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<!DOCTYPE html><html lang='en'><head><title>[qty.non.mem.conv]</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'>5.6</a> Quantity <a class='abbr_ref' href='qty.html#non.mem.conv'>[qty]</a></h2><h3 ><a class='secnum' style='min-width:80pt'>5.6.15</a> Non-member conversions <a class='abbr_ref'>[qty.non.mem.conv]</a></h3><div class='texpara'><div id='lib:sudo-cast,quantity'><div id='lib:quantity,sudo-cast'><div class='itemdecl'><div class='marginalizedparent'><a class='itemDeclLink' href='#lib:quantity,sudo-cast'>🔗</a></div><code class='itemdeclcode'><span class='keyword'>template</span><span class='anglebracket'>&lt;</span><a href='qty.syn.html#concept:Quantity' title='5.6.3&emsp;Class template quantity&emsp;[qty.syn]'><span id='conceptref:Quantity'><span class='texttt'>Quantity</span></span></a> To, <span class='keyword'>typename</span> FwdFrom, <a href='qty.syn.html#concept:Quantity' title='5.6.3&emsp;Class template quantity&emsp;[qty.syn]'><span id='conceptref:Quantity_'><span class='texttt'>Quantity</span></span></a> From <span class='operator'>=</span> std<span class='operator'>::</span>remove_cvref_t<span class='anglebracket'>&lt;</span>FwdFrom<span class='anglebracket'>&gt;</span><span class='anglebracket'>&gt;</span>
<span class='keyword'>requires</span> <i ><span class='texttt'>see below</span></i>
<span class='keyword'>constexpr</span> To <span class='texttt'><i >sudo-cast</i></span><span class='parenthesis'>(</span>FwdFrom<span class='operator'>&amp;</span><span class='operator'>&amp;</span> q<span class='parenthesis'>)</span>; <span class='comment'>// <i >exposition only</i></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-units329ffdaacb1bd7bed08565ac80d9eef7ccb1e5b5/source/quantities.tex#L5308'>#</a></div><div class='texpara'><div id='1.sentence-1' class='sentence'><i >Returns</i>: TBD<a class='hidden_link' href='#1.sentence-1'>.</a></div></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-units329ffdaacb1bd7bed08565ac80d9eef7ccb1e5b5/source/quantities.tex#L5313'>#</a></div><div class='texpara'><div id='2.sentence-1' class='sentence'><span class='texttt'>value_<span class='shy'></span>cast</span> is an explicit cast that allows truncation<a class='hidden_link' href='#2.sentence-1'>.</a></div></div></div><div class='texpara'><div id='lib:quantity,value_cast'><div id='lib:value_cast,quantity'><div class='itemdecl'><div class='marginalizedparent'><a class='itemDeclLink' href='#lib:value_cast,quantity'>🔗</a></div><code class='itemdeclcode'><span class='keyword'>template</span><span class='anglebracket'>&lt;</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> ToU, <span class='keyword'>typename</span> FwdQ, <a href='qty.syn.html#concept:Quantity' title='5.6.3&emsp;Class template quantity&emsp;[qty.syn]'><span id='conceptref:Quantity__'><span class='texttt'>Quantity</span></span></a> Q <span class='operator'>=</span> std<span class='operator'>::</span>remove_cvref_t<span class='anglebracket'>&lt;</span>FwdQ<span class='anglebracket'>&gt;</span><span class='anglebracket'>&gt;</span>
<span class='keyword'>requires</span><span class='parenthesis'>(</span>convertible<span class='parenthesis'>(</span>Q<span class='operator'>::</span>reference, ToU<span class='parenthesis'>)</span><span class='parenthesis'>)</span>
<span class='keyword'>constexpr</span> <a href='qty.syn.html#concept:Quantity' title='5.6.3&emsp;Class template quantity&emsp;[qty.syn]'><span id='conceptref:Quantity___'><span class='texttt'>Quantity</span></span></a> <span class='keyword'>auto</span> value_cast<span class='parenthesis'>(</span>FwdQ<span class='operator'>&amp;</span><span class='operator'>&amp;</span> q<span class='parenthesis'>)</span>;
</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-units329ffdaacb1bd7bed08565ac80d9eef7ccb1e5b5/source/quantities.tex#L5324'>#</a></div><div class='texpara'><div id='3.sentence-1' class='sentence'><i >Effects</i>: Equivalent to:
<span class='codeblock'><span class='keyword'>return</span> <i >sudo-cast</i><span class='anglebracket'>&lt;</span>quantity<span class='anglebracket'>&lt;</span><i >make-reference</i><span class='parenthesis'>(</span>Q<span class='operator'>::</span>quantity_spec, ToU<span class='parenthesis'>)</span>, <span class='keyword'>typename</span> Q<span class='operator'>::</span>rep<span class='anglebracket'>&gt;</span><span class='anglebracket'>&gt;</span><span class='parenthesis'>(</span>
std<span class='operator'>::</span>forward<span class='anglebracket'>&lt;</span>FwdQ<span class='anglebracket'>&gt;</span><span class='parenthesis'>(</span>q<span class='parenthesis'>)</span><span class='parenthesis'>)</span>;
</span></div></div></div></div><div class='texpara'><div id='lib:quantity,value_cast_'><div id='lib:value_cast,quantity_'><div class='itemdecl'><div class='marginalizedparent'><a class='itemDeclLink' href='#lib:value_cast,quantity_'>🔗</a></div><code class='itemdeclcode'><span class='keyword'>template</span><span class='anglebracket'>&lt;</span><a href='qty.rep.concepts.html#concept:Representation' title='5.5.4&emsp;Concepts&emsp;[qty.rep.concepts]'><span id='conceptref:Representation'><span class='texttt'>Representation</span></span></a> ToRep, <span class='keyword'>typename</span> FwdQ, <a href='qty.syn.html#concept:Quantity' title='5.6.3&emsp;Class template quantity&emsp;[qty.syn]'><span id='conceptref:Quantity____'><span class='texttt'>Quantity</span></span></a> Q <span class='operator'>=</span> std<span class='operator'>::</span>remove_cvref_t<span class='anglebracket'>&lt;</span>FwdQ<span class='anglebracket'>&gt;</span><span class='anglebracket'>&gt;</span>
<span class='keyword'>requires</span> <a href='qty.rep.concepts.html#concept:RepresentationOf' title='5.5.4&emsp;Concepts&emsp;[qty.rep.concepts]'><span id='conceptref:RepresentationOf'><span class='texttt'>RepresentationOf</span></span></a><span class='anglebracket'>&lt;</span>ToRep, Q<span class='operator'>::</span>quantity_spec<span class='anglebracket'>&gt;</span> <span class='operator'>&amp;</span><span class='operator'>&amp;</span>
std<span class='operator'>::</span>constructible_from<span class='anglebracket'>&lt;</span>ToRep, <span class='keyword'>typename</span> Q<span class='operator'>::</span>rep<span class='anglebracket'>&gt;</span>
<span class='keyword'>constexpr</span> quantity<span class='anglebracket'>&lt;</span>Q<span class='operator'>::</span>reference, ToRep<span class='anglebracket'>&gt;</span> value_cast<span class='parenthesis'>(</span>FwdQ<span class='operator'>&amp;</span><span class='operator'>&amp;</span> q<span class='parenthesis'>)</span>;
</code></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-units329ffdaacb1bd7bed08565ac80d9eef7ccb1e5b5/source/quantities.tex#L5342'>#</a></div><div class='texpara'><div id='4.sentence-1' class='sentence'><i >Effects</i>: Equivalent to:
<span class='codeblock'><span class='keyword'>return</span> <i >sudo-cast</i><span class='anglebracket'>&lt;</span>quantity<span class='anglebracket'>&lt;</span>Q<span class='operator'>::</span>reference, ToRep<span class='anglebracket'>&gt;</span><span class='anglebracket'>&gt;</span><span class='parenthesis'>(</span>std<span class='operator'>::</span>forward<span class='anglebracket'>&lt;</span>FwdQ<span class='anglebracket'>&gt;</span><span class='parenthesis'>(</span>q<span class='parenthesis'>)</span><span class='parenthesis'>)</span>;
</span></div></div></div></div><div class='texpara'><div id='lib:quantity,value_cast__'><div id='lib:value_cast,quantity__'><div class='itemdecl'><div class='marginalizedparent'><a class='itemDeclLink' href='#lib:value_cast,quantity__'>🔗</a></div><code class='itemdeclcode'><span class='keyword'>template</span><span class='anglebracket'>&lt;</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> ToU, <a href='qty.rep.concepts.html#concept:Representation' title='5.5.4&emsp;Concepts&emsp;[qty.rep.concepts]'><span id='conceptref:Representation_'><span class='texttt'>Representation</span></span></a> ToRep, <span class='keyword'>typename</span> FwdQ,
<a href='qty.syn.html#concept:Quantity' title='5.6.3&emsp;Class template quantity&emsp;[qty.syn]'><span id='conceptref:Quantity_____'><span class='texttt'>Quantity</span></span></a> Q <span class='operator'>=</span> std<span class='operator'>::</span>remove_cvref_t<span class='anglebracket'>&lt;</span>FwdQ<span class='anglebracket'>&gt;</span><span class='anglebracket'>&gt;</span>
<span class='keyword'>requires</span> <i ><span class='texttt'>see below</span></i>
<span class='keyword'>constexpr</span> <a href='qty.syn.html#concept:Quantity' title='5.6.3&emsp;Class template quantity&emsp;[qty.syn]'><span id='conceptref:Quantity______'><span class='texttt'>Quantity</span></span></a> <span class='keyword'>auto</span> value_cast<span class='parenthesis'>(</span>FwdQ<span class='operator'>&amp;</span><span class='operator'>&amp;</span> q<span class='parenthesis'>)</span>;
<span class='keyword'>template</span><span class='anglebracket'>&lt;</span><a href='qty.rep.concepts.html#concept:Representation' title='5.5.4&emsp;Concepts&emsp;[qty.rep.concepts]'><span id='conceptref:Representation__'><span class='texttt'>Representation</span></span></a> ToRep, <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> ToU, <span class='keyword'>typename</span> FwdQ,
<a href='qty.syn.html#concept:Quantity' title='5.6.3&emsp;Class template quantity&emsp;[qty.syn]'><span id='conceptref:Quantity_______'><span class='texttt'>Quantity</span></span></a> Q <span class='operator'>=</span> std<span class='operator'>::</span>remove_cvref_t<span class='anglebracket'>&lt;</span>FwdQ<span class='anglebracket'>&gt;</span><span class='anglebracket'>&gt;</span>
<span class='keyword'>requires</span> <i ><span class='texttt'>see below</span></i>
<span class='keyword'>constexpr</span> <a href='qty.syn.html#concept:Quantity' title='5.6.3&emsp;Class template quantity&emsp;[qty.syn]'><span id='conceptref:Quantity________'><span class='texttt'>Quantity</span></span></a> <span class='keyword'>auto</span> value_cast<span class='parenthesis'>(</span>FwdQ<span class='operator'>&amp;</span><span class='operator'>&amp;</span> q<span class='parenthesis'>)</span>;
</code></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-units329ffdaacb1bd7bed08565ac80d9eef7ccb1e5b5/source/quantities.tex#L5363'>#</a></div><div class='texpara'><div id='5.sentence-1' class='sentence'><i >Effects</i>: Equivalent to:
<span class='codeblock'><span class='keyword'>return</span> <i >sudo-cast</i><span class='anglebracket'>&lt;</span>quantity<span class='anglebracket'>&lt;</span><i >make-reference</i><span class='parenthesis'>(</span>Q<span class='operator'>::</span>quantity_spec, ToU<span class='parenthesis'>)</span>, ToRep<span class='anglebracket'>&gt;</span><span class='anglebracket'>&gt;</span><span class='parenthesis'>(</span>
std<span class='operator'>::</span>forward<span class='anglebracket'>&lt;</span>FwdQ<span class='anglebracket'>&gt;</span><span class='parenthesis'>(</span>q<span class='parenthesis'>)</span><span class='parenthesis'>)</span>;
</span></div></div></div></div><div class='para' id='6'><div class='marginalizedparent'><a class='marginalized' href='#6'>6</a></div><div class='itemdescr'><div class='sourceLinkParent'><a class='sourceLink' href='https://github.com/mpusz/mp-units329ffdaacb1bd7bed08565ac80d9eef7ccb1e5b5/source/quantities.tex#L5371'>#</a></div><div class='texpara'><div id='6.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>convertible<span class='parenthesis'>(</span>Q<span class='operator'>::</span>reference, ToU<span class='parenthesis'>)</span><span class='parenthesis'>)</span> <span class='operator'>&amp;</span><span class='operator'>&amp;</span> <a href='qty.rep.concepts.html#concept:RepresentationOf' title='5.5.4&emsp;Concepts&emsp;[qty.rep.concepts]'><span id='conceptref:RepresentationOf_'><span class='tcode_in_codeblock'>RepresentationOf</span></span></a><span class='anglebracket'>&lt;</span>ToRep, Q<span class='operator'>::</span>quantity_spec<span class='anglebracket'>&gt;</span> <span class='operator'>&amp;</span><span class='operator'>&amp;</span>
std<span class='operator'>::</span>constructible_from<span class='anglebracket'>&lt;</span>ToRep, <span class='keyword'>typename</span> Q<span class='operator'>::</span>rep<span class='anglebracket'>&gt;</span>
</span></div></div></div></div><div class='texpara'><div id='lib:quantity,value_cast___'><div id='lib:value_cast,quantity___'><div class='itemdecl'><div class='marginalizedparent'><a class='itemDeclLink' href='#lib:value_cast,quantity___'>🔗</a></div><code class='itemdeclcode'><span class='keyword'>template</span><span class='anglebracket'>&lt;</span><a href='qty.syn.html#concept:Quantity' title='5.6.3&emsp;Class template quantity&emsp;[qty.syn]'><span id='conceptref:Quantity_________'><span class='texttt'>Quantity</span></span></a> ToQ, <span class='keyword'>typename</span> FwdQ, <a href='qty.syn.html#concept:Quantity' title='5.6.3&emsp;Class template quantity&emsp;[qty.syn]'><span id='conceptref:Quantity__________'><span class='texttt'>Quantity</span></span></a> Q <span class='operator'>=</span> std<span class='operator'>::</span>remove_cvref_t<span class='anglebracket'>&lt;</span>FwdQ<span class='anglebracket'>&gt;</span><span class='anglebracket'>&gt;</span>
<span class='keyword'>requires</span><span class='parenthesis'>(</span>convertible<span class='parenthesis'>(</span>Q<span class='operator'>::</span>reference, ToQ<span class='operator'>::</span>unit<span class='parenthesis'>)</span><span class='parenthesis'>)</span> <span class='operator'>&amp;</span><span class='operator'>&amp;</span> <span class='parenthesis'>(</span>ToQ<span class='operator'>::</span>quantity_spec <span class='operator'>=</span><span class='operator'>=</span> Q<span class='operator'>::</span>quantity_spec<span class='parenthesis'>)</span> <span class='operator'>&amp;</span><span class='operator'>&amp;</span>
std<span class='operator'>::</span>constructible_from<span class='anglebracket'>&lt;</span><span class='keyword'>typename</span> ToQ<span class='operator'>::</span>rep, <span class='keyword'>typename</span> Q<span class='operator'>::</span>rep<span class='anglebracket'>&gt;</span>
<span class='keyword'>constexpr</span> <a href='qty.syn.html#concept:Quantity' title='5.6.3&emsp;Class template quantity&emsp;[qty.syn]'><span id='conceptref:Quantity___________'><span class='texttt'>Quantity</span></span></a> <span class='keyword'>auto</span> value_cast<span class='parenthesis'>(</span>FwdQ<span class='operator'>&amp;</span><span class='operator'>&amp;</span> q<span class='parenthesis'>)</span>;
</code></div></div></div></div><div class='para' id='7'><div class='marginalizedparent'><a class='marginalized' href='#7'>7</a></div><div class='itemdescr'><div class='sourceLinkParent'><a class='sourceLink' href='https://github.com/mpusz/mp-units329ffdaacb1bd7bed08565ac80d9eef7ccb1e5b5/source/quantities.tex#L5389'>#</a></div><div class='texpara'><div id='7.sentence-1' class='sentence'><i >Effects</i>: Equivalent to: <span class='texttt'><span class='keyword'>return</span> <span class='texttt'><i >sudo-cast</i></span><span class='anglebracket'>&lt;</span>ToQ<span class='anglebracket'>&gt;</span><span class='parenthesis'>(</span>std<span class='operator'>&#x200b;::&#x200b;</span>forward<span class='anglebracket'>&lt;</span>FwdQ<span class='anglebracket'>&gt;</span><span class='parenthesis'>(</span>q<span class='parenthesis'>)</span><span class='parenthesis'>)</span>;</span></div></div></div></div><div class='para' id='8'><div class='marginalizedparent'><a class='marginalized' href='#8'>8</a></div><div class='sourceLinkParent'><a class='sourceLink' href='https://github.com/mpusz/mp-units329ffdaacb1bd7bed08565ac80d9eef7ccb1e5b5/source/quantities.tex#L5394'>#</a></div><div class='texpara'><div id='8.sentence-1' class='sentence'><span class='texttt'>quantity_<span class='shy'></span>cast</span> is an explicit cast that allows converting to more specific quantities<a class='hidden_link' href='#8.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'>auto</span> length <span class='operator'>=</span> isq<span class='operator'>::</span>length<span class='parenthesis'>(</span><span class='literal'>42</span> <span class='operator'>*</span> m<span class='parenthesis'>)</span>;
<span class='keyword'>auto</span> distance <span class='operator'>=</span> quantity_cast<span class='anglebracket'>&lt;</span>isq<span class='operator'>::</span>distance<span class='anglebracket'>&gt;</span><span class='parenthesis'>(</span>length<span class='parenthesis'>)</span>;
</span> —&nbsp;<i>end example</i>]</div></div></div></div><div class='texpara'><div id='lib:quantity,quantity_cast'><div id='lib:quantity_cast,quantity'><div class='itemdecl'><div class='marginalizedparent'><a class='itemDeclLink' href='#lib:quantity_cast,quantity'>🔗</a></div><code class='itemdeclcode'><span class='keyword'>template</span><span class='anglebracket'>&lt;</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> ToQS, <span class='keyword'>typename</span> FwdQ, <a href='qty.syn.html#concept:Quantity' title='5.6.3&emsp;Class template quantity&emsp;[qty.syn]'><span id='conceptref:Quantity____________'><span class='texttt'>Quantity</span></span></a> Q <span class='operator'>=</span> std<span class='operator'>::</span>remove_cvref_t<span class='anglebracket'>&lt;</span>FwdQ<span class='anglebracket'>&gt;</span><span class='anglebracket'>&gt;</span>
<span class='keyword'>requires</span> <a class='index' id='conceptref:QuantitySpecCastableTo'></a><span class='texttt'><i >QuantitySpecCastableTo</i></span><span class='anglebracket'>&lt;</span>Q<span class='operator'>::</span>quantity_spec, ToQS<span class='anglebracket'>&gt;</span>
<span class='keyword'>constexpr</span> <a href='qty.syn.html#concept:Quantity' title='5.6.3&emsp;Class template quantity&emsp;[qty.syn]'><span id='conceptref:Quantity_____________'><span class='texttt'>Quantity</span></span></a> <span class='keyword'>auto</span> quantity_cast<span class='parenthesis'>(</span>FwdQ<span class='operator'>&amp;</span><span class='operator'>&amp;</span> q<span class='parenthesis'>)</span>;
</code></div></div></div></div><div class='para' id='9'><div class='marginalizedparent'><a class='marginalized' href='#9'>9</a></div><div class='itemdescr'><div class='sourceLinkParent'><a class='sourceLink' href='https://github.com/mpusz/mp-units329ffdaacb1bd7bed08565ac80d9eef7ccb1e5b5/source/quantities.tex#L5411'>#</a></div><div class='texpara'><div id='9.sentence-1' class='sentence'><i >Effects</i>: Equivalent to:
<span class='codeblock'><span class='keyword'>return</span> quantity<span class='curlybracket'>{</span>std<span class='operator'>::</span>forward<span class='anglebracket'>&lt;</span>FwdQ<span class='anglebracket'>&gt;</span><span class='parenthesis'>(</span>q<span class='parenthesis'>)</span><span class='operator'>.</span><i >numerical-value</i>, <i >make-reference</i><span class='parenthesis'>(</span>ToQS, Q<span class='operator'>::</span>unit<span class='parenthesis'>)</span><span class='curlybracket'>}</span>;
</span></div></div></div></div></div></body></html>