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<title>Issue 4106: basic_format_args should not be default-constructible</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#WP">WP</a> status.</em></p>
<h3 id="4106"><a href="lwg-defects.html#4106">4106</a>. <code>basic_format_args</code> should not be default-constructible</h3>
<p><b>Section:</b> 28.5.8.3 <a href="https://wg21.link/format.args">[format.args]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2024-05-17 <b>Last modified:</b> 2024-07-08</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#format.args">issues</a> in [format.args].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
It's unclear why <code>basic_format_args</code> originally provided a default constructor and its actual use cases,
all three major libraries declare them as <code>default</code>, which allows value initialization of 
<code>basic_format_args</code> at compile time.
<p/>
This does not fully conform to the current wording as its signature does not have the <code>constexpr</code>
specifier.
<p/>
Additionally, the current wording only initializes the <code>size_</code> member in the default constructor, which
may lead to undefined behavior when copying <code>basic_format_args</code> as uninitialized <code>data_</code> member 
is copied. There is also an implementation divergence (<a href="https://godbolt.org/z/nYEje7zzj">demo</a>):
</p>
<blockquote><pre>
#include &lt;format&gt;
constexpr std::format_args fmt_args; // <span style="color:red;font-weight:bolder">only well-formed in MSVC-STL</span>
</pre></blockquote>
<p>
One option is to add default member initializers for all members and <code>default</code> the default constructor 
to best match the status quo, which guarantees that <code>basic_format_args</code> is <code>constexpr</code>-able.
<p/>
However, given that <code>basic_format_args</code> has different implementation details in the three libraries, 
its actual members may not be <code>size_</code> and <code>data_</code>. It is unnecessary to ensure that all
the internal members are initialized when default-constructed <code>basic_format_args</code>, indicating that not
providing one is reasonable.
<p/>
The proposed solution is to prefer the more aggressive one.
</p>

<p><i>[2024-05-19; Daniel comments]</i></p>

<p>
The here suggested proposal to remove the default constructor implicitly depends on the decision of
LWG <a href="lwg-defects.html#4061" title="Should std::basic_format_context be default-constructible/copyable/movable? (Status: WP)">4061</a><sup><a href="https://cplusplus.github.io/LWG/issue4061" title="Latest snapshot">(i)</a></sup> to remove <code>basic_format_context</code>'s default constructor, since its usage
would require that the exposition-only member <code>args_</code> of type <code>basic_format_args&lt;basic_format_context&gt;</code> 
can be default-constructed as well.
</p>

<p><i>[2024-06-24; Reflector poll]</i></p>

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

<p><i>[St. Louis 2024-06-29; Status changed: Voting &rarr; WP.]</i></p>



<p id="res-4106"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4981" title=" Working Draft, Programming Languages — C++">N4981</a>.
</p>

<ol>

<li><p>Modify 28.5.8.3 <a href="https://wg21.link/format.args">[format.args]</a> as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std {
  template&lt;class Context&gt;
  class basic_format_args {
    size_t size_;                               // <i>exposition only</i>
    const basic_format_arg&lt;Context&gt;* data_;     // <i>exposition only</i>

  public:
    <del>basic_format_args() noexcept;</del>

    template&lt;class... Args&gt;
      basic_format_args(const <i>format-arg-store</i>&lt;Context, Args...&gt;&amp; store) noexcept;

    basic_format_arg&lt;Context&gt; get(size_t i) const noexcept;
  };
  [&hellip;]
}
</pre>
</blockquote>
[&hellip;]
<pre>
<del>basic_format_args() noexcept;</del>
</pre>
<blockquote>
<p>
<del>-2- <i>Effects</i> Initializes <code>size_</code> with <code>0</code>.</del>
</p>
</blockquote>
</blockquote>

</li>
</ol>





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