<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 4221: Cannot format const-iterable only ranges</title>
<meta property="og:title" content="Issue 4221: Cannot format const-iterable only ranges">
<meta property="og:description" content="C++ library issue. Status: New">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue4221.html">
<meta property="og:type" content="website">
<meta property="og:image" content="http://cplusplus.github.io/LWG/images/cpp_logo.png">
<meta property="og:image:alt" content="C++ logo">
<style>
  p {text-align:justify}
  li {text-align:justify}
  pre code.backtick::before { content: "`" }
  pre code.backtick::after { content: "`" }
  blockquote.note
  {
    background-color:#E0E0E0;
    padding-left: 15px;
    padding-right: 15px;
    padding-top: 1px;
    padding-bottom: 1px;
  }
  ins {background-color:#A0FFA0}
  del {background-color:#FFA0A0}
  table.issues-index { border: 1px solid; border-collapse: collapse; }
  table.issues-index th { text-align: center; padding: 4px; border: 1px solid; }
  table.issues-index td { padding: 4px; border: 1px solid; }
  table.issues-index td:nth-child(1) { text-align: right; }
  table.issues-index td:nth-child(2) { text-align: left; }
  table.issues-index td:nth-child(3) { text-align: left; }
  table.issues-index td:nth-child(4) { text-align: left; }
  table.issues-index td:nth-child(5) { text-align: center; }
  table.issues-index td:nth-child(6) { text-align: center; }
  table.issues-index td:nth-child(7) { text-align: left; }
  table.issues-index td:nth-child(5) span.no-pr { color: red; }
  @media (prefers-color-scheme: dark) {
     html {
        color: #ddd;
        background-color: black;
     }
     ins {
        background-color: #225522
     }
     del {
        background-color: #662222
     }
     a {
        color: #6af
     }
     a:visited {
        color: #6af
     }
     blockquote.note
     {
        background-color: rgba(255, 255, 255, .10)
     }
  }
</style>
</head>
<body>
<hr>
<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="4221"><a href="lwg-active.html#4221">4221</a>. Cannot format const-iterable only ranges</h3>
<p><b>Section:</b> 28.5.7 <a href="https://wg21.link/format.range">[format.range]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2025-03-06 <b>Last modified:</b> 2025-03-09</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The standard does not explicitly prohibit ranges that are only const-iterable, i.e. a range with
<code>const begin()</code> and deleted or invalid non-<code>const begin()</code>.
</p>
<p>
Unfortunately, those ranges cannot be formatted because the <code>R</code> in
<code>formatter&lt;R&gt;</code> is always without the <code>const</code>-qualifier,
which makes it never satisfy the <code>range</code> concept
(<a href="https://godbolt.org/z/68T6EjKG4">demo</a>):
</p>
<blockquote><pre>
#include &lt;print&gt;
#include &lt;ranges&gt;

struct R {
  int* begin() = delete;
  int* end() = delete;
  const int* begin() const;
  const int* end() const;
};

int main() {
  const R r;
  static_assert(std::ranges::contiguous_range&lt;decltype(r)&gt;);

  for (auto&amp;&amp; elem : r)
    std::print("{} ", elem); // ok

  std::ranges::for_each(
    r, [](auto&amp;&amp; elem) { std::print("{} ", elem); }
  );                         // ok

  std::print("{}", r);       // <span style="color:red;font-weight:bolder">not ok</span>
}
</pre></blockquote>
<p>
Although such type might be relatively rare, it does reflect an inconsistency in the general usage of formatting
ranges, which do not support all valid ranges.
</p>


<p id="res-4221"><b>Proposed resolution:</b></p>





</body>
</html>
