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<!DOCTYPE html><html lang='en'><head><title>[qty.unit.obs]</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#qty.unit.obs'>[qty.ref]</a></h2><h3 ><a class='secnum' style='min-width:80pt'>5.4.4</a> Unit <a class='abbr_ref' href='qty.unit.html#obs'>[qty.unit]</a></h3><h4 ><a class='secnum' style='min-width:95pt'>5.4.4.8</a> Observers <a class='abbr_ref'>[qty.unit.obs]</a></h4><div class='texpara'><div id='lib:AssociatedUnit,get_quantity_spec'><div id='lib:get_quantity_spec,AssociatedUnit'><div class='itemdecl'><div class='marginalizedparent'><a class='itemDeclLink' href='#lib:get_quantity_spec,AssociatedUnit'>🔗</a></div><code class='itemdeclcode'><span class='keyword'>consteval</span> <a href='qty.spec.concepts.html#concept:QuantitySpec' title='5.4.3.2&emsp;Concepts&emsp;[qty.spec.concepts]'><span id='conceptref:QuantitySpec'><span class='texttt'>QuantitySpec</span></span></a> <span class='keyword'>auto</span> get_quantity_spec<span class='parenthesis'>(</span><a href='qty.unit.concepts.html#concept:AssociatedUnit' title='5.4.4.4&emsp;Concepts&emsp;[qty.unit.concepts]'><span id='conceptref:AssociatedUnit'><span class='texttt'>AssociatedUnit</span></span></a> <span class='keyword'>auto</span> u<span class='parenthesis'>)</span>;
</code></div></div></div></div><div class='para' id='1'><div class='marginalizedparent'><a class='marginalized' href='#1'>1</a></div><div class='itemdescr'><div class='sourceLinkParent'><a class='sourceLink' href='https://github.com/mpusz/mp-unitse9c5f7236ebfe1b287a5a4a49aaf57fbefa87233/source/quantities.tex#L3141'>#</a></div><div class='texpara'><div id='1.sentence-1' class='sentence'><i >Returns</i>: <span class='texttt'>kind_<span class='shy'></span>of<span class='anglebracket'>&lt;</span><span class='texttt'><i >get-associated-quantity</i></span><span class='parenthesis'>(</span>u<span class='parenthesis'>)</span><span class='anglebracket'>&gt;</span></span><a class='hidden_link' href='#1.sentence-1'>.</a></div></div></div></div><div class='texpara'><div id='lib:AssociatedUnit,get_unit'><div id='lib:get_unit,AssociatedUnit'><div class='itemdecl'><div class='marginalizedparent'><a class='itemDeclLink' href='#lib:get_unit,AssociatedUnit'>🔗</a></div><code class='itemdeclcode'><span class='keyword'>consteval</span> <a href='qty.unit.concepts.html#concept:Unit' title='5.4.4.4&emsp;Concepts&emsp;[qty.unit.concepts]'><span id='conceptref:Unit'><span class='texttt'>Unit</span></span></a> <span class='keyword'>auto</span> get_unit<span class='parenthesis'>(</span><a href='qty.unit.concepts.html#concept:AssociatedUnit' title='5.4.4.4&emsp;Concepts&emsp;[qty.unit.concepts]'><span id='conceptref:AssociatedUnit_'><span class='texttt'>AssociatedUnit</span></span></a> <span class='keyword'>auto</span> u<span class='parenthesis'>)</span>;
</code></div></div></div></div><div class='para' id='2'><div class='marginalizedparent'><a class='marginalized' href='#2'>2</a></div><div class='itemdescr'><div class='sourceLinkParent'><a class='sourceLink' href='https://github.com/mpusz/mp-unitse9c5f7236ebfe1b287a5a4a49aaf57fbefa87233/source/quantities.tex#L3152'>#</a></div><div class='texpara'><div id='2.sentence-1' class='sentence'><i >Returns</i>: <span class='texttt'>u</span><a class='hidden_link' href='#2.sentence-1'>.</a></div></div></div></div><div class='texpara'><div id='lib:Unit,get_common_unit'><div id='lib:get_common_unit,Unit'><div class='itemdecl'><div class='marginalizedparent'><a class='itemDeclLink' href='#lib:get_common_unit,Unit'>🔗</a></div><code class='itemdeclcode'><span class='keyword'>consteval</span> <a href='qty.unit.concepts.html#concept:Unit' title='5.4.4.4&emsp;Concepts&emsp;[qty.unit.concepts]'><span id='conceptref:Unit_'><span class='texttt'>Unit</span></span></a> <span class='keyword'>auto</span> get_common_unit<span class='parenthesis'>(</span><a href='qty.unit.concepts.html#concept:Unit' title='5.4.4.4&emsp;Concepts&emsp;[qty.unit.concepts]'><span id='conceptref:Unit__'><span class='texttt'>Unit</span></span></a> <span class='keyword'>auto</span><span class='operator'>.</span><span class='operator'>.</span><span class='operator'>.</span> us<span class='parenthesis'>)</span>
<span class='keyword'>requires</span> <i ><span class='texttt'>see below</span></i>;
</code></div></div></div></div><div class='para' id='3'><div class='marginalizedparent'><a class='marginalized' href='#3'>3</a></div><div class='itemdescr'><div class='sourceLinkParent'><a class='sourceLink' href='https://github.com/mpusz/mp-unitse9c5f7236ebfe1b287a5a4a49aaf57fbefa87233/source/quantities.tex#L3164'>#</a></div><div class='texpara'><div id='3.sentence-1' class='sentence'>Let
<ul class='itemize'><li id='3.1'><div class='marginalizedparent' style='left:-39mm'><a class='marginalized' href='#3.1'>(3.1)</a></div><span class='texttt'>u1</span> be <span class='texttt'>us<span class='operator'>.</span><span class='operator'>.</span><span class='operator'>.</span><span class='squarebracket'>[</span><span class='literal'>0</span><span class='squarebracket'>]</span></span>,</li><li id='3.2'><div class='marginalizedparent' style='left:-39mm'><a class='marginalized' href='#3.2'>(3.2)</a></div><span class='texttt'>u2</span> be <span class='texttt'>us<span class='operator'>.</span><span class='operator'>.</span><span class='operator'>.</span><span class='squarebracket'>[</span><span class='literal'>1</span><span class='squarebracket'>]</span></span>,</li><li id='3.3'><div class='marginalizedparent' style='left:-39mm'><a class='marginalized' href='#3.3'>(3.3)</a></div><span class='texttt'>U1</span> be <span class='texttt'><span class='keyword'>decltype</span><span class='parenthesis'>(</span>u1<span class='parenthesis'>)</span></span>,</li><li id='3.4'><div class='marginalizedparent' style='left:-39mm'><a class='marginalized' href='#3.4'>(3.4)</a></div><span class='texttt'>U2</span> be <span class='texttt'><span class='keyword'>decltype</span><span class='parenthesis'>(</span>u2<span class='parenthesis'>)</span></span>, and</li><li id='3.5'><div class='marginalizedparent' style='left:-39mm'><a class='marginalized' href='#3.5'>(3.5)</a></div><span class='texttt'>rest</span> be a pack denoting the elements of <span class='texttt'>us</span> without <span class='texttt'>u1</span> and <span class='texttt'>u2</span><a class='hidden_link' href='#3.sentence-1'>.</a></li></ul></div></div></div></div><div class='para' id='4'><div class='marginalizedparent'><a class='marginalized' href='#4'>4</a></div><div class='itemdescr'><div class='sourceLinkParent'><a class='sourceLink' href='https://github.com/mpusz/mp-unitse9c5f7236ebfe1b287a5a4a49aaf57fbefa87233/source/quantities.tex#L3179'>#</a></div><div class='texpara'><div id='4.sentence-1' class='sentence'><i >Effects</i>: Equivalent to:
<span class='codeblock'><span class='keyword'>if</span> <span class='keyword'>constexpr</span> <span class='parenthesis'>(</span><span class='keyword'>sizeof</span><span class='operator'>.</span><span class='operator'>.</span><span class='operator'>.</span><span class='parenthesis'>(</span>us<span class='parenthesis'>)</span> <span class='operator'>=</span><span class='operator'>=</span> <span class='literal'>1</span><span class='parenthesis'>)</span>
<span class='keyword'>return</span> u1;
<span class='keyword'>else</span> <span class='keyword'>if</span> <span class='keyword'>constexpr</span> <span class='parenthesis'>(</span><span class='keyword'>sizeof</span><span class='operator'>.</span><span class='operator'>.</span><span class='operator'>.</span><span class='parenthesis'>(</span>us<span class='parenthesis'>)</span> <span class='operator'>=</span><span class='operator'>=</span> <span class='literal'>2</span><span class='parenthesis'>)</span> <span class='curlybracket'>{</span>
<span class='keyword'>if</span> <span class='keyword'>constexpr</span> <span class='parenthesis'>(</span><i >is-derived-from-specialization-of</i><span class='anglebracket'>&lt;</span>U1, common_unit<span class='anglebracket'>&gt;</span><span class='parenthesis'>(</span><span class='parenthesis'>)</span><span class='parenthesis'>)</span> <span class='curlybracket'>{</span>
<span class='keyword'>return</span> <i >TBD.</i>;
<span class='curlybracket'>}</span> <span class='keyword'>else</span> <span class='keyword'>if</span> <span class='keyword'>constexpr</span> <span class='parenthesis'>(</span><i >is-derived-from-specialization-of</i><span class='anglebracket'>&lt;</span>U2, common_unit<span class='anglebracket'>&gt;</span><span class='parenthesis'>(</span><span class='parenthesis'>)</span><span class='parenthesis'>)</span>
<span class='keyword'>return</span> get_common_unit<span class='parenthesis'>(</span>u2, u1<span class='parenthesis'>)</span>;
<span class='keyword'>else</span> <span class='keyword'>if</span> <span class='keyword'>constexpr</span> <span class='parenthesis'>(</span>std<span class='operator'>::</span>is_same_v<span class='anglebracket'>&lt;</span>U1, U2<span class='anglebracket'>&gt;</span><span class='parenthesis'>)</span>
<span class='keyword'>return</span> u1;
<span class='keyword'>else</span> <span class='keyword'>if</span> <span class='keyword'>constexpr</span> <span class='parenthesis'>(</span>equivalent<span class='parenthesis'>(</span>U1<span class='curlybracket'>{</span><span class='curlybracket'>}</span>, U2<span class='curlybracket'>{</span><span class='curlybracket'>}</span><span class='parenthesis'>)</span><span class='parenthesis'>)</span> <span class='curlybracket'>{</span>
<span class='keyword'>if</span> <span class='keyword'>constexpr</span> <span class='parenthesis'>(</span>std<span class='operator'>::</span>derived_from<span class='anglebracket'>&lt;</span>U1, <span class='keyword'>typename</span> U2<span class='operator'>::</span><i >base-type</i><span class='anglebracket'>&gt;</span><span class='parenthesis'>)</span>
<span class='keyword'>return</span> u1;
<span class='keyword'>else</span> <span class='keyword'>if</span> <span class='keyword'>constexpr</span> <span class='parenthesis'>(</span>std<span class='operator'>::</span>derived_from<span class='anglebracket'>&lt;</span>U2, <span class='keyword'>typename</span> U1<span class='operator'>::</span><i >base-type</i><span class='anglebracket'>&gt;</span><span class='parenthesis'>)</span>
<span class='keyword'>return</span> u2;
<span class='keyword'>else</span>
<span class='keyword'>return</span> std<span class='operator'>::</span>conditional_t<span class='anglebracket'>&lt;</span><i >type-less-impl</i><span class='anglebracket'>&lt;</span>U1, U2<span class='anglebracket'>&gt;</span><span class='parenthesis'>(</span><span class='parenthesis'>)</span>, U1, U2<span class='anglebracket'>&gt;</span><span class='curlybracket'>{</span><span class='curlybracket'>}</span>;
<span class='curlybracket'>}</span> <span class='keyword'>else</span> <span class='curlybracket'>{</span>
<span class='keyword'>constexpr</span> <span class='keyword'>auto</span> canonical_lhs <span class='operator'>=</span> <i >get-canonical-unit</i><span class='parenthesis'>(</span>U1<span class='curlybracket'>{</span><span class='curlybracket'>}</span><span class='parenthesis'>)</span>;
<span class='keyword'>constexpr</span> <span class='keyword'>auto</span> canonical_rhs <span class='operator'>=</span> <i >get-canonical-unit</i><span class='parenthesis'>(</span>U2<span class='curlybracket'>{</span><span class='curlybracket'>}</span><span class='parenthesis'>)</span>;
<span class='keyword'>if</span> <span class='keyword'>constexpr</span> <span class='parenthesis'>(</span><i >is-positive-integral-power</i><span class='parenthesis'>(</span>canonical_lhs<span class='operator'>.</span>mag <span class='operator'>/</span> canonical_rhs<span class='operator'>.</span>mag<span class='parenthesis'>)</span><span class='parenthesis'>)</span>
<span class='keyword'>return</span> u2;
<span class='keyword'>else</span> <span class='keyword'>if</span> <span class='keyword'>constexpr</span> <span class='parenthesis'>(</span><i >is-positive-integral-power</i><span class='parenthesis'>(</span>canonical_rhs<span class='operator'>.</span>mag <span class='operator'>/</span> canonical_lhs<span class='operator'>.</span>mag<span class='parenthesis'>)</span><span class='parenthesis'>)</span>
<span class='keyword'>return</span> u1;
<span class='keyword'>else</span> <span class='curlybracket'>{</span>
<span class='keyword'>if</span> <span class='keyword'>constexpr</span> <span class='parenthesis'>(</span><i >type-less</i><span class='anglebracket'>&lt;</span>U1, U2<span class='anglebracket'>&gt;</span><span class='curlybracket'>{</span><span class='curlybracket'>}</span><span class='parenthesis'>)</span>
<span class='keyword'>return</span> common_unit<span class='anglebracket'>&lt;</span>U1, U2<span class='anglebracket'>&gt;</span><span class='curlybracket'>{</span><span class='curlybracket'>}</span>;
<span class='keyword'>else</span>
<span class='keyword'>return</span> common_unit<span class='anglebracket'>&lt;</span>U2, U1<span class='anglebracket'>&gt;</span><span class='curlybracket'>{</span><span class='curlybracket'>}</span>;
<span class='curlybracket'>}</span>
<span class='curlybracket'>}</span>
<span class='curlybracket'>}</span> <span class='keyword'>else</span>
<span class='keyword'>return</span> get_common_unit<span class='parenthesis'>(</span>get_common_unit<span class='parenthesis'>(</span>u1, u2<span class='parenthesis'>)</span>, rest<span class='operator'>.</span><span class='operator'>.</span><span class='operator'>.</span><span class='parenthesis'>)</span>;
</span></div></div></div></div><div class='para' id='5'><div class='marginalizedparent'><a class='marginalized' href='#5'>5</a></div><div class='itemdescr'><div class='sourceLinkParent'><a class='sourceLink' href='https://github.com/mpusz/mp-unitse9c5f7236ebfe1b287a5a4a49aaf57fbefa87233/source/quantities.tex#L3218'>#</a></div><div class='texpara'><div id='5.sentence-1' class='sentence'><i >Remarks</i>: The expression in the <span class='textsf'><i >requires-clause</i></span> is equivalent to:
<span class='codeblock'><span class='parenthesis'>(</span><span class='keyword'>sizeof</span><span class='operator'>.</span><span class='operator'>.</span><span class='operator'>.</span><span class='parenthesis'>(</span>us<span class='parenthesis'>)</span> <span class='operator'>!</span><span class='operator'>=</span> <span class='literal'>0</span> <span class='operator'>&amp;</span><span class='operator'>&amp;</span> <span class='parenthesis'>(</span><span class='keyword'>sizeof</span><span class='operator'>.</span><span class='operator'>.</span><span class='operator'>.</span><span class='parenthesis'>(</span>us<span class='parenthesis'>)</span> <span class='operator'>=</span><span class='operator'>=</span> <span class='literal'>1</span> <span class='operator'>|</span><span class='operator'>|</span> <span class='comment'>//</span>
<span class='parenthesis'>(</span><span class='keyword'>sizeof</span><span class='operator'>.</span><span class='operator'>.</span><span class='operator'>.</span><span class='parenthesis'>(</span>us<span class='parenthesis'>)</span> <span class='operator'>=</span><span class='operator'>=</span> <span class='literal'>2</span> <span class='operator'>&amp;</span><span class='operator'>&amp;</span> convertible<span class='parenthesis'>(</span>U1<span class='curlybracket'>{</span><span class='curlybracket'>}</span>, U2<span class='curlybracket'>{</span><span class='curlybracket'>}</span><span class='parenthesis'>)</span><span class='parenthesis'>)</span> <span class='operator'>|</span><span class='operator'>|</span>
<span class='keyword'>requires</span> <span class='curlybracket'>{</span> get_common_unit<span class='parenthesis'>(</span>get_common_unit<span class='parenthesis'>(</span>u1, u2<span class='parenthesis'>)</span>, rest<span class='operator'>.</span><span class='operator'>.</span><span class='operator'>.</span><span class='parenthesis'>)</span>; <span class='curlybracket'>}</span><span class='parenthesis'>)</span><span class='parenthesis'>)</span>
</span></div></div></div></div></div></body></html>
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