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  <title>std::from_chars should work with std::string_view</title>
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<header id="title-block-header">
<h1 class="title" style="text-align:center"><code>std::from_chars</code> should work with <code>std::string_view</code></h1>

<table style="border:none;float:right">
  <tr>
    <td>Document #: </td>
    <td>P2007R0</td>
  </tr>
  <tr>
    <td>Date: </td>
    <td>2020-01-10</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project: </td>
    <td>Programming Language C++<br>
      Library Evolution Working Group Incubator<br>
      Library Evolution Working Group<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to: </td>
    <td>
      Mateusz Pusz (<a href="http://www.epam.com">Epam Systems</a>)<br>&lt;<a href="mailto:mateusz.pusz@gmail.com" class="email">mateusz.pusz@gmail.com</a>&gt;<br>
    </td>
  </tr>
</table>

</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#introduction"><span class="toc-section-number">1</span> Introduction<span></span></a></li>
<li><a href="#motivation-and-scope"><span class="toc-section-number">2</span> Motivation and Scope<span></span></a>
<ul>
<li><a href="#a-naive-approach"><span class="toc-section-number">2.1</span> A naive approach<span></span></a></li>
<li><a href="#a-hackers-approach"><span class="toc-section-number">2.2</span> A hacker’s approach<span></span></a></li>
<li><a href="#a-correct-approach"><span class="toc-section-number">2.3</span> A correct approach<span></span></a></li>
</ul></li>
<li><a href="#beforeafter-comparison"><span class="toc-section-number">3</span> Before/After Comparison<span></span></a></li>
<li><a href="#proposed-wording"><span class="toc-section-number">4</span> Proposed Wording<span></span></a></li>
<li><a href="#acknowledgements"><span class="toc-section-number">5</span> Acknowledgements<span></span></a></li>
<li><a href="#references"><span class="toc-section-number">6</span> References<span></span></a></li>
</ul>
</div>
<section id="introduction" data-number="1">
<h1 data-number="1"><span class="header-section-number">1</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p><code>std::from_chars</code> accepts only a pair of raw <code>char</code> pointers in its overloads to provide a range of characters for conversion. The author proposes to add overloads taking <code>std::string_view</code> which is the main C++ vocabulary type to denote a range of characters.</p>
</section>
<section id="motivation-and-scope" data-number="2">
<h1 data-number="2"><span class="header-section-number">2</span> Motivation and Scope<a href="#motivation-and-scope" class="self-link"></a></h1>
<section id="a-naive-approach" data-number="2.1">
<h2 data-number="2.1"><span class="header-section-number">2.1</span> A naive approach<a href="#a-naive-approach" class="self-link"></a></h2>
<p>Unfortunately, it is common for an average C++ developer to provide the following implementation for converting a <code>std::string_view</code> to <code>int</code> via a <code>std::from_chars</code>.</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a><span class="dt">void</span> foo<span class="op">(</span>std<span class="op">::</span>string_view txt<span class="op">)</span></span>
<span id="cb1-2"><a href="#cb1-2"></a><span class="op">{</span></span>
<span id="cb1-3"><a href="#cb1-3"></a>  <span class="dt">int</span> result;</span>
<span id="cb1-4"><a href="#cb1-4"></a>  <span class="kw">auto</span> <span class="op">[</span>ptr, ec<span class="op">]</span> <span class="op">=</span> std<span class="op">::</span>from_chars<span class="op">(</span>txt<span class="op">.</span>begin<span class="op">()</span>, txt<span class="op">.</span>end<span class="op">()</span>, result<span class="op">)</span>;</span>
<span id="cb1-5"><a href="#cb1-5"></a>  <span class="cf">if</span> <span class="op">(</span>ec <span class="op">==</span> std<span class="op">::</span>errc<span class="op">())</span></span>
<span id="cb1-6"><a href="#cb1-6"></a>    <span class="co">// ...</span></span>
<span id="cb1-7"><a href="#cb1-7"></a><span class="op">}</span></span></code></pre></div>
<p>The above will work for some implementations and build modes but will fail for others. This is caused by the fact that even though some vendors implement <code>std::string_view::const_iterator</code> in terms of <code>const char*</code>, such code is not portable. The C++ standard specifies <code>std::string_view::const_iterator</code> as <em>implementation defined</em> and only requires that the iterator’s <code>value_type</code> is <code>char</code>.</p>
</section>
<section id="a-hackers-approach" data-number="2.2">
<h2 data-number="2.2"><span class="header-section-number">2.2</span> A hacker’s approach<a href="#a-hackers-approach" class="self-link"></a></h2>
<p>After realizing the portability issue programmers often fix it in the following way which is far from what we want to teach:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1"></a><span class="dt">void</span> foo<span class="op">(</span>std<span class="op">::</span>string_view txt<span class="op">)</span></span>
<span id="cb2-2"><a href="#cb2-2"></a><span class="op">{</span></span>
<span id="cb2-3"><a href="#cb2-3"></a>  <span class="dt">int</span> result;</span>
<span id="cb2-4"><a href="#cb2-4"></a>  <span class="kw">auto</span> <span class="op">[</span>ptr, ec<span class="op">]</span> <span class="op">=</span> std<span class="op">::</span>from_chars<span class="op">(&amp;*</span>txt<span class="op">.</span>begin<span class="op">()</span>, <span class="op">&amp;*</span>txt<span class="op">.</span>end<span class="op">()</span>, result<span class="op">)</span>;</span>
<span id="cb2-5"><a href="#cb2-5"></a>  <span class="cf">if</span> <span class="op">(</span>ec <span class="op">==</span> std<span class="op">::</span>errc<span class="op">())</span></span>
<span id="cb2-6"><a href="#cb2-6"></a>    <span class="co">// ...</span></span>
<span id="cb2-7"><a href="#cb2-7"></a><span class="op">}</span></span></code></pre></div>
</section>
<section id="a-correct-approach" data-number="2.3">
<h2 data-number="2.3"><span class="header-section-number">2.3</span> A correct approach<a href="#a-correct-approach" class="self-link"></a></h2>
<p>A correct approach works on the underlying data rather than on iterators which is counter intuitive as we always taught to use iterators to denote ranges in C++:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a><span class="dt">void</span> foo<span class="op">(</span>std<span class="op">::</span>string_view txt<span class="op">)</span></span>
<span id="cb3-2"><a href="#cb3-2"></a><span class="op">{</span></span>
<span id="cb3-3"><a href="#cb3-3"></a>  <span class="dt">int</span> result;</span>
<span id="cb3-4"><a href="#cb3-4"></a>  <span class="kw">auto</span> <span class="op">[</span>ptr, ec<span class="op">]</span> <span class="op">=</span> std<span class="op">::</span>from_chars<span class="op">(</span>txt<span class="op">.</span>data<span class="op">()</span>, txt<span class="op">.</span>data<span class="op">()</span> <span class="op">+</span> txt<span class="op">.</span>size<span class="op">()</span>, result<span class="op">)</span>;</span>
<span id="cb3-5"><a href="#cb3-5"></a>  <span class="cf">if</span> <span class="op">(</span>ec <span class="op">==</span> std<span class="op">::</span>errc<span class="op">())</span></span>
<span id="cb3-6"><a href="#cb3-6"></a>    <span class="co">// ...</span></span>
<span id="cb3-7"><a href="#cb3-7"></a><span class="op">}</span></span></code></pre></div>
</section>
</section>
<section id="beforeafter-comparison" data-number="3">
<h1 data-number="3"><span class="header-section-number">3</span> Before/After Comparison<a href="#beforeafter-comparison" class="self-link"></a></h1>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>Before</strong>
</div></th>
<th><div style="text-align:center">
<strong>After</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1"></a><span class="dt">void</span> foo<span class="op">(</span>std<span class="op">::</span>string_view txt<span class="op">)</span></span>
<span id="cb4-2"><a href="#cb4-2"></a><span class="op">{</span></span>
<span id="cb4-3"><a href="#cb4-3"></a>  <span class="dt">int</span> result;</span>
<span id="cb4-4"><a href="#cb4-4"></a>  <span class="kw">auto</span> <span class="op">[</span>ptr, ec<span class="op">]</span> <span class="op">=</span> std<span class="op">::</span>from_chars<span class="op">(</span>txt<span class="op">.</span>data<span class="op">()</span>, txt<span class="op">.</span>data<span class="op">()</span> <span class="op">+</span> txt<span class="op">.</span>size<span class="op">()</span>, result<span class="op">)</span>;</span>
<span id="cb4-5"><a href="#cb4-5"></a>  <span class="cf">if</span> <span class="op">(</span>ec <span class="op">==</span> std<span class="op">::</span>errc<span class="op">())</span></span>
<span id="cb4-6"><a href="#cb4-6"></a>    <span class="co">// ...</span></span>
<span id="cb4-7"><a href="#cb4-7"></a><span class="op">}</span></span></code></pre></div></td>
<td><div class="sourceCode" id="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1"></a><span class="dt">void</span> foo<span class="op">(</span>std<span class="op">::</span>string_view txt<span class="op">)</span></span>
<span id="cb5-2"><a href="#cb5-2"></a><span class="op">{</span></span>
<span id="cb5-3"><a href="#cb5-3"></a>  <span class="dt">int</span> result;</span>
<span id="cb5-4"><a href="#cb5-4"></a>  <span class="kw">auto</span> <span class="op">[</span>ptr, ec<span class="op">]</span> <span class="op">=</span> std<span class="op">::</span>from_chars<span class="op">(</span>txt, result<span class="op">)</span>;</span>
<span id="cb5-5"><a href="#cb5-5"></a>  <span class="cf">if</span> <span class="op">(</span>ec <span class="op">==</span> std<span class="op">::</span>errc<span class="op">())</span></span>
<span id="cb5-6"><a href="#cb5-6"></a>    <span class="co">// ...</span></span>
<span id="cb5-7"><a href="#cb5-7"></a><span class="op">}</span></span></code></pre></div></td>
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</section>
<section id="proposed-wording" data-number="4">
<h1 data-number="4"><span class="header-section-number">4</span> Proposed Wording<a href="#proposed-wording" class="self-link"></a></h1>
<p>The proposed changes are relative to the working draft of the standard as of <span class="citation" data-cites="N4830">[<a href="#ref-N4830" role="doc-biblioref">N4830</a>]</span>.</p>
<p>Add the following to the end of synopsis in <strong>20.19.1 [charconv.syn]</strong>:</p>
<div class="sourceCode" id="cb6"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb6-1"><a href="#cb6-1"></a>from_chars_result from_chars<span class="op">(</span>string_view txt, <em>see below</em><span class="op">&amp;</span> value, <span class="dt">int</span> base <span class="op">=</span> <span class="dv">10</span><span class="op">)</span>;</span>
<span id="cb6-2"><a href="#cb6-2"></a>from_chars_result from_chars<span class="op">(</span>string_view txt, <span class="dt">float</span><span class="op">&amp;</span> value, chars_format fmt <span class="op">=</span> chars_format<span class="op">::</span>general<span class="op">)</span>;</span>
<span id="cb6-3"><a href="#cb6-3"></a>from_chars_result from_chars<span class="op">(</span>string_view txt, <span class="dt">double</span><span class="op">&amp;</span> value, chars_format fmt <span class="op">=</span> chars_format<span class="op">::</span>general<span class="op">)</span>;</span>
<span id="cb6-4"><a href="#cb6-4"></a>from_chars_result from_chars<span class="op">(</span>string_view txt, <span class="dt">long</span> <span class="dt">double</span><span class="op">&amp;</span> value, chars_format fmt <span class="op">=</span> chars_format<span class="op">::</span>general<span class="op">)</span>;</span></code></pre></div>
<p>Update the following paragraphs in <strong>20.19.3 [charconv.from.chars]</strong>:</p>
<p><span class="marginalizedparent"><a class="marginalized">1</a></span> All functions named <code>from_chars</code> analyze the <span class="add" style="color: #006e28"><ins>provided</ins></span> string <span class="rm" style="color: #bf0303"><del><span><code>[first, last)</code></span></del></span> for a pattern, where <span class="add" style="color: #006e28"><ins>string is either a <span><code>txt</code></span> or <span><code>[first, last)</code></span> denoting</ins></span><span class="rm" style="color: #bf0303"><del><span><code>[first, last)</code></span> is required to be</del></span> a valid range.</p>
<div>
<div class="sourceCode" id="cb7"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb7-1"><a href="#cb7-1"></a>  from_chars_result from_chars(const char* first, const char* last,</span>
<span id="cb7-2"><a href="#cb7-2"></a>                               <em>see below</em>&amp; value, int base = 10);</span>
<span id="cb7-3"><a href="#cb7-3"></a><span class="va">+ from_chars_result from_chars(string_view txt,</span></span>
<span id="cb7-4"><a href="#cb7-4"></a><span class="va">+                              <em>see below</em>&amp; value, int base = 10);</span></span></code></pre></div>
</div>
<div>
<div class="sourceCode" id="cb8"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb8-1"><a href="#cb8-1"></a>  from_chars_result from_chars(const char* first, const char* last, float&amp; value,</span>
<span id="cb8-2"><a href="#cb8-2"></a>                               chars_format fmt = chars_format::general);</span>
<span id="cb8-3"><a href="#cb8-3"></a><span class="va">+ from_chars_result from_chars(string_view txt, float&amp; value,</span></span>
<span id="cb8-4"><a href="#cb8-4"></a><span class="va">+                              chars_format fmt = chars_format::general);</span></span>
<span id="cb8-5"><a href="#cb8-5"></a>  from_chars_result from_chars(const char* first, const char* last, double&amp; value,</span>
<span id="cb8-6"><a href="#cb8-6"></a>                               chars_format fmt = chars_format::general);</span>
<span id="cb8-7"><a href="#cb8-7"></a><span class="va">+ from_chars_result from_chars(string_view txt, double&amp; value,</span></span>
<span id="cb8-8"><a href="#cb8-8"></a><span class="va">+                              chars_format fmt = chars_format::general);</span></span>
<span id="cb8-9"><a href="#cb8-9"></a>  from_chars_result from_chars(const char* first, const char* last, long double&amp; value,</span>
<span id="cb8-10"><a href="#cb8-10"></a>                               chars_format fmt = chars_format::general);</span>
<span id="cb8-11"><a href="#cb8-11"></a><span class="va">+ from_chars_result from_chars(string_view txt, long double&amp; value,</span></span>
<span id="cb8-12"><a href="#cb8-12"></a><span class="va">+                              chars_format fmt = chars_format::general);</span></span></code></pre></div>
</div>
</section>
<section id="acknowledgements" data-number="5">
<h1 data-number="5"><span class="header-section-number">5</span> Acknowledgements<a href="#acknowledgements" class="self-link"></a></h1>
<p>Special thanks and recognition goes to <a href="http://www.epam.com">Epam Systems</a> for supporting my membership in the ISO C++ Committee and the production of this proposal.</p>
</section>
<section id="references" data-number="6">
<h1 data-number="6"><span class="header-section-number">6</span> References<a href="#references" class="self-link"></a></h1>
<div id="refs" class="references" role="doc-bibliography">
<div id="ref-N4830">
<p>[N4830] Richard Smith. 2019. Committee Draft, Standard for Programming Language C++. <br />
<a href="https://wg21.link/n4830">https://wg21.link/n4830</a></p>
</div>
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