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<title>Issue 3612: Inconsistent pointer alignment in std::format</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#C++23">C++23</a> status.</em></p>
<h3 id="3612"><a href="lwg-defects.html#3612">3612</a>. Inconsistent pointer alignment in <code>std::format</code></h3>
<p><b>Section:</b> 28.5.2.2 <a href="https://wg21.link/format.string.std">[format.string.std]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Victor Zverovich <b>Opened:</b> 2021-10-02 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#format.string.std">active issues</a> in [format.string.std].</p>
<p><b>View all other</b> <a href="lwg-index.html#format.string.std">issues</a> in [format.string.std].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
According to [tab:format.type.ptr] pointers are formatted as hexadecimal integers (at least in the 
common case when <code>uintptr_t</code> is available). However, it appears that they have left alignment 
by default according to [tab:format.align]:
</p>
<blockquote><p>
Forces the field to be aligned to the start of the available space. This is the default for non-arithmetic 
types, <code>charT</code>, and <code>bool</code>, unless an integer presentation type is specified.
</p></blockquote>
<p>
because pointers are not arithmetic types.
<p/>
For example:
</p>
<blockquote><pre>
void* p = &hellip;
std::format("{:#16x}", std::bit_cast&lt;uintptr_t&gt;(p));
std::format("{:16}", p);
</pre></blockquote>
<p>
may produce <code>"  0x7fff88716c84"</code> and <code>"0x7fff88716c84  "</code> (the actual output depends 
on the value of <code>p</code>).
<p/>
This is inconsistent and clearly a bug in specification that should have included pointers together 
with arithmetic types in [tab:format.align].
</p>

<p><i>[2021-10-14; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after seven votes in favour during reflector poll.
</p>

<p><i>[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3612"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4892">N4892</a>.
</p>

<ol>
<li><p>Modify 28.5.2.2 <a href="https://wg21.link/format.string.std">[format.string.std]</a>, Table [tab:format.align], as indicated:</p>

<blockquote>
<table border="1">
<caption>Table 59 &mdash; Meaning of <i>align</i> options  [tab:format.align]</caption>
<tr style="text-align:center">
<th>Option</th>
<th>Meaning</th>
</tr>
<tr>
<td><code>&lt;</code></td>
<td>Forces the field to be aligned to the start of the available space. This is the default for
non-arithmetic <ins>non-pointer</ins> types, <code>charT</code>, and <code>bool</code>, unless an integer 
presentation type is specified.</td>
</tr>
<tr>
<td><code>&gt;</code></td>
<td>Forces the field to be aligned to the end of the available space. This is the default for
arithmetic types other than <code>charT</code> and <code>bool</code><ins>, pointer types</ins> or when an 
integer presentation type is specified.</td>
</tr>
<tr>
<td colspan="2" align="center">
<code>[&hellip;]</code>
</td>
</tr>
</table>
</blockquote>

<blockquote class="note">
<p>
[<i>Drafting note:</i> The wording above touches a similar area as LWG <a href="lwg-active.html#3586" title="Formatting character alignment inconsistencies (Status: New)">3586</a><sup><a href="https://cplusplus.github.io/LWG/issue3586" title="Latest snapshot">(i)</a></sup>. To help solving the
merge conflict the following shows the delta of this proposed wording on top of the LWG <a href="lwg-active.html#3586" title="Formatting character alignment inconsistencies (Status: New)">3586</a><sup><a href="https://cplusplus.github.io/LWG/issue3586" title="Latest snapshot">(i)</a></sup>
merge result]
</p>
</blockquote>

<blockquote>
<table border="1">
<caption>Table 59 &mdash; Meaning of <i>align</i> options  [tab:format.align]</caption>
<tr style="text-align:center">
<th>Option</th>
<th>Meaning</th>
</tr>
<tr>
<td><code>&lt;</code></td>
<td>Forces the field to be aligned to the start of the available space. This is the default when 
the presentation type is a non-arithmetic <ins>non-pointer</ins> type.</td>
</tr>
<tr>
<td><code>&gt;</code></td>
<td>Forces the field to be aligned to the end of the available space. This is the default when 
the presentation type is an arithmetic <ins>or pointer</ins> type.</td>
</tr>
<tr>
<td colspan="2" align="center">
<code>[&hellip;]</code>
</td>
</tr>
</table>
</blockquote>

</li>
</ol>





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