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‎issue4221.html‎

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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="utf-8">
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<title>Issue 4221: Cannot format const-iterable only ranges</title>
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<meta property="og:title" content="Issue 4221: Cannot format const-iterable only ranges">
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<meta property="og:description" content="C++ library issue. Status: New">
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<meta property="og:url" content="https://cplusplus.github.io/LWG/issue4221.html">
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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>
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<h3 id="4221"><a href="lwg-active.html#4221">4221</a>. Cannot format const-iterable only ranges</h3>
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<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>
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<b>Submitter:</b> Hewill Kang <b>Opened:</b> 2025-03-06 <b>Last modified:</b> 2025-03-09</p>
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<p><b>Priority: </b>Not Prioritized
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</p>
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<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
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<p><b>Discussion:</b></p>
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<p>
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The standard does not explicitly prohibit ranges that are only const-iterable, i.e. a range with
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<code>const begin()</code> and deleted or invalid non-<code>const begin()</code>.
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</p>
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<p>
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Unfortunately, those ranges cannot be formatted because the <code>R</code> in
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<code>formatter&lt;R&gt;</code> is always without the <code>const</code>-qualifier,
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which makes it never satisfy the <code>range</code> concept
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(<a href="https://godbolt.org/z/68T6EjKG4">demo</a>):
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</p>
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<blockquote><pre>
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#include &lt;print&gt;
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#include &lt;ranges&gt;
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struct R {
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int* begin() = delete;
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int* end() = delete;
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const int* begin() const;
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const int* end() const;
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};
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int main() {
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const R r;
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static_assert(std::ranges::contiguous_range&lt;decltype(r)&gt;);
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for (auto&amp;&amp; elem : r)
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std::print("{} ", elem); // ok
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std::ranges::for_each(
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r, [](auto&amp;&amp; elem) { std::print("{} ", elem); }
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); // ok
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std::print("{}", r); // <span style="color:red;font-weight:bolder">not ok</span>
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}
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</pre></blockquote>
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<p>
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Although such type might be relatively rare, it does reflect an inconsistency in the general usage of formatting
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ranges, which do not support all valid ranges.
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</p>
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<p id="res-4221"><b>Proposed resolution:</b></p>
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</body>
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</html>

‎lwg-active.html‎

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‎lwg-closed.html‎

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‎lwg-defects.html‎

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‎lwg-immediate.html‎

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<td align="left">Date:</td>
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<td align="left">Revised 2025-03-09 at 10:46:46 UTC
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<td align="left">Revised 2025-03-09 at 10:56:36 UTC
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<h2>Index by Section (non-Ready active issues only)</h2>
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<p><a href="lwg-index.html">(view all issues)</a></p>
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<p>Revised 2025-03-09 at 10:46:46 UTC
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<p>Revised 2025-03-09 at 10:56:36 UTC
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</p><h2 id="Section_3">Section 3 (2 issues)</h2>
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<td id="4221"><a href="lwg-active.html#4221" title="Cannot format const-iterable only ranges (Status: New)">4221</a><sup><a href="https://cplusplus.github.io/LWG/issue4221">(i)</a></sup></td>
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<td><a href="lwg-active.html#New">New</a></td>
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<td>28.5.7 [format.range]<a id="format.range"></a></td>
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<td>Cannot format const-iterable only ranges</td>
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<td><span class="no-pr">No</span></td>
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<td></td>
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<td id="4107"><a href="lwg-active.html#4107" title="Map formatter may conflict with user-defined specializations of pair/tuple formatters (Status: New)">4107</a><sup><a href="https://cplusplus.github.io/LWG/issue4107">(i)</a></sup></td>
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<td><a href="lwg-active.html#New">New</a></td>
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<td>28.5.7.4 [format.range.fmtmap]<a id="format.range.fmtmap"></a></td>

‎lwg-index.html‎

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<p>This document is the Index by Section for the <a href="lwg-active.html">Library Active Issues List</a>, <a href="lwg-defects.html">Library Defect Reports and Accepted Issues</a>, and <a href="lwg-closed.html">Library Closed Issues List</a>.</p>
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<h2>Index by Section</h2>
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<p>Revised 2025-03-09 at 10:46:46 UTC
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<p>Revised 2025-03-09 at 10:56:36 UTC
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<td id="4221"><a href="lwg-active.html#4221" title="Cannot format const-iterable only ranges (Status: New)">4221</a><sup><a href="https://cplusplus.github.io/LWG/issue4221">(i)</a></sup></td>
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<td><a href="lwg-active.html#New">New</a></td>
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<td>28.5.7 [format.range]<a id="format.range"></a></td>
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<td>Cannot format const-iterable only ranges</td>
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<td><span class="no-pr">No</span></td>
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<td id="3892"><a href="lwg-defects.html#3892" title="Incorrect formatting of nested ranges and tuples (Status: WP)">3892</a><sup><a href="https://cplusplus.github.io/LWG/issue3892">(i)</a></sup></td>
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<td><a href="lwg-active.html#WP">WP</a></td>
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<td>28.5.7.2 [format.range.formatter]<a id="format.range.formatter"></a></td>

‎lwg-ready.html‎

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<td align="left">Date:</td>
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<td align="left">Revised 2025-03-09 at 10:46:46 UTC
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This document is the Index by Status and Date for the <a href="lwg-active.html">Library Active Issues List</a>,
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<a href="lwg-defects.html">Library Defect Reports and Accepted Issues</a>, and <a href="lwg-closed.html">Library Closed Issues List</a>.
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</p>
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<p>Revised 2025-03-09 at 10:46:46 UTC
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<p>Revised 2025-03-09 at 10:56:36 UTC
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</p><h2 id="Ready">Ready (7 issues)</h2>
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<td id="4221"><a href="lwg-active.html#4221" title="Cannot format const-iterable only ranges (Status: New)">4221</a><sup><a href="https://cplusplus.github.io/LWG/issue4221">(i)</a></sup></td>
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<td><a href="lwg-active.html#New">New</a></td>
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<td>28.5.7 [format.range]</td>
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<td>Cannot format const-iterable only ranges</td>
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<td><span class="no-pr">No</span></td>
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<td></td>
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<td></td>
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<td id="4216"><a href="lwg-active.html#4216" title="num_put::do_put and void pointers (Status: New)">4216</a><sup><a href="https://cplusplus.github.io/LWG/issue4216">(i)</a></sup></td>
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‎lwg-status.html‎

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<a href="lwg-defects.html">Library Defect Reports and Accepted Issues</a>, and <a href="lwg-closed.html">Library Closed Issues List</a>.
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</p><h2 id="Ready">Ready (7 issues)</h2>
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<td id="4221"><a href="lwg-active.html#4221" title="Cannot format const-iterable only ranges (Status: New)">4221</a><sup><a href="https://cplusplus.github.io/LWG/issue4221">(i)</a></sup></td>
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<td>28.5.7 [format.range]</td>
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<td>Cannot format const-iterable only ranges</td>
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<td><span class="no-pr">No</span></td>
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<td id="4107"><a href="lwg-active.html#4107" title="Map formatter may conflict with user-defined specializations of pair/tuple formatters (Status: New)">4107</a><sup><a href="https://cplusplus.github.io/LWG/issue4107">(i)</a></sup></td>
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<td>28.5.7.4 [format.range.fmtmap]</td>

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