the representation type. In case the target
|
|
quantity ends up with a value bigger than the representation type can handle, you will be facing
|
|
Undefined Behavior.</p>
|
|
</div>
|
|
<p>In case a user would like to perform an opposite transformation:</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="k">auto</span><span class="w"> </span><span class="n">q1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">5</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">m</span><span class="p">;</span>
|
|
</span><span id="__span-1-2"><a id="__codelineno-1-2" name="__codelineno-1-2" href="#__codelineno-1-2"></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="n">q1</span><span class="p">.</span><span class="n">in</span><span class="p">(</span><span class="n">km</span><span class="p">)</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="sc">'\n'</span><span class="p">;</span>
|
|
</span><span id="__span-1-3"><a id="__codelineno-1-3" name="__codelineno-1-3" href="#__codelineno-1-3"></a><span class="n">quantity</span><span class="o"><</span><span class="n">si</span><span class="o">::</span><span class="n">kilo</span><span class="o"><</span><span class="n">si</span><span class="o">::</span><span class="n">metre</span><span class="o">></span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">q2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">q1</span><span class="p">;</span>
|
|
</span></code></pre></div>
|
|
<p>Both conversions will fail to compile.</p>
|
|
<p>There are two ways to make the above work. The first solution is to use a floating-point
|
|
representation type:</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">auto</span><span class="w"> </span><span class="n">q1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mf">5.</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">m</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">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="n">q1</span><span class="p">.</span><span class="n">in</span><span class="p">(</span><span class="n">km</span><span class="p">)</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="sc">'\n'</span><span class="p">;</span>
|
|
</span><span id="__span-2-3"><a id="__codelineno-2-3" name="__codelineno-2-3" href="#__codelineno-2-3"></a><span class="n">quantity</span><span class="o"><</span><span class="n">si</span><span class="o">::</span><span class="n">kilo</span><span class="o"><</span><span class="n">si</span><span class="o">::</span><span class="n">metre</span><span class="o">>></span><span class="w"> </span><span class="n">q2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">q1</span><span class="p">;</span>
|
|
</span></code></pre></div>
|
|
<p>or</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">auto</span><span class="w"> </span><span class="n">q1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">5</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">m</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="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="n">value_cast</span><span class="o"><</span><span class="kt">double</span><span class="o">></span><span class="p">(</span><span class="n">q1</span><span class="p">).</span><span class="n">in</span><span class="p">(</span><span class="n">km</span><span class="p">)</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="sc">'\n'</span><span class="p">;</span>
|
|
</span><span id="__span-3-3"><a id="__codelineno-3-3" name="__codelineno-3-3" href="#__codelineno-3-3"></a><span class="n">quantity</span><span class="o"><</span><span class="n">si</span><span class="o">::</span><span class="n">kilo</span><span class="o"><</span><span class="n">si</span><span class="o">::</span><span class="n">metre</span><span class="o">>></span><span class="w"> </span><span class="n">q2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">q1</span><span class="p">;</span><span class="w"> </span><span class="c1">// double by default</span>
|
|
</span></code></pre></div>
|
|
<div class="admonition important">
|
|
<p class="admonition-title">Important</p>
|
|
<p>The <strong>mp-units</strong> library follows <a href="https://en.cppreference.com/w/cpp/chrono/duration"><code>std::chrono::duration</code></a>
|
|
logic and treats floating-point types as value-preserving.</p>
|
|
</div>
|
|
<h2 id="value-truncating-conversions">Value-truncating conversions<a class="headerlink" href="#value-truncating-conversions" title="Permanent link">¶</a></h2>
|
|
<p>The second solution is to force a truncating conversion:</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="k">auto</span><span class="w"> </span><span class="n">q1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">5</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">m</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="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="n">value_cast</span><span class="o"><</span><span class="n">km</span><span class="o">></span><span class="p">(</span><span class="n">q1</span><span class="p">)</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="sc">'\n'</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="n">quantity</span><span class="o"><</span><span class="n">si</span><span class="o">::</span><span class="n">kilo</span><span class="o"><</span><span class="n">si</span><span class="o">::</span><span class="n">metre</span><span class="o">></span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">q2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">q1</span><span class="p">.</span><span class="n">force_in</span><span class="p">(</span><span class="n">km</span><span class="p">);</span>
|
|
</span></code></pre></div>
|
|
<p>This explicit cast makes it clear that something unsafe is going on. It is easy to spot in code
|
|
reviews or while chasing a bug in the source code.</p>
|
|
<div class="admonition note">
|
|
<p class="admonition-title">Note</p>
|
|
<p><code>q.force_in(U)</code> is just a shortcut to run <code>value_cast<U>(q)</code>. There is no difference in behavior
|
|
between those two interfaces. <code>q.force_in(U)</code> was added for consistency with <code>q.in(U)</code> and
|
|
<code>q.force_numerical_value_in(U)</code>.</p>
|
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</div>
|
|
<p>Another place where this cast is useful is when a user wants to convert a quantity with
|
|
a floating-point representation to the one using an integral one. Again this is a truncating
|
|
conversion, so an explicit cast is needed:</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">si</span><span class="o">::</span><span class="n">metre</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">q3</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">value_cast</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">(</span><span class="mf">3.14</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">m</span><span class="p">);</span>
|
|
</span></code></pre></div>
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|
<div class="admonition info">
|
|
<p class="admonition-title">Info</p>
|
|
<p>It is often fine to use an integral as a representation type, but in general, floating-point
|
|
types provide better precision and are privileged in the library as they are considered
|
|
to be value-preserving.</p>
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