mp-units/HEAD/api_reference/gen/derived.qty.html

<!DOCTYPE html><html lang='en'><head><title>[derived.qty]</title><meta charset='UTF-8'/><link rel='stylesheet' type='text/css' href='14882.css'/><link rel='stylesheet' type='text/css' href='https://fonts.googleapis.com/css2?family=Noto+Serif'/><link rel='stylesheet' type='text/css' href='https://fonts.googleapis.com/css2?family=Noto+Sans'/><link rel='stylesheet' type='text/css' href='https://fonts.googleapis.com/css2?family=Noto+Sans+Mono'/><link rel='icon' href='icon.png'/><link rel='stylesheet' type='text/css' href='expanded.css' title='Normal'/><link rel='alternate stylesheet' type='text/css' href='colored.css' title='Notes and examples colored'/><link rel='alternate stylesheet' type='text/css' href='normative-only.css' title='Notes and examples hidden'/></head><body><div class='wrapper'><h1 ><a class='secnum' style='min-width:50pt'>5</a> Quantities and units library <a class='abbr_ref' href='index.html#quantities'>[quantities]</a></h1><h2 ><a class='secnum' style='min-width:65pt'>5.4</a> Reference <a class='abbr_ref' href='qty.ref.html#derived.qty'>[qty.ref]</a></h2><h3 ><a class='secnum' style='min-width:80pt'>5.4.3</a> Quantity specification <a class='abbr_ref' href='qty.spec.html#derived.qty'>[qty.spec]</a></h3><h4 ><a class='secnum' style='min-width:95pt'>5.4.3.3</a> Types <a class='abbr_ref' href='qty.spec.types.html#derived.qty'>[qty.spec.types]</a></h4><h4 ><a class='secnum' style='min-width:110pt'>5.4.3.3.2</a> Derived <a class='abbr_ref'>[derived.qty]</a></h4><div class='texpara'><span id='lib:derived_quantity_spec'><div class='marginalizedparent'><a class='itemDeclLink' href='#lib:derived_quantity_spec'>🔗</a></div><span class='codeblock'><span class='keyword'>namespace</span> mp_units <span class='curlybracket'>{</span>

<span class='keyword'>template</span><span class='anglebracket'>&lt;</span><i >NamedQuantitySpec</i><a class='index' id='concept:NamedQuantitySpec'></a> Q<span class='anglebracket'>&gt;</span>
<span class='keyword'>using</span> <i >to-dimension</i> <span class='operator'>=</span> <span class='keyword'>decltype</span><span class='parenthesis'>(</span><span class='keyword'>auto</span><span class='parenthesis'>(</span>Q<span class='operator'>::</span>dimension<span class='parenthesis'>)</span><span class='parenthesis'>)</span>;  <span class='comment'>// <i >exposition only</i></span>

<span class='keyword'>template</span><span class='anglebracket'>&lt;</span><span class='keyword'>typename</span><span class='operator'>.</span><span class='operator'>.</span><span class='operator'>.</span> Expr<span class='anglebracket'>&gt;</span>
<span class='keyword'>struct</span> <i >derived-quantity-spec-impl</i> <span class='operator'>:</span>                 <span class='comment'>// <i >exposition only</i></span>
    <i >quantity-spec-interface</i>,
    <i >expr-fractions</i><span class='anglebracket'>&lt;</span><span class='keyword'>struct</span> dimensionless, Expr<span class='operator'>.</span><span class='operator'>.</span><span class='operator'>.</span><span class='anglebracket'>&gt;</span> <span class='curlybracket'>{</span>
  <span class='keyword'>using</span> <i >base-type</i> <span class='operator'>=</span> <i >derived-quantity-spec-impl</i>;
  <span class='keyword'>using</span> <i >base</i> <span class='operator'>=</span> <i >expr-fractions</i><span class='anglebracket'>&lt;</span><span class='keyword'>struct</span> dimensionless, Expr<span class='operator'>.</span><span class='operator'>.</span><span class='operator'>.</span><span class='anglebracket'>&gt;</span>;

  <span class='keyword'>static</span> <span class='keyword'>constexpr</span> <a href='qty.dim.concepts.html#concept:Dimension' title='5.4.2.2&emsp;Concepts&emsp;[qty.dim.concepts]'><span id='conceptref:Dimension'><span class='tcode_in_codeblock'>Dimension</span></span></a> <span class='keyword'>auto</span> dimension <span class='operator'>=</span>
    <i >expr-map</i><span class='anglebracket'>&lt;</span><i >to-dimension</i>, derived_dimension, <span class='keyword'>struct</span> dimension_one<span class='anglebracket'>&gt;</span><span class='parenthesis'>(</span><i >base</i><span class='curlybracket'>{</span><span class='curlybracket'>}</span><span class='parenthesis'>)</span>;
  <span class='keyword'>static</span> <span class='keyword'>constexpr</span> quantity_character character <span class='operator'>=</span> <i ><span class='texttt'>see below</span></i>;
<span class='curlybracket'>}</span>;

<span class='keyword'>template</span><span class='anglebracket'>&lt;</span><a class='index' id='conceptref:SymbolicConstant'></a><i >SymbolicConstant</i><span class='operator'>.</span><span class='operator'>.</span><span class='operator'>.</span> Expr<span class='anglebracket'>&gt;</span>
<span class='keyword'>struct</span> derived_quantity_spec <span class='keyword'>final</span> <span class='operator'>:</span> <i >derived-quantity-spec-impl</i><span class='anglebracket'>&lt;</span>Expr<span class='operator'>.</span><span class='operator'>.</span><span class='operator'>.</span><span class='anglebracket'>&gt;</span> <span class='curlybracket'>{</span><span class='curlybracket'>}</span>;

<span class='curlybracket'>}</span>
</span></span></div><div class='para' id='1'><div class='marginalizedparent'><a class='marginalized' href='#1'>1</a></div><div class='sourceLinkParent'><a class='sourceLink' href='https://github.com/mpusz/mp-unitse9c5f7236ebfe1b287a5a4a49aaf57fbefa87233/source/quantities.tex#L1768'>#</a></div><div class='texpara'><div id='1.sentence-1' class='sentence'><span class='texttt'>derived_<span class='shy'></span>quantity_<span class='shy'></span>spec</span> is used by the library
to represent the result of a quantity calculus not equal to a named quantity<a class='hidden_link' href='#1.sentence-1'>.</a></div> <div id='example-1' class='example'><div class='texpara'>[<i>Example&nbsp;<a href='#example-1'>1</a></i>:&ensp;<span class='codeblock'><span class='keyword'>constexpr</span> <span class='keyword'>auto</span> area <span class='operator'>=</span> pow<span class='anglebracket'>&lt;</span><span class='literal'>2</span><span class='anglebracket'>&gt;</span><span class='parenthesis'>(</span>isq<span class='operator'>::</span>length<span class='parenthesis'>)</span>;
<span class='keyword'>int</span> x <span class='operator'>=</span> area;  <span class='comment'>// error: cannot construct from <span class='tcode_in_codeblock'>derived_<span class='shy'></span>quantity_<span class='shy'></span>spec&lt;power&lt;isq&#x200b;::&#x200b;length, 2&gt;&gt;</span></span>
</span> —&nbsp;<i>end example</i>]</div></div> <div id='1.sentence-2' class='sentence'>
A program that instantiates a specialization of <span class='texttt'>derived_<span class='shy'></span>quantity_<span class='shy'></span>spec</span>
that is not a possible result of the library specifications
is ill-formed, no diagnostic required<a class='hidden_link' href='#1.sentence-2'>.</a></div></div></div><div class='para' id='2'><div class='marginalizedparent'><a class='marginalized' href='#2'>2</a></div><div class='sourceLinkParent'><a class='sourceLink' href='https://github.com/mpusz/mp-unitse9c5f7236ebfe1b287a5a4a49aaf57fbefa87233/source/quantities.tex#L1781'>#</a></div><div class='texpara'><div id='2.sentence-1' class='sentence'>Let
<ul class='itemize'><li id='2.1'><div class='marginalizedparent' style='left:-27mm'><a class='marginalized' href='#2.1'>(2.1)</a></div><span class='texttt'>Nums</span> and <span class='texttt'>Dens</span>
be packs denoting the template arguments of
<span class='texttt'><span class='texttt'><i >base</i></span><span class='operator'>&#x200b;::&#x200b;</span><span class='texttt'><i >nums</i></span></span> and <span class='texttt'><span class='texttt'><i >base</i></span><span class='operator'>&#x200b;::&#x200b;</span><span class='texttt'><i >dens</i></span></span>, respectively,</li><li id='2.2'><div class='marginalizedparent' style='left:-27mm'><a class='marginalized' href='#2.2'>(2.2)</a></div><span class='texttt'><i >QUANTITY-CHARACTER-OF</i><span class='parenthesis'>(</span>Pack<span class='parenthesis'>)</span></span> be
<span class='codeblock'>std<span class='operator'>::</span>max<span class='parenthesis'>(</span><span class='curlybracket'>{</span>quantity_character<span class='operator'>::</span>real_scalar, <i >expr-type</i><span class='anglebracket'>&lt;</span>Pack<span class='anglebracket'>&gt;</span><span class='operator'>::</span>character<span class='operator'>.</span><span class='operator'>.</span><span class='operator'>.</span><span class='curlybracket'>}</span><span class='parenthesis'>)</span>
</span>
and</li><li id='2.3'><div class='marginalizedparent' style='left:-27mm'><a class='marginalized' href='#2.3'>(2.3)</a></div><span class='texttt'>num_<span class='shy'></span>char</span> be <span class='texttt'><i >QUANTITY-CHARACTER-OF</i><span class='parenthesis'>(</span>Nums<span class='parenthesis'>)</span></span> and
<span class='texttt'>den_<span class='shy'></span>char</span> be <span class='texttt'><i >QUANTITY-CHARACTER-OF</i><span class='parenthesis'>(</span>Dens<span class='parenthesis'>)</span></span><a class='hidden_link' href='#2.sentence-1'>.</a></li></ul></div> <div id='2.sentence-2' class='sentence'>
The member <span class='texttt'>character</span> is equal to
<span class='texttt'>quantity_<span class='shy'></span>character<span class='operator'>&#x200b;::&#x200b;</span>real_<span class='shy'></span>scalar</span> if <span class='texttt'>num_<span class='shy'></span>char <span class='operator'>=</span><span class='operator'>=</span> den_<span class='shy'></span>char</span> is <span class='texttt'><span class='literal'>true</span></span>, and
<span class='texttt'>std<span class='operator'>&#x200b;::&#x200b;</span>max<span class='parenthesis'>(</span>num_<span class='shy'></span>char, den_<span class='shy'></span>char<span class='parenthesis'>)</span></span> otherwise<a class='hidden_link' href='#2.sentence-2'>.</a></div></div></div></div></body></html>