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<title>Issue 4365: boyer_moore_searcher and boyer_moore_horspool_searcher should be constexpr-friendly</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="4365"><a href="lwg-active.html#4365">4365</a>. <code class='backtick'>boyer_moore_searcher</code> and <code class='backtick'>boyer_moore_horspool_searcher</code> should be <code class='backtick'>constexpr</code>-friendly</h3>
<p><b>Section:</b> 22.10.18.3 <a href="https://wg21.link/func.search.bm">[func.search.bm]</a>, 22.10.18.4 <a href="https://wg21.link/func.search.bmh">[func.search.bmh]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2025-09-03 <b>Last modified:</b> 2025-09-15</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#func.search.bm">active issues</a> in [func.search.bm].</p>
<p><b>View all other</b> <a href="lwg-index.html#func.search.bm">issues</a> in [func.search.bm].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently, <code class='backtick'>boyer_moore_searcher</code> and <code class='backtick'>boyer_moore_horspool_searcher</code> are not
<code class='backtick'>constexpr</code>-friendly because their underlying implementation needs to precompute 
the shift table, which usually requires a <code class='backtick'>vector</code> or <code class='backtick'>unordered_map</code> to store.
<p/>
Thanks to <a href="https://wg21.link/P3372R3" title=" constexpr containers and adapters">P3372R3</a>, unordered containers are now <code class='backtick'>constexpr</code>-friendly. 
Although <code class='backtick'>std::hash</code> still lacks <code class='backtick'>constexpr</code> support, users can provide their own 
hash functions to use unordered containers at compile time.
<p/>
Given that both <code class='backtick'>boyer_moore_searcher</code> and <code class='backtick'>boyer_moore_horspool_searcher</code> can 
take a custom hash, it makes perfect sense that they could be <code class='backtick'>constexpr</code>-friendly.
<p/>
Not to mention that library implementations usually simply use arrays instead of 
hash tables for the common string case because characters only have 256 values, 
so <code class='backtick'>unordered_map</code> is not actually used.
</p>


<p id="res-4365"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5014">N5014</a>.
</p>

<ol>

<li><p>Modify 22.10.18.3 <a href="https://wg21.link/func.search.bm">[func.search.bm]</a> as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std {
  template&lt;class RandomAccessIterator1,
           class Hash = hash&lt;typename iterator_traits&lt;RandomAccessIterator1&gt;::value_type&gt;,
           class BinaryPredicate = equal_to&lt;&gt;&gt;
  class boyer_moore_searcher {
  public:
    <ins>constexpr</ins> boyer_moore_searcher(RandomAccessIterator1 pat_first,
                                   RandomAccessIterator1 pat_last,
                                   Hash hf = Hash(),
                                   BinaryPredicate pred = BinaryPredicate());

    template&lt;class RandomAccessIterator2&gt;
      <ins>constexpr</ins> pair&lt;RandomAccessIterator2, RandomAccessIterator2&gt;
        operator()(RandomAccessIterator2 first, RandomAccessIterator2 last) const;

  private:
    RandomAccessIterator1 pat_first_;   // <i>exposition only</i>
    RandomAccessIterator1 pat_last_;    // <i>exposition only</i>
    Hash hash_;                         // <i>exposition only</i>
    BinaryPredicate pred_;              // <i>exposition only</i>
  };
}
</pre>
</blockquote>
<pre>
<ins>constexpr</ins> boyer_moore_searcher(RandomAccessIterator1 pat_first,
                               RandomAccessIterator1 pat_last,
                               Hash hf = Hash(),
                               BinaryPredicate pred = BinaryPredicate());
</pre>
<blockquote>
<p>
-1- <i>Preconditions</i>: The value type of <code>RandomAccessIterator1</code> meets the
   <i>Cpp17DefaultConstructible</i>, <i>Cpp17CopyConstructible</i>, and <i>Cpp17CopyAssignable</i>
   requirements.
</p>
</blockquote>
<blockquote>
[&hellip;]
</blockquote>
<pre>
template&lt;class RandomAccessIterator2&gt;
  <ins>constexpr</ins> pair&lt;RandomAccessIterator2, RandomAccessIterator2&gt;
    operator()(RandomAccessIterator2 first, RandomAccessIterator2 last) const;
</pre>
<blockquote>
<p>
-5- <i>Mandates</i>: <code>RandomAccessIterator1</code> and <code>RandomAccessIterator2</code>
have the same value type.
</p>
</blockquote>
</blockquote>

</li>

<li><p>Modify 22.10.18.4 <a href="https://wg21.link/func.search.bmh">[func.search.bmh]</a> as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std {
  template&lt;class RandomAccessIterator1,
           class Hash = hash&lt;typename iterator_traits&lt;RandomAccessIterator1&gt;::value_type&gt;,
           class BinaryPredicate = equal_to&lt;&gt;&gt;
  class boyer_moore_horspool_searcher {
  public:
   <ins>constexpr</ins> boyer_moore_horspool_searcher(RandomAccessIterator1 pat_first,
                                           RandomAccessIterator1 pat_last,
                                           Hash hf = Hash(),
                                           BinaryPredicate pred = BinaryPredicate());

    template&lt;class RandomAccessIterator2&gt;
      <ins>constexpr</ins> pair&lt;RandomAccessIterator2, RandomAccessIterator2&gt;
        operator()(RandomAccessIterator2 first, RandomAccessIterator2 last) const;

  private:
    RandomAccessIterator1 pat_first_;   // <i>exposition only</i>
    RandomAccessIterator1 pat_last_;    // <i>exposition only</i>
    Hash hash_;                         // <i>exposition only</i>
    BinaryPredicate pred_;              // <i>exposition only</i>
  };
}
</pre>
</blockquote>
<pre>
<ins>constexpr</ins> boyer_moore_horspool_searcher(RandomAccessIterator1 pat_first,
                                        RandomAccessIterator1 pat_last,
                                        Hash hf = Hash(),
                                        BinaryPredicate pred = BinaryPredicate());
</pre>
<blockquote>
<p>
-1- <i>Preconditions</i>: The value type of <code>RandomAccessIterator1</code> meets the
<i>Cpp17DefaultConstructible</i>, <i>Cpp17CopyConstructible</i>, and <i>Cpp17CopyAssignable</i>
requirements.
</p>
</blockquote>
<blockquote>
[&hellip;]
</blockquote>
<pre>
template&lt;class RandomAccessIterator2&gt;
  <ins>constexpr</ins> pair&lt;RandomAccessIterator2, RandomAccessIterator2&gt;
    operator()(RandomAccessIterator2 first, RandomAccessIterator2 last) const;
</pre>
<blockquote>
<p>
-5- <i>Mandates</i>: <code>RandomAccessIterator1</code> and <code>RandomAccessIterator2</code>
have the same value type.
</p>
</blockquote>
</blockquote>

</li>

</ol>




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