feedc0de/mp-units
1
0
Fork 0
forked from mpusz/mp-units
Code Pull Requests Activity
Files
5092f01adb21dba94de76b708e55582c10f2eac4
mp-units/2.2/getting_started/faq/index.html
T
Mateusz Pusz 32808bef78 Reinit docs to shrink git repo
2024-11-05 18:52:17 +01:00

2448 lines
68 KiB
HTML
Raw Blame History

<!doctype html>
<html lang="en" class="no-js">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width,initial-scale=1">
<meta name="description" content="The quantities and units library for C++">
<meta name="author" content="mp-units Team">
<link rel="canonical" href="https://mpusz.github.io/mp-units/2.2/getting_started/faq/">
<link rel="prev" href="../installation_and_usage/">
<link rel="next" href="../../users_guide/terms_and_definitions/">
<link rel="alternate" type="application/rss+xml" title="RSS feed" href="../../feed_rss_created.xml">
<link rel="alternate" type="application/rss+xml" title="RSS feed of updated content" href="../../feed_rss_updated.xml">
<link rel="icon" href="../../assets/images/mp-units-color.svg">
<meta name="generator" content="mkdocs-1.6.0, mkdocs-material-9.5.26">
<title>FAQ - mp-units</title>
<link rel="stylesheet" href="../../assets/stylesheets/main.6543a935.min.css">
<link rel="stylesheet" href="../../assets/stylesheets/palette.06af60db.min.css">
<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
<link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Roboto:300,300i,400,400i,700,700i%7CRoboto+Mono:400,400i,700,700i&display=fallback">
<style>:root{--md-text-font:"Roboto";--md-code-font:"Roboto Mono"}</style>
<link rel="stylesheet" href="../../stylesheets/extra.css">
<script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce((e,_)=>(e<<5)-e+_.charCodeAt(0),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script>
<meta property="og:type" content="website" >
<meta property="og:title" content="FAQ - mp-units" >
<meta property="og:description" content="The quantities and units library for C++" >
<meta property="og:image" content="https://mpusz.github.io/mp-units/2.2/assets/images/social/getting_started/faq.png" >
<meta property="og:image:type" content="image/png" >
<meta property="og:image:width" content="1200" >
<meta property="og:image:height" content="630" >
<meta property="og:url" content="https://mpusz.github.io/mp-units/2.2/getting_started/faq/" >
<meta name="twitter:card" content="summary_large_image" >
<meta name="twitter:title" content="FAQ - mp-units" >
<meta name="twitter:description" content="The quantities and units library for C++" >
<meta name="twitter:image" content="https://mpusz.github.io/mp-units/2.2/assets/images/social/getting_started/faq.png" >
</head>
<body dir="ltr" data-md-color-scheme="default" data-md-color-primary="indigo" data-md-color-accent="indigo">
<input class="md-toggle" data-md-toggle="drawer" type="checkbox" id="__drawer" autocomplete="off">
<input class="md-toggle" data-md-toggle="search" type="checkbox" id="__search" autocomplete="off">
<label class="md-overlay" for="__drawer"></label>
<div data-md-component="skip">
<a href="#frequently-asked-questions" class="md-skip">
Skip to content
</a>
</div>
<div data-md-component="announce">
</div>
<div data-md-color-scheme="default" data-md-component="outdated" hidden>
<aside class="md-banner md-banner--warning">
<div class="md-banner__inner md-grid md-typeset">
You're not viewing the latest version.
<a href="../../..">
<strong>Click here to go to latest.</strong>
</a>
</div>
<script>var el=document.querySelector("[data-md-component=outdated]"),outdated=__md_get("__outdated",sessionStorage);!0===outdated&&el&&(el.hidden=!1)</script>
</aside>
</div>
<header class="md-header" data-md-component="header">
<nav class="md-header__inner md-grid" aria-label="Header">
<a href="../.." title="mp-units" class="md-header__button md-logo" aria-label="mp-units" data-md-component="logo">
<img src="../../assets/images/mp-units-transparent-white.svg" alt="logo">
</a>
<label class="md-header__button md-icon" for="__drawer">
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M3 6h18v2H3V6m0 5h18v2H3v-2m0 5h18v2H3v-2Z"/></svg>
</label>
<div class="md-header__title" data-md-component="header-title">
<div class="md-header__ellipsis">
<div class="md-header__topic">
<span class="md-ellipsis">
mp-units
</span>
</div>
<div class="md-header__topic" data-md-component="header-topic">
<span class="md-ellipsis">
FAQ
</span>
</div>
</div>
</div>
<form class="md-header__option" data-md-component="palette">
<input class="md-option" data-md-color-media="(prefers-color-scheme)" data-md-color-scheme="default" data-md-color-primary="indigo" data-md-color-accent="indigo" aria-label="Switch to light mode" type="radio" name="__palette" id="__palette_0">
<label class="md-header__button md-icon" title="Switch to light mode" for="__palette_1" hidden>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M3.9 12c0-1.71 1.39-3.1 3.1-3.1h4V7H7a5 5 0 0 0-5 5 5 5 0 0 0 5 5h4v-1.9H7c-1.71 0-3.1-1.39-3.1-3.1M8 13h8v-2H8v2m9-6h-4v1.9h4c1.71 0 3.1 1.39 3.1 3.1 0 1.71-1.39 3.1-3.1 3.1h-4V17h4a5 5 0 0 0 5-5 5 5 0 0 0-5-5Z"/></svg>
</label>
<input class="md-option" data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme="default" data-md-color-primary="custom" data-md-color-accent="custom" aria-label="Switch to dark mode" type="radio" name="__palette" id="__palette_1">
<label class="md-header__button md-icon" title="Switch to dark mode" for="__palette_2" hidden>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M12 7a5 5 0 0 1 5 5 5 5 0 0 1-5 5 5 5 0 0 1-5-5 5 5 0 0 1 5-5m0 2a3 3 0 0 0-3 3 3 3 0 0 0 3 3 3 3 0 0 0 3-3 3 3 0 0 0-3-3m0-7 2.39 3.42C13.65 5.15 12.84 5 12 5c-.84 0-1.65.15-2.39.42L12 2M3.34 7l4.16-.35A7.2 7.2 0 0 0 5.94 8.5c-.44.74-.69 1.5-.83 2.29L3.34 7m.02 10 1.76-3.77a7.131 7.131 0 0 0 2.38 4.14L3.36 17M20.65 7l-1.77 3.79a7.023 7.023 0 0 0-2.38-4.15l4.15.36m-.01 10-4.14.36c.59-.51 1.12-1.14 1.54-1.86.42-.73.69-1.5.83-2.29L20.64 17M12 22l-2.41-3.44c.74.27 1.55.44 2.41.44.82 0 1.63-.17 2.37-.44L12 22Z"/></svg>
</label>
<input class="md-option" data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme="slate" data-md-color-primary="black" data-md-color-accent="custom" aria-label="Switch to system preference" type="radio" name="__palette" id="__palette_2">
<label class="md-header__button md-icon" title="Switch to system preference" for="__palette_0" hidden>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="m17.75 4.09-2.53 1.94.91 3.06-2.63-1.81-2.63 1.81.91-3.06-2.53-1.94L12.44 4l1.06-3 1.06 3 3.19.09m3.5 6.91-1.64 1.25.59 1.98-1.7-1.17-1.7 1.17.59-1.98L15.75 11l2.06-.05L18.5 9l.69 1.95 2.06.05m-2.28 4.95c.83-.08 1.72 1.1 1.19 1.85-.32.45-.66.87-1.08 1.27C15.17 23 8.84 23 4.94 19.07c-3.91-3.9-3.91-10.24 0-14.14.4-.4.82-.76 1.27-1.08.75-.53 1.93.36 1.85 1.19-.27 2.86.69 5.83 2.89 8.02a9.96 9.96 0 0 0 8.02 2.89m-1.64 2.02a12.08 12.08 0 0 1-7.8-3.47c-2.17-2.19-3.33-5-3.49-7.82-2.81 3.14-2.7 7.96.31 10.98 3.02 3.01 7.84 3.12 10.98.31Z"/></svg>
</label>
</form>
<script>var media,input,key,value,palette=__md_get("__palette");if(palette&&palette.color){"(prefers-color-scheme)"===palette.color.media&&(media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']"),palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent"));for([key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script>
<label class="md-header__button md-icon" for="__search">
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.516 6.516 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5Z"/></svg>
</label>
<div class="md-search" data-md-component="search" role="dialog">
<label class="md-search__overlay" for="__search"></label>
<div class="md-search__inner" role="search">
<form class="md-search__form" name="search">
<input type="text" class="md-search__input" name="query" aria-label="Search" placeholder="Search" autocapitalize="off" autocorrect="off" autocomplete="off" spellcheck="false" data-md-component="search-query" required>
<label class="md-search__icon md-icon" for="__search">
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.516 6.516 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5Z"/></svg>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg>
</label>
<nav class="md-search__options" aria-label="Search">
<a href="javascript:void(0)" class="md-search__icon md-icon" title="Share" aria-label="Share" data-clipboard data-clipboard-text="" data-md-component="search-share" tabindex="-1">
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M18 16.08c-.76 0-1.44.3-1.96.77L8.91 12.7c.05-.23.09-.46.09-.7 0-.24-.04-.47-.09-.7l7.05-4.11c.54.5 1.25.81 2.04.81a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3c0 .24.04.47.09.7L8.04 9.81C7.5 9.31 6.79 9 6 9a3 3 0 0 0-3 3 3 3 0 0 0 3 3c.79 0 1.5-.31 2.04-.81l7.12 4.15c-.05.21-.08.43-.08.66 0 1.61 1.31 2.91 2.92 2.91 1.61 0 2.92-1.3 2.92-2.91A2.92 2.92 0 0 0 18 16.08Z"/></svg>
</a>
<button type="reset" class="md-search__icon md-icon" title="Clear" aria-label="Clear" tabindex="-1">
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M19 6.41 17.59 5 12 10.59 6.41 5 5 6.41 10.59 12 5 17.59 6.41 19 12 13.41 17.59 19 19 17.59 13.41 12 19 6.41Z"/></svg>
</button>
</nav>
<div class="md-search__suggest" data-md-component="search-suggest"></div>
</form>
<div class="md-search__output">
<div class="md-search__scrollwrap" tabindex="0" data-md-scrollfix>
<div class="md-search-result" data-md-component="search-result">
<div class="md-search-result__meta">
Initializing search
</div>
<ol class="md-search-result__list" role="presentation"></ol>
</div>
</div>
</div>
</div>
</div>
<div class="md-header__source">
<a href="https://github.com/mpusz/mp-units" title="Go to repository" class="md-source" data-md-component="source">
<div class="md-source__icon md-icon">
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 448 512"><!--! Font Awesome Free 6.5.2 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2024 Fonticons, Inc.--><path d="M439.55 236.05 244 40.45a28.87 28.87 0 0 0-40.81 0l-40.66 40.63 51.52 51.52c27.06-9.14 52.68 16.77 43.39 43.68l49.66 49.66c34.23-11.8 61.18 31 35.47 56.69-26.49 26.49-70.21-2.87-56-37.34L240.22 199v121.85c25.3 12.54 22.26 41.85 9.08 55a34.34 34.34 0 0 1-48.55 0c-17.57-17.6-11.07-46.91 11.25-56v-123c-20.8-8.51-24.6-30.74-18.64-45L142.57 101 8.45 235.14a28.86 28.86 0 0 0 0 40.81l195.61 195.6a28.86 28.86 0 0 0 40.8 0l194.69-194.69a28.86 28.86 0 0 0 0-40.81z"/></svg>
</div>
<div class="md-source__repository">
mpusz/mp-units
</div>
</a>
</div>
</nav>
</header>
<div class="md-container" data-md-component="container">
<nav class="md-tabs" aria-label="Tabs" data-md-component="tabs">
<div class="md-grid">
<ul class="md-tabs__list">
<li class="md-tabs__item">
<a href="../.." class="md-tabs__link">
Home
</a>
</li>
<li class="md-tabs__item md-tabs__item--active">
<a href="../introduction/" class="md-tabs__link">
Getting Started
</a>
</li>
<li class="md-tabs__item">
<a href="../../users_guide/terms_and_definitions/" class="md-tabs__link">
User's Guide
</a>
</li>
<li class="md-tabs__item">
<a href="../../blog/" class="md-tabs__link">
Blog
</a>
</li>
<li class="md-tabs__item">
<a href="../../release_notes/" class="md-tabs__link">
Release Notes
</a>
</li>
</ul>
</div>
</nav>
<main class="md-main" data-md-component="main">
<div class="md-main__inner md-grid">
<div class="md-sidebar md-sidebar--primary" data-md-component="sidebar" data-md-type="navigation" >
<div class="md-sidebar__scrollwrap">
<div class="md-sidebar__inner">
<nav class="md-nav md-nav--primary md-nav--lifted md-nav--integrated" aria-label="Navigation" data-md-level="0">
<label class="md-nav__title" for="__drawer">
<a href="../.." title="mp-units" class="md-nav__button md-logo" aria-label="mp-units" data-md-component="logo">
<img src="../../assets/images/mp-units-transparent-white.svg" alt="logo">
</a>
mp-units
</label>
<div class="md-nav__source">
<a href="https://github.com/mpusz/mp-units" title="Go to repository" class="md-source" data-md-component="source">
<div class="md-source__icon md-icon">
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 448 512"><!--! Font Awesome Free 6.5.2 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2024 Fonticons, Inc.--><path d="M439.55 236.05 244 40.45a28.87 28.87 0 0 0-40.81 0l-40.66 40.63 51.52 51.52c27.06-9.14 52.68 16.77 43.39 43.68l49.66 49.66c34.23-11.8 61.18 31 35.47 56.69-26.49 26.49-70.21-2.87-56-37.34L240.22 199v121.85c25.3 12.54 22.26 41.85 9.08 55a34.34 34.34 0 0 1-48.55 0c-17.57-17.6-11.07-46.91 11.25-56v-123c-20.8-8.51-24.6-30.74-18.64-45L142.57 101 8.45 235.14a28.86 28.86 0 0 0 0 40.81l195.61 195.6a28.86 28.86 0 0 0 40.8 0l194.69-194.69a28.86 28.86 0 0 0 0-40.81z"/></svg>
</div>
<div class="md-source__repository">
mpusz/mp-units
</div>
</a>
</div>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../.." class="md-nav__link">
<span class="md-ellipsis">
Home
</span>
</a>
</li>
<li class="md-nav__item md-nav__item--active md-nav__item--section md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_2" checked>
<label class="md-nav__link" for="__nav_2" id="__nav_2_label" tabindex="">
<span class="md-ellipsis">
Getting Started
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="1" aria-labelledby="__nav_2_label" aria-expanded="true">
<label class="md-nav__title" for="__nav_2">
<span class="md-nav__icon md-icon"></span>
Getting Started
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../introduction/" class="md-nav__link">
<span class="md-ellipsis">
Introduction
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../quick_start/" class="md-nav__link">
<span class="md-ellipsis">
Quick Start
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../look_and_feel/" class="md-nav__link">
<span class="md-ellipsis">
Look and Feel
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../cpp_compiler_support/" class="md-nav__link">
<span class="md-ellipsis">
C++ compiler support (API/ABI)
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../installation_and_usage/" class="md-nav__link">
<span class="md-ellipsis">
Installation and Usage
</span>
</a>
</li>
<li class="md-nav__item md-nav__item--active">
<input class="md-nav__toggle md-toggle" type="checkbox" id="__toc">
<label class="md-nav__link md-nav__link--active" for="__toc">
<span class="md-ellipsis">
FAQ
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<a href="./" class="md-nav__link md-nav__link--active">
<span class="md-ellipsis">
FAQ
</span>
</a>
<nav class="md-nav md-nav--secondary" aria-label="Table of contents">
<label class="md-nav__title" for="__toc">
<span class="md-nav__icon md-icon"></span>
Table of contents
</label>
<ul class="md-nav__list" data-md-component="toc" data-md-scrollfix>
<li class="md-nav__item">
<a href="#why-do-we-spell-metre-instead-of-meter" class="md-nav__link">
<span class="md-ellipsis">
Why do we spell metre instead of meter?
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#why-dont-we-use-udls-to-create-quantities" class="md-nav__link">
<span class="md-ellipsis">
Why don't we use UDLs to create quantities?
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#why-cant-i-create-a-quantity-by-passing-a-number-to-a-constructor" class="md-nav__link">
<span class="md-ellipsis">
Why can't I create a quantity by passing a number to a constructor?
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#why-a-dimensionless-quantity-is-not-just-a-fundamental-arithmetic-type" class="md-nav__link">
<span class="md-ellipsis">
Why a dimensionless quantity is not just a fundamental arithmetic type?
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#why-do-the-identifiers-for-concepts-in-the-library-use-camelcase" class="md-nav__link">
<span class="md-ellipsis">
Why do the identifiers for concepts in the library use CamelCase?
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#why-unicode-quantity-symbols-are-used-by-default-instead-of-ascii-only-characters" class="md-nav__link">
<span class="md-ellipsis">
Why Unicode quantity symbols are used by default instead of ASCII-only characters?
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#why-dont-we-have-cmake-options-to-disable-the-building-of-tests-and-examples" class="md-nav__link">
<span class="md-ellipsis">
Why don't we have CMake options to disable the building of tests and examples?
</span>
</a>
</li>
</ul>
</nav>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_3" >
<label class="md-nav__link" for="__nav_3" id="__nav_3_label" tabindex="0">
<span class="md-ellipsis">
User's Guide
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="1" aria-labelledby="__nav_3_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_3">
<span class="md-nav__icon md-icon"></span>
User's Guide
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../users_guide/terms_and_definitions/" class="md-nav__link">
<span class="md-ellipsis">
Terms and Definitions
</span>
</a>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_3_2" >
<label class="md-nav__link" for="__nav_3_2" id="__nav_3_2_label" tabindex="0">
<span class="md-ellipsis">
Framework Basics
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_3_2_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_3_2">
<span class="md-nav__icon md-icon"></span>
Framework Basics
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../users_guide/framework_basics/interface_introduction/" class="md-nav__link">
<span class="md-ellipsis">
Interface Introduction
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/framework_basics/design_overview/" class="md-nav__link">
<span class="md-ellipsis">
Design Overview
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/framework_basics/systems_of_quantities/" class="md-nav__link">
<span class="md-ellipsis">
Systems of Quantities
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/framework_basics/systems_of_units/" class="md-nav__link">
<span class="md-ellipsis">
Systems of Units
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/framework_basics/simple_and_typed_quantities/" class="md-nav__link">
<span class="md-ellipsis">
Simple and Typed Quantities
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/framework_basics/value_conversions/" class="md-nav__link">
<span class="md-ellipsis">
Value Conversions
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/framework_basics/character_of_a_quantity/" class="md-nav__link">
<span class="md-ellipsis">
Character of a Quantity
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/framework_basics/dimensionless_quantities/" class="md-nav__link">
<span class="md-ellipsis">
Dimensionless Quantities
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/framework_basics/quantity_arithmetics/" class="md-nav__link">
<span class="md-ellipsis">
Quantity Arithmetics
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/framework_basics/generic_interfaces/" class="md-nav__link">
<span class="md-ellipsis">
Generic Interfaces
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/framework_basics/faster_than_lightspeed_constants/" class="md-nav__link">
<span class="md-ellipsis">
Faster-than-lightspeed Constants
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/framework_basics/the_affine_space/" class="md-nav__link">
<span class="md-ellipsis">
The Affine Space
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/framework_basics/obtaining_metadata/" class="md-nav__link">
<span class="md-ellipsis">
Obtaining Metadata
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/framework_basics/concepts/" class="md-nav__link">
<span class="md-ellipsis">
Concepts
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/framework_basics/text_output/" class="md-nav__link">
<span class="md-ellipsis">
Text Output
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_3_3" >
<label class="md-nav__link" for="__nav_3_3" id="__nav_3_3_label" tabindex="0">
<span class="md-ellipsis">
Defining Systems
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_3_3_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_3_3">
<span class="md-nav__icon md-icon"></span>
Defining Systems
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../users_guide/defining_systems/introduction/" class="md-nav__link">
<span class="md-ellipsis">
Introduction
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/defining_systems/isq/" class="md-nav__link">
<span class="md-ellipsis">
International System of Quantities (ISQ)
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/defining_systems/si/" class="md-nav__link">
<span class="md-ellipsis">
International System of Units (SI)
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/defining_systems/strong_angular_system/" class="md-nav__link">
<span class="md-ellipsis">
Strong Angular System
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/defining_systems/natural_units/" class="md-nav__link">
<span class="md-ellipsis">
Natural Units
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_3_4" >
<label class="md-nav__link" for="__nav_3_4" id="__nav_3_4_label" tabindex="0">
<span class="md-ellipsis">
Use Cases
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_3_4_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_3_4">
<span class="md-nav__icon md-icon"></span>
Use Cases
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../users_guide/use_cases/pure_dimensional_analysis/" class="md-nav__link">
<span class="md-ellipsis">
Pure Dimensional Analysis
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/use_cases/working_with_legacy_interfaces/" class="md-nav__link">
<span class="md-ellipsis">
Working with Legacy Interfaces
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/use_cases/using_custom_representation_types/" class="md-nav__link">
<span class="md-ellipsis">
Using Custom Representation Types
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/use_cases/interoperability_with_other_libraries/" class="md-nav__link">
<span class="md-ellipsis">
Interoperability with Other Libraries
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/use_cases/extending_the_library/" class="md-nav__link">
<span class="md-ellipsis">
Extending the Library
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/use_cases/wide_compatibility/" class="md-nav__link">
<span class="md-ellipsis">
Wide Compatibility
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_3_5" >
<label class="md-nav__link" for="__nav_3_5" id="__nav_3_5_label" tabindex="0">
<span class="md-ellipsis">
Examples
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_3_5_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_3_5">
<span class="md-nav__icon md-icon"></span>
Examples
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../users_guide/examples/tags_index/" class="md-nav__link">
<span class="md-ellipsis">
Tags Index
</span>
</a>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_3_5_2" >
<label class="md-nav__link" for="__nav_3_5_2" id="__nav_3_5_2_label" tabindex="0">
<span class="md-ellipsis">
All Examples
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="3" aria-labelledby="__nav_3_5_2_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_3_5_2">
<span class="md-nav__icon md-icon"></span>
All Examples
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../users_guide/examples/hello_units/" class="md-nav__link">
<span class="md-ellipsis">
hello_units
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/examples/avg_speed/" class="md-nav__link">
<span class="md-ellipsis">
avg_speed
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../users_guide/examples/si_constants/" class="md-nav__link">
<span class="md-ellipsis">
si_constants
</span>
</a>
</li>
</ul>
</nav>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_3_6" >
<label class="md-nav__link" for="__nav_3_6" id="__nav_3_6_label" tabindex="0">
<span class="md-ellipsis">
Appendix
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_3_6_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_3_6">
<span class="md-nav__icon md-icon"></span>
Appendix
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../appendix/glossary/" class="md-nav__link">
<span class="md-ellipsis">
Glossary
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../appendix/references/" class="md-nav__link">
<span class="md-ellipsis">
References
</span>
</a>
</li>
</ul>
</nav>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_4" >
<div class="md-nav__link md-nav__container">
<a href="../../blog/" class="md-nav__link ">
<span class="md-ellipsis">
Blog
</span>
</a>
<label class="md-nav__link " for="__nav_4" id="__nav_4_label" tabindex="0">
<span class="md-nav__icon md-icon"></span>
</label>
</div>
<nav class="md-nav" data-md-level="1" aria-labelledby="__nav_4_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_4">
<span class="md-nav__icon md-icon"></span>
Blog
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_4_2" >
<label class="md-nav__link" for="__nav_4_2" id="__nav_4_2_label" tabindex="0">
<span class="md-ellipsis">
Archive
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_4_2_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_4_2">
<span class="md-nav__icon md-icon"></span>
Archive
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../blog/archive/2024/" class="md-nav__link">
<span class="md-ellipsis">
2024
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../blog/archive/2023/" class="md-nav__link">
<span class="md-ellipsis">
2023
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_4_3" >
<label class="md-nav__link" for="__nav_4_3" id="__nav_4_3_label" tabindex="0">
<span class="md-ellipsis">
Categories
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_4_3_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_4_3">
<span class="md-nav__icon md-icon"></span>
Categories
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../blog/category/releases/" class="md-nav__link">
<span class="md-ellipsis">
Releases
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../blog/category/wg21/" class="md-nav__link">
<span class="md-ellipsis">
WG21
</span>
</a>
</li>
</ul>
</nav>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item">
<a href="../../release_notes/" class="md-nav__link">
<span class="md-ellipsis">
Release Notes
</span>
</a>
</li>
</ul>
</nav>
</div>
</div>
</div>
<div class="md-content" data-md-component="content">
<article class="md-content__inner md-typeset">
<a href="https://github.com/mpusz/mp-units/edit/master/docs/getting_started/faq.md" title="Edit this page" class="md-content__button md-icon">
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M10 20H6V4h7v5h5v3.1l2-2V8l-6-6H6c-1.1 0-2 .9-2 2v16c0 1.1.9 2 2 2h4v-2m10.2-7c.1 0 .3.1.4.2l1.3 1.3c.2.2.2.6 0 .8l-1 1-2.1-2.1 1-1c.1-.1.2-.2.4-.2m0 3.9L14.1 23H12v-2.1l6.1-6.1 2.1 2.1Z"/></svg>
</a>
<a href="https://github.com/mpusz/mp-units/raw/master/docs/getting_started/faq.md" title="View source of this page" class="md-content__button md-icon">
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M17 18c.56 0 1 .44 1 1s-.44 1-1 1-1-.44-1-1 .44-1 1-1m0-3c-2.73 0-5.06 1.66-6 4 .94 2.34 3.27 4 6 4s5.06-1.66 6-4c-.94-2.34-3.27-4-6-4m0 6.5a2.5 2.5 0 0 1-2.5-2.5 2.5 2.5 0 0 1 2.5-2.5 2.5 2.5 0 0 1 2.5 2.5 2.5 2.5 0 0 1-2.5 2.5M9.27 20H6V4h7v5h5v4.07c.7.08 1.36.25 2 .49V8l-6-6H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h4.5a8.15 8.15 0 0 1-1.23-2Z"/></svg>
</a>
<h1 id="frequently-asked-questions">Frequently Asked Questions<a class="headerlink" href="#frequently-asked-questions" title="Permanent link">&para;</a></h1>
<h2 id="why-do-we-spell-metre-instead-of-meter">Why do we spell <code>metre</code> instead of <code>meter</code>?<a class="headerlink" href="#why-do-we-spell-metre-instead-of-meter" title="Permanent link">&para;</a></h2>
<p>This is how the BIPM defines it in the <a href="../../appendix/references/#SIBrochure">SI Brochure</a>
(British English spelling by default).</p>
<h2 id="why-dont-we-use-udls-to-create-quantities">Why don't we use UDLs to create quantities?<a class="headerlink" href="#why-dont-we-use-udls-to-create-quantities" title="Permanent link">&para;</a></h2>
<p>Many reasons make UDLs a poor choice for a physical units library:</p>
<ol>
<li>UDLs work only with literals (compile-time known values). Our observation is that besides
the unit tests, there are only a few compile-time known quantity values used in the production
code. Please note that for physical constants, we recommend using
<a href="../../users_guide/framework_basics/faster_than_lightspeed_constants/">Faster-than-lightspeed Constants</a>.</li>
<li>
<p>Typical implementations of UDLs tend to always use the widest representation type available.
In the case of <code>std::chrono::duration</code>, the following is true:</p>
<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">using</span><span class="w"> </span><span class="k">namespace</span><span class="w"> </span><span class="nn">std</span><span class="o">::</span><span class="nn">chrono_literals</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="k">auto</span><span class="w"> </span><span class="n">d1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">42</span><span class="n">s</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="k">auto</span><span class="w"> </span><span class="n">d2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mf">42.</span><span class="n">s</span><span class="p">;</span>
</span><span id="__span-0-4"><a id="__codelineno-0-4" name="__codelineno-0-4" href="#__codelineno-0-4"></a><span class="k">static_assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_same_v</span><span class="o">&lt;</span><span class="k">decltype</span><span class="p">(</span><span class="n">d1</span><span class="p">)</span><span class="o">::</span><span class="n">rep</span><span class="p">,</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="kt">int64_t</span><span class="o">&gt;</span><span class="p">);</span>
</span><span id="__span-0-5"><a id="__codelineno-0-5" name="__codelineno-0-5" href="#__codelineno-0-5"></a><span class="k">static_assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_same_v</span><span class="o">&lt;</span><span class="k">decltype</span><span class="p">(</span><span class="n">d2</span><span class="p">)</span><span class="o">::</span><span class="n">rep</span><span class="p">,</span><span class="w"> </span><span class="kt">long</span><span class="w"> </span><span class="kt">double</span><span class="o">&gt;</span><span class="p">);</span>
</span></code></pre></div>
</li>
</ol>
<p>When such UDL is intermixed in arithmetics with any quantity type of a shorter representation
type, it will always expand it to the longest one. In other words, such long type spreads until
all types use it everywhere.</p>
<ol>
<li>
<p>While increasing the coverage for the library, we learned that many unit symbols conflict with
built-in types or numeric extensions. A few of those are: <code>F</code> (farad), <code>J</code> (joule), <code>W</code> (watt),
<code>K</code> (kelvin), <code>d</code> (day), <code>l</code> or <code>L</code> (litre), <code>erg</code>, <code>ergps</code>. Usage of the <code>_</code> prefix would make
it work for <strong>mp-units</strong>, but in case the library is standardized, those naming collisions would
be a big issue. This is why we came up with the <code>_q_</code> prefix that would become <code>q_</code> after
standardization (e.g., <code>42q_s</code>), which is not that nice anymore.</p>
</li>
<li>
<p>UDLs with the same identifiers defined in different namespace can't be disambiguated in the C++
language. If both SI and CGS systems define <code>_q_s</code> UDL for a second unit, then it would not be possible
to specify which one to use in case both namespaces are "imported" with using directives.</p>
</li>
<li>
<p>Another bad property of UDLs is that they do not compose. A coherent unit of angular momentum would
have a UDL specified as <code>_q_kg_m2_per_s</code>. Now imagine that we want to make every possible user happy.
How many variations of that unit would we predefine for differently scaled versions of all unit
ingredients?</p>
</li>
<li>
<p>UDLs are also really expensive to define and specify. Typically, for each unit, we need two
definitions. One for integral and another one for floating-point representation. Before the
V2 framework, the coherent unit of angular momentum was defined as:</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">constexpr</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="k">operator</span><span class="s">&quot;&quot;</span><span class="w"> </span><span class="n">_q_kg_m2_per_s</span><span class="p">(</span><span class="kt">unsigned</span><span class="w"> </span><span class="kt">long</span><span class="w"> </span><span class="kt">long</span><span class="w"> </span><span class="n">l</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="p">{</span>
</span><span id="__span-1-3"><a id="__codelineno-1-3" name="__codelineno-1-3" href="#__codelineno-1-3"></a><span class="w"> </span><span class="n">gsl_Expects</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">in_range</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="kt">int64_t</span><span class="o">&gt;</span><span class="p">(</span><span class="n">l</span><span class="p">));</span>
</span><span id="__span-1-4"><a id="__codelineno-1-4" name="__codelineno-1-4" href="#__codelineno-1-4"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">angular_momentum</span><span class="o">&lt;</span><span class="n">kilogram_metre_sq_per_second</span><span class="p">,</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="kt">int64_t</span><span class="o">&gt;</span><span class="p">(</span><span class="k">static_cast</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="kt">int64_t</span><span class="o">&gt;</span><span class="p">(</span><span class="n">l</span><span class="p">));</span>
</span><span id="__span-1-5"><a id="__codelineno-1-5" name="__codelineno-1-5" href="#__codelineno-1-5"></a><span class="p">}</span>
</span><span id="__span-1-6"><a id="__codelineno-1-6" name="__codelineno-1-6" href="#__codelineno-1-6"></a>
</span><span id="__span-1-7"><a id="__codelineno-1-7" name="__codelineno-1-7" href="#__codelineno-1-7"></a><span class="k">constexpr</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="k">operator</span><span class="s">&quot;&quot;</span><span class="w"> </span><span class="n">_q_kg_m2_per_s</span><span class="p">(</span><span class="kt">long</span><span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="n">l</span><span class="p">)</span>
</span><span id="__span-1-8"><a id="__codelineno-1-8" name="__codelineno-1-8" href="#__codelineno-1-8"></a><span class="p">{</span>
</span><span id="__span-1-9"><a id="__codelineno-1-9" name="__codelineno-1-9" href="#__codelineno-1-9"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">angular_momentum</span><span class="o">&lt;</span><span class="n">kilogram_metre_sq_per_second</span><span class="p">,</span><span class="w"> </span><span class="kt">long</span><span class="w"> </span><span class="kt">double</span><span class="o">&gt;</span><span class="p">(</span><span class="n">l</span><span class="p">);</span>
</span><span id="__span-1-10"><a id="__codelineno-1-10" name="__codelineno-1-10" href="#__codelineno-1-10"></a><span class="p">}</span>
</span></code></pre></div>
</li>
</ol>
<h2 id="why-cant-i-create-a-quantity-by-passing-a-number-to-a-constructor">Why can't I create a quantity by passing a number to a constructor?<a class="headerlink" href="#why-cant-i-create-a-quantity-by-passing-a-number-to-a-constructor" title="Permanent link">&para;</a></h2>
<p>A quantity class template in the <strong>mp-units</strong> library has no publicly available constructor taking
a raw value. Such support is provided by the <code>std::chrono::duration</code> and was pointed out to us as
a red flag safety issue by a few parties already.</p>
<p>Consider the following structure and a code using it:</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">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </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="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">chrono</span><span class="o">::</span><span class="n">milliseconds</span><span class="o">&gt;</span><span class="w"> </span><span class="n">vec</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="w"> </span><span class="c1">// ...</span>
</span><span id="__span-2-4"><a id="__codelineno-2-4" name="__codelineno-2-4" href="#__codelineno-2-4"></a><span class="p">};</span>
</span></code></pre></div>
<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="n">X</span><span class="w"> </span><span class="n">x</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">x</span><span class="p">.</span><span class="n">vec</span><span class="p">.</span><span class="n">emplace_back</span><span class="p">(</span><span class="mi">42</span><span class="p">);</span>
</span></code></pre></div>
<p>Everything works fine for years until, at some point, someone changes the structure to:</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">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </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="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">chrono</span><span class="o">::</span><span class="n">microseconds</span><span class="o">&gt;</span><span class="w"> </span><span class="n">vec</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="w"> </span><span class="c1">// ...</span>
</span><span id="__span-4-4"><a id="__codelineno-4-4" name="__codelineno-4-4" href="#__codelineno-4-4"></a><span class="p">};</span>
</span></code></pre></div>
<p>The code continues to compile just fine, but all the calculations are off now. This is why we decided
to not follow this path.</p>
<p>In the <strong>mp-units</strong> library, both a number and a unit have to always be explicitly provided in order
to form a quantity.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>The same applies to the construction of <code>quantity_point</code> using an explicit point origin.
To prevent similar safety issues during maintenance, the initialization always requires
providing both a <code>quantity</code> and a <a href="../../users_guide/framework_basics/concepts/#PointOrigin"><code>PointOrigin</code></a>
that we use as a reference point.</p>
</div>
<h2 id="why-a-dimensionless-quantity-is-not-just-a-fundamental-arithmetic-type">Why a dimensionless quantity is not just a fundamental arithmetic type?<a class="headerlink" href="#why-a-dimensionless-quantity-is-not-just-a-fundamental-arithmetic-type" title="Permanent link">&para;</a></h2>
<p>In the initial design of this library, the resulting type of division of two quantities was their
common representation type:</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="k">static_assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_same_v</span><span class="o">&lt;</span><span class="k">decltype</span><span class="p">(</span><span class="mi">10</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="p">(</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 class="w"> </span><span class="kt">int</span><span class="o">&gt;</span><span class="p">);</span>
</span></code></pre></div>
<p>First of all, this was consistent with
<a href="https://en.cppreference.com/w/cpp/chrono/duration/operator_arith4"><code>std::chrono::duration</code></a> behavior.
Additional reasoning behind it was not providing a false impression of a strong <code>quantity</code> type for
something that looks and feels like a regular number. Also, all of the mathematic and trigonometric functions
were working fine out of the box with such representation types, so we did not have to rewrite
<code>sin()</code>, <code>cos()</code>, <code>exp()</code>, and others.</p>
<p>However, the feedback we got from the production usage was that such an approach is really bad for generic
programming. It is hard to handle the result of the two quantities' division (or multiplication) as
it might be either a quantity or a fundamental type. If we want to raise such a result to some power, we
must use <code>units::pow</code> or <code>std::pow</code> depending on the resulting type. Those are only a few issues related
to such an approach.</p>
<p>Moreover, suppose we divide quantities of the same dimension but with units of significantly different
magnitudes. In that case, we may end up with a really small or a huge floating-point value, which may result
in losing lots of precision. Returning a dimensionless quantity from such cases allows us to benefit from
all the properties of scaled units and is consistent with the rest of the library.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>More information on the current design can be found in
<a href="../../users_guide/framework_basics/dimensionless_quantities/">the Dimensionless Quantities chapter</a>.</p>
</div>
<h2 id="why-do-the-identifiers-for-concepts-in-the-library-use-camelcase">Why do the identifiers for concepts in the library use <code>CamelCase</code>?<a class="headerlink" href="#why-do-the-identifiers-for-concepts-in-the-library-use-camelcase" title="Permanent link">&para;</a></h2>
<p>Initially, C++20 was meant to use <code>CamelCase</code> for all the concept identifiers. All the concepts
from the <code>std::ranges</code> library were merged with such names into the standard document draft.
Frustratingly, <code>CamelCase</code> concepts got dropped from the C++ standard at the last moment before
releasing C++20. Now, we are facing the predictable consequences of running out of names.</p>
<p>As long as some concepts in the library could be easily named with a <code>standard_case</code> there are
some that are hard to distinguish from the corresponding type names, such as <code>Quantity</code>,
<code>QuantityPoint</code>, <code>QuantitySpec</code>, or <code>Reference</code>. This is why we decided to use <code>CamelCase</code>
consistently for all the concept identifiers to make it clear when we are talking about a type
or concept identifier.</p>
<p>However, we are aware that this might be a temporary solution. In case the library gets
standardized, we can expect the ISO C++ Committee to bikeshed/rename all of
the concept identifiers to a <code>standard_case</code>, even if it will result in a harder to understand
code.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>In case you have a good idea of how to rename
<a href="../../users_guide/framework_basics/concepts/">existing concepts</a> to the <code>standard_case</code>,
please let us know in the associated <a href="https://github.com/mpusz/mp-units/issues/93">GitHub Issue</a>.</p>
</div>
<h2 id="why-unicode-quantity-symbols-are-used-by-default-instead-of-ascii-only-characters">Why Unicode quantity symbols are used by default instead of ASCII-only characters?<a class="headerlink" href="#why-unicode-quantity-symbols-are-used-by-default-instead-of-ascii-only-characters" title="Permanent link">&para;</a></h2>
<p>Both C++ and <a href="../../appendix/references/#ISO80000">ISO 80000</a> are standardized by the ISO.
<a href="../../appendix/references/#ISO80000">ISO 80000</a> and the <a href="../../appendix/references/#SIBrochure">SI</a>
standards specify Unicode symbols as the official unit names for some quantities
(e.g. <code>Ω</code> symbol for the resistance quantity).
As the <strong>mp-units</strong> library will be proposed for standardization as a part of the C++ Standard Library
we have to obey the rules and be consistent with ISO specifications.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>We do understand engineering reality and the constraints of some environments. This is why the library
has the option of <a href="../../users_guide/framework_basics/text_output/#unit_symbol_formatting">ASCII-only Quantity Symbols</a>.</p>
</div>
<h2 id="why-dont-we-have-cmake-options-to-disable-the-building-of-tests-and-examples">Why don't we have CMake options to disable the building of tests and examples?<a class="headerlink" href="#why-dont-we-have-cmake-options-to-disable-the-building-of-tests-and-examples" title="Permanent link">&para;</a></h2>
<p>Over time, many people provided PRs proposing adding options to build tests and examples conditionally.
Here are a few examples:</p>
<ul>
<li><a href="https://github.com/mpusz/mp-units/pull/124">Add CMake options for disabling docs, examples and tests</a></li>
<li><a href="https://github.com/mpusz/mp-units/pull/402">build: add options to disable part of the build</a></li>
<li><a href="https://github.com/mpusz/mp-units/pull/456">CMake Refactoring and Option Cleanup</a></li>
</ul>
<p>We admit this is a common practice in the industry, but we also believe this is a bad pattern.</p>
<p>First, the only need for such options comes when a user wants to use <code>add_subdirectory()</code> in CMake
to handle dependencies. Such an approach does not scale and should be discouraged. There is little
use for such a practice in times when we have dedicated package managers like Conan.</p>
<p>The second thing is that our observation is that many people are fixed on disabling "unneeded" subdirectories
from compilation, but they do not see or address the biggest issue, which is polluting user's build
environment with our development-specific settings. Propagating our restrictive compilation flags to user's
project is not the best idea as it might cause a lot of harm if this project stops to compile
because of that.</p>
<p>Last but not least, not having those options is on purpose. Top level <em>CMakeLists.txt</em> file should only
be used by <strong>mp-units</strong> developers and contributors as an entry point for the project's development.
We want to ensure that everyone will build <strong>ALL</strong> the code correctly before pushing a commit. Having
such options would allow unintended issues to leak to PRs and CI.</p>
<p>This is why our projects have two entry points:</p>
<ul>
<li><em>./CMakeLists.txt</em> is to be used by projects developers to build <strong>ALL</strong> the project code with really
restrictive compilation flags,</li>
<li><em>./src/CMakeLists.txt</em> contains only a pure library definition and should be used by the customers
that prefer to use CMake's <code>add_subdirectory()</code> to handle the dependencies.</li>
</ul>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>For more details on this please refer to the
<a href="https://youtu.be/mrSwJBJ-0z8?t=1931">CMake + Conan: 3 Years Later - Mateusz Pusz</a>
lecture that Mateusz Pusz provided at the C++Now 2021 conference.</p>
</div>
</article>
</div>
<script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script>
</div>
<button type="button" class="md-top md-icon" data-md-component="top" hidden>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg>
Back to top
</button>
</main>
<footer class="md-footer">
<nav class="md-footer__inner md-grid" aria-label="Footer" >
<a href="../installation_and_usage/" class="md-footer__link md-footer__link--prev" aria-label="Previous: Installation and Usage">
<div class="md-footer__button md-icon">
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg>
</div>
<div class="md-footer__title">
<span class="md-footer__direction">
Previous
</span>
<div class="md-ellipsis">
Installation and Usage
</div>
</div>
</a>
<a href="../../users_guide/terms_and_definitions/" class="md-footer__link md-footer__link--next" aria-label="Next: Terms and Definitions">
<div class="md-footer__title">
<span class="md-footer__direction">
Next
</span>
<div class="md-ellipsis">
Terms and Definitions
</div>
</div>
<div class="md-footer__button md-icon">
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg>
</div>
</a>
</nav>
<div class="md-footer-meta md-typeset">
<div class="md-footer-meta__inner md-grid">
<div class="md-copyright">
<div class="md-copyright__highlight">
Copyright &copy; 2018-2023 Mateusz Pusz
</div>
Made with
<a href="https://squidfunk.github.io/mkdocs-material/" target="_blank" rel="noopener">
Material for MkDocs
</a>
</div>
</div>
</div>
</footer>
</div>
<div class="md-dialog" data-md-component="dialog">
<div class="md-dialog__inner md-typeset"></div>
</div>
<script id="__config" type="application/json">{"base": "../..", "features": ["content.action.edit", "content.action.view", "content.code.annotate", "content.code.copy", "navigation.indexes", "navigation.tabs", "navigation.footer", "navigation.top", "search.highlight", "search.share", "search.suggest", "toc.follow", "toc.integrate"], "search": "../../assets/javascripts/workers/search.b8dbb3d2.min.js", "translations": {"clipboard.copied": "Copied to clipboard", "clipboard.copy": "Copy to clipboard", "search.result.more.one": "1 more on this page", "search.result.more.other": "# more on this page", "search.result.none": "No matching documents", "search.result.one": "1 matching document", "search.result.other": "# matching documents", "search.result.placeholder": "Type to start searching", "search.result.term.missing": "Missing", "select.version": "Select version"}, "version": {"provider": "mike"}}</script>
<script src="../../assets/javascripts/bundle.ad660dcc.min.js"></script>
<script src="../../javascripts/mathjax.js"></script>
<script src="https://polyfill.io/v3/polyfill.min.js?features=es6"></script>
<script src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js"></script>
</body>
</html>
View Git Blame Copy Permalink