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<title>Issue 3126: There's no std::sub_match::compare(string_view) overload</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#New">New</a> status.</em></p>
<h3 id="3126"><a href="lwg-active.html#3126">3126</a>. There's no <code>std::sub_match::compare(string_view)</code> overload</h3>
<p><b>Section:</b> 28.6.8 <a href="https://wg21.link/re.submatch">[re.submatch]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2018-06-26 <b>Last modified:</b> 2024-10-03</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#re.submatch">issues</a> in [re.submatch].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>std::sub_match::compare</code> can be called with a <code>basic_string</code> or a pointer to a null-terminated
character sequence, but can't be called with a <code>basic_string_view</code>. To compare to a <code>string_view</code>
requires either conversion to <code>basic_string</code> (with a potential allocation) or a redundant call to
<code>traits_type::length</code> to calculate a length that is already known.
</p>

<p><i>[2018-07-02, Jonathan comments and completes proposed wording]</i></p>

<p>
To make the relational and equality operators for <code>sub_match</code> support string views I propose specifying
the semantics, not adding another 12 overloaded operators to namespace <code>std</code>, in addition to the 42
already there. This allows them to be implemented as "hidden friends" if the implementation so desires, or to
retain namespace-scope declaration if backwards compatibility with C++11 - C++17 is preferred.
</p>

<p><i>[2018-07-20 Priority set to 3 after reflector discussion]</i></p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">

<p>
This wording is relative to <a href="https://wg21.link/n4750">N4750</a>.
</p>

<ol>
<li>
<p>Change 28.6.3 <a href="https://wg21.link/re.syn">[re.syn]</a>, header <code>&lt;regex&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
#include &lt;initializer_list&gt;

namespace std {
  [&hellip;]
  using csub_match = sub_match&lt;const char*&gt;;
  using wcsub_match = sub_match&lt;const wchar_t*&gt;;
  using ssub_match = sub_match&lt;string::const_iterator&gt;;
  using wssub_match = sub_match&lt;wstring::const_iterator&gt;;

  <del><i>// 28.6.8.3 <a href="https://wg21.link/re.submatch.op">[re.submatch.op]</a>, sub_match non-member operators</i>
  template&lt;class BiIter&gt;
  bool operator==(const sub_match&lt;BiIter&gt;&amp; lhs, const sub_match&lt;BiIter&gt;&amp; rhs);
  [&hellip;]
  template&lt;class BiIter&gt;
    bool operator&gt;=(const sub_match&lt;BiIter&gt;&amp; lhs,
                   const typename iterator_traits&lt;BiIter&gt;::value_type&amp; rhs);</del>

  template&lt;class charT, class ST, class BiIter&gt;
  basic_ostream&lt;charT, ST&gt;&amp;
  operator&lt;&lt;(basic_ostream&lt;charT, ST&gt;&amp; os, const sub_match&lt;BiIter&gt;&amp; m);
  [&hellip;]
}
</pre>
</blockquote>
</li>

<li>
<p>Change 28.6.8 <a href="https://wg21.link/re.submatch">[re.submatch]</a>, class template <code>sub_match</code> synopsis, as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;class BidirectionalIterator&gt;
  class sub_match : public pair&lt;BidirectionalIterator, BidirectionalIterator&gt; {
  public:
    using value_type =
      typename iterator_traits&lt;BidirectionalIterator&gt;::value_type;
    [&hellip;]
    int compare(const sub_match&amp; s) const;
    int compare(const string_type&amp; s) const;
    int compare(const value_type* s) const;
    <ins>int compare(basic_string_view&lt;value_type&gt; s) const;</ins>
  };
}
</pre>
</blockquote>
</li>

<li>
<p>Change 28.6.8.2 <a href="https://wg21.link/re.submatch.members">[re.submatch.members]</a> as indicated:</p>

<blockquote>
<pre>
int compare(const value_type* s) const;
</pre>
<blockquote>
<p>
-7- <i>Returns:</i> <code>str().compare(s)</code>.
</p>
</blockquote>
<pre>
<ins>int compare(basic_string_view&lt;value_type&gt; s) const;</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Returns:</i> <code>str().compare(s)</code>.</ins>
</p>
</blockquote>
</blockquote>
</li>

<li>
<p>Change sub-clause 28.6.8.3 <a href="https://wg21.link/re.submatch.op">[re.submatch.op]</a> as indicated:</p>

<blockquote>
<p>
<b>31.9.2 <code>sub_match</code> non-member operators [re.submatch.op]</b>
</p>
<pre><del>template&lt;class BiIter&gt;
bool operator==(const sub_match&lt;BiIter&gt;&amp; lhs, const sub_match&lt;BiIter&gt;&amp; rhs);</del></pre>
<blockquote>
<p>
<del>-1- <i>Returns:</i> <code>lhs.compare(rhs) == 0</code>.</del>
</p>
</blockquote>
<del>[&hellip;]</del>
<pre>
<del>template&lt;class BiIter&gt;
  bool operator&gt;=(const sub_match&lt;BiIter&gt;&amp; lhs,
    const typename iterator_traits&lt;BiIter&gt;::value_type&amp; rhs);</del></pre>
<blockquote>
<p>
<del>-42- <i>Returns:</i> <code>!(lhs &lt; rhs)</code>.</del>
</p>
</blockquote>
<pre>
template&lt;class charT, class ST, class BiIter&gt;
  basic_ostream&lt;charT, ST&gt;&amp;
    operator&lt;&lt;(basic_ostream&lt;charT, ST&gt;&amp; os, const sub_match&lt;BiIter&gt;&amp; m);</pre>
<blockquote>
<p>
-43- <i>Returns:</i> <code>os &lt;&lt; m.str()</code>.
</p>
</blockquote>
<p>
<ins>Class template <code>sub_match</code> provides overloaded relational operators (7.6.9 <a href="https://wg21.link/expr.rel">[expr.rel]</a>) and
equality operators (7.6.10 <a href="https://wg21.link/expr.eq">[expr.eq]</a>) for comparisons with another <code>sub_match</code>, with a <code>string</code>,
or with a single character. The expressions shown in Table ?? are valid when one of the operands is a type <code>S</code>,
that is a specialization of <code>sub_match</code>, and the other expression is one of:</ins>
</p>
<ol style="list-style-type: none">
<li><p><ins>(?.?) &mdash; a value <code>x</code> of a type <code>S</code>, in which case <code><i>STR</i>(x)</code> is
<code>x.str()</code>;</ins></p></li>
<li><p><ins>(?.?) &mdash; a value <code>x</code> of type <code>basic_string&lt;S::value_type, T, A&gt;</code> for any types
<code>T</code> and <code>A</code>, in which case <code><i>STR</i>(x)</code> is
<code>basic_string_view&lt;S::value_type&gt;(x.data(), x.length())</code>;</ins></p></li>
<li><p><ins>(?.?) &mdash; a value <code>x</code> of type <code>basic_string_view&lt;S::value_type, T&gt;</code> for
any type <code>T</code>, in which case <code><i>STR</i>(x)</code> is
<code>basic_string_view&lt;S::value_type&gt;(x.data(), x.length())</code>;</ins></p></li>
<li><p><ins>(?.?) &mdash; a value <code>x</code> of a type convertible to <code>const S::value_type*</code>, in which
case <code><i>STR</i>(x)</code> is <code>basic_string_view&lt;S::value_type&gt;(x)</code>;</ins></p></li>
<li><p><ins>(?.?) &mdash; a value <code>x</code> of type convertible to <code>S::value_type</code>, in which case
<code><i>STR</i>(x)</code> is <code>basic_string_view&lt;S::value_type&gt;(&amp;x, 1)</code>.</ins></p></li>
</ol>
<table border="1">
<caption><ins>Table ?? &mdash; <code>sub_match</code> comparisons</ins></caption>
<tr style="text-align:center">
<th><ins>Expression</ins></th>
<th><ins>Return type</ins></th>
<th><ins>Operational<br/>semantics</ins></th>
</tr>
<tr>
<td>
<ins><code>s == t</code></ins>
</td>
<td>
<ins><code>bool</code></ins>
</td>
<td>
<ins><code><i>STR</i>(s).compare(<i>STR</i>(t)) == 0</code></ins>
</td>
</tr>
<tr>
<td>
<ins><code>s != t</code></ins>
</td>
<td>
<ins><code>bool</code></ins>
</td>
<td>
<ins><code><i>STR</i>(s).compare(<i>STR</i>(t)) != 0</code></ins>
</td>
</tr>
<tr>
<td>
<ins><code>s &lt; t</code></ins>
</td>
<td>
<ins><code>bool</code></ins>
</td>
<td>
<ins><code><i>STR</i>(s).compare(<i>STR</i>(t)) &lt; 0</code></ins>
</td>
</tr>
<tr>
<td>
<ins><code>s &gt; t</code></ins>
</td>
<td>
<ins><code>bool</code></ins>
</td>
<td>
<ins><code><i>STR</i>(s).compare(<i>STR</i>(t)) &gt; 0</code></ins>
</td>
</tr>
<tr>
<td>
<ins><code>s &lt;= t</code></ins>
</td>
<td>
<ins><code>bool</code></ins>
</td>
<td>
<ins><code><i>STR</i>(s).compare(<i>STR</i>(t)) &lt;= 0</code></ins>
</td>
</tr>
<tr>
<td>
<ins><code>s &gt;= t</code></ins>
</td>
<td>
<ins><code>bool</code></ins>
</td>
<td>
<ins><code><i>STR</i>(s).compare(<i>STR</i>(t)) &gt;= 0</code></ins>
</td>
</tr>
</table>

</blockquote>
</li>

</ol>
</blockquote>

<p><i>[2024-10-03; Jonathan rebases the wording on the latest WP]</i></p>

<p>The proposed resolution has been implemented and tested in libstdc++.</p>



<p id="res-3126"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/N4988" title=" Working Draft, Programming Languages — C++">N4988</a>.</p>

<ol>
<li>
<p>Change 28.6.3 <a href="https://wg21.link/re.syn">[re.syn]</a>, header <code>&lt;regex&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
  using csub_match = sub_match&lt;const char*&gt;;
  using wcsub_match = sub_match&lt;const wchar_t*&gt;;
  using ssub_match = sub_match&lt;string::const_iterator&gt;;
  using wssub_match = sub_match&lt;wstring::const_iterator&gt;;

  <i>// 28.6.8.3 <a href="https://wg21.link/re.submatch.op">[re.submatch.op]</a>, sub_match non-member operators</i>
  template&lt;class BiIter&gt;
    bool operator==(const sub_match&lt;BiIter&gt;&amp; lhs, const sub_match&lt;BiIter&gt;&amp; rhs);
  template&lt;class BiIter&gt;
    bool operator&lt;=&gt;(const sub_match&lt;BiIter&gt;&amp; lhs, const sub_match&lt;BiIter&gt;&amp; rhs);

  template&lt;class BiIter, class ST, class SA&gt;
    bool operator==(
      const sub_match&lt;BiIter&gt;&amp; lhs,
      const basic_string&lt;typename iterator_traits&lt;BiIter&gt;::value_type, ST, SA&gt;&amp; rhs);
  template&lt;class BiIter, class ST, class SA&gt;
    bool operator&lt;=&gt;(
      const sub_match&lt;BiIter&gt;&amp; lhs,
      const basic_string&lt;typename iterator_traits&lt;BiIter&gt;::value_type, ST, SA&gt;&amp; rhs);
<ins>
  template&lt;class BiIter, class ST&gt;
    bool operator==(
      const sub_match&lt;BiIter&gt;&amp; lhs,
      const basic_string_view&lt;typename iterator_traits&lt;BiIter&gt;::value_type, ST&gt;&amp; rhs);
  template&lt;class BiIter, class ST&gt;
    bool operator&lt;=&gt;(
      const sub_match&lt;BiIter&gt;&amp; lhs,
      const basic_string_view&lt;typename iterator_traits&lt;BiIter&gt;::value_type, ST&gt;&amp; rhs);
</ins>
  template&lt;class BiIter&gt;
    bool operator==(const sub_match&lt;BiIter&gt;&amp; lhs,
                    const typename iterator_traits&lt;BiIter&gt;::value_type* rhs);
  template&lt;class BiIter&gt;
    bool operator&lt;=&gt;(const sub_match&lt;BiIter&gt;&amp; lhs,
                     const typename iterator_traits&lt;BiIter&gt;::value_type* rhs);

  template&lt;class BiIter&gt;
    bool operator==(const sub_match&lt;BiIter&gt;&amp; lhs,
                    const typename iterator_traits&lt;BiIter&gt;::value_type&amp; rhs);
  template&lt;class BiIter&gt;
    bool operator&lt;=&gt;(const sub_match&lt;BiIter&gt;&amp; lhs,
                     const typename iterator_traits&lt;BiIter&gt;::value_type&amp; rhs);

  template&lt;class charT, class ST, class BiIter&gt;
    basic_ostream&lt;charT, ST&gt;&amp;
      operator&lt;&lt;(basic_ostream&lt;charT, ST&gt;&amp; os, const sub_match&lt;BiIter&gt;&amp; m);
</pre>
</blockquote>
</li>

<li>
<p>Change 28.6.8 <a href="https://wg21.link/re.submatch">[re.submatch]</a>, class template <code>sub_match</code> synopsis, as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;class BidirectionalIterator&gt;
  class sub_match : public pair&lt;BidirectionalIterator, BidirectionalIterator&gt; {
  public:
    using value_type =
      typename iterator_traits&lt;BidirectionalIterator&gt;::value_type;
    [&hellip;]
    int compare(const sub_match&amp; s) const;
    int compare(const string_type&amp; s) const;
    int compare(const value_type* s) const;
    <ins>int compare(basic_string_view&lt;value_type&gt; s) const;</ins>
  };
}
</pre>
</blockquote>
</li>

<li>
<p>Change 28.6.8.2 <a href="https://wg21.link/re.submatch.members">[re.submatch.members]</a> as indicated:</p>

<blockquote>
<pre>
int compare(const value_type* s) const;
</pre>
<blockquote>
<p>
-7- <i>Returns:</i> <code>str().compare(s)</code>.
</p>
</blockquote>
<pre>
<ins>int compare(basic_string_view&lt;value_type&gt; s) const;</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Returns:</i> <code>str().compare(s)</code>.</ins>
</p>
</blockquote>
</blockquote>
</li>

<li>
<p>Change sub-clause 28.6.8.3 <a href="https://wg21.link/re.submatch.op">[re.submatch.op]</a> as indicated:</p>

<blockquote>
<p>
<b>32.8.3 <code>sub_match</code> non-member operators [re.submatch.op]</b>
</p>
<p>-1-
Let
<ins>
<code><i>SV</i>(I)</code> be
<code>basic_string_view&lt;typename iterator_traits&lt;I&gt;::value_type&gt;</code>
and let
</ins>
<code><i>SM-CAT</i>(I)</code> be
<code>compare_three_way_result_t&lt;<del>basic_string&lt;typename iterator_traits&lt;I&gt;::value_type&gt;</del><ins><i>SV</i>(I)</ins>&gt;</code>
</p>

<pre>template&lt;class BiIter&gt;
  bool operator==(const sub_match&lt;BiIter&gt;&amp; lhs, const sub_match&lt;BiIter&gt;&amp; rhs);</pre>
<blockquote>
<p>
-2- <i>Returns:</i>
<code>lhs.compare(rhs) == 0</code>.
</p>
</blockquote>

<pre>template&lt;class BiIter&gt;
  bool operator&lt;=&gt;(const sub_match&lt;BiIter&gt;&amp; lhs, const sub_match&lt;BiIter&gt;&amp; rhs);</pre>
<blockquote>
<p>
-3- <i>Returns:</i>
<code>static_cast&lt;<i>SM-CAT</i>(BiIter)&gt;(lhs.compare(rhs) &lt;=&gt; 0)</code>.
</p>
</blockquote>

<pre>template&lt;class BiIter, class ST, class SA&gt;
  bool operator==(
    const sub_match&lt;BiIter&gt;&amp; lhs,
    const basic_string&lt;typename iterator_traits&lt;BiIter&gt;::value_type, ST, SA&gt;&amp; rhs);</pre>
<blockquote>
<p>
-4- <i>Returns:</i>
<code>lhs.compare(<del>typename sub_match&lt;BiIter&gt;::string_type</del><ins><i>SV</i>(BiIter)</ins>(rhs.data(), rhs.size())) == 0</code>.
</p>
</blockquote>

<pre>template&lt;class BiIter, class ST, class SA&gt;
  auto operator&lt;=&gt;(
    const sub_match&lt;BiIter&gt;&amp; lhs,
    const basic_string&lt;typename iterator_traits&lt;BiIter&gt;::value_type, ST, SA&gt;&amp; rhs);</pre>
<blockquote>
<p>
-5- <i>Returns:</i>
<pre><code>static_cast&lt;<i>SM-CAT</i>(BiIter)&gt;(lhs.compare(
    <del>typename sub_match&lt;BiIter&gt;::string_type</del><ins><i>SV</i>(BiIter)</ins>(rhs.data(), rhs.size()))
      &lt;=&gt; 0
    )
</code></pre>
</p>
</blockquote>


<pre><ins>template&lt;class BiIter, class ST&gt;
  bool operator==(
    const sub_match&lt;BiIter&gt;&amp; lhs,
    const basic_string_view&lt;typename iterator_traits&lt;BiIter&gt;::value_type, ST&gt;&amp; rhs);</ins></pre>
<blockquote>
<p>
<ins>
-?- <i>Returns:</i>
<code>lhs.compare(<i>SV</i>(BiIter)(rhs.data(), rhs.size())) == 0</code>.
</ins>
</p>
</blockquote>

<pre><ins>template&lt;class BiIter, class ST&gt;
  auto operator&lt;=&gt;(
    const sub_match&lt;BiIter&gt;&amp; lhs,
    const basic_string_view&lt;typename iterator_traits&lt;BiIter&gt;::value_type, ST&gt;&amp; rhs);</ins></pre>
<blockquote>
<p>
<ins>
-?- <i>Returns:</i>
<code>
static_cast&lt;<i>SM-CAT</i>(BiIter)&gt;(lhs.compare(<i>SV</i>(BiIter)(rhs.data(), rhs.size())) &lt;=&gt; 0)
</code>.
</ins>
</p>
</blockquote>

</blockquote>
</li>
</ol>





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