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<title>Issue 3839: range_formatter's set_separator, set_brackets, and underlying functions should be noexcept</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="3839"><a href="lwg-defects.html#3839">3839</a>. <code>range_formatter</code>'s <code>set_separator</code>, <code>set_brackets</code>, and <code>underlying</code> functions should be <code>noexcept</code></h3>
<p><b>Section:</b> 28.5.7.2 <a href="https://wg21.link/format.range.formatter">[format.range.formatter]</a>, 28.5.7.3 <a href="https://wg21.link/format.range.fmtdef">[format.range.fmtdef]</a>, 28.5.9 <a href="https://wg21.link/format.tuple">[format.tuple]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-12-11 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#format.range.formatter">issues</a> in [format.range.formatter].</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>
The <code>set_separator</code> and <code>set_brackets</code> of <code>range_formatter</code> only invoke 
<code>basic_string_view</code>'s assignment operator, which is <code>noexcept</code>, we should add <code>noexcept</code> 
specifications for them.
<p/>
In addition, its <code>underlying</code> function returns a reference to the underlying <code>formatter</code>, 
which never throws, they should also be <code>noexcept</code>.
<p/>
Similar rules apply to <code><i>range-default-formatter</i></code> and <code>formatter</code>'s <code>tuple</code> specialization.
</p>

<p><i>[2023-01-06; Reflector poll]</i></p>

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

<p><i>[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3839"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</a>.
</p>

<ol>
<li><p>Modify 28.5.7.2 <a href="https://wg21.link/format.range.formatter">[format.range.formatter]</a> as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std {
  template&lt;class T, class charT = char&gt;
    requires same_as&lt;remove_cvref_t&lt;T&gt;, T&gt; &amp;&amp; formattable&lt;T, charT&gt;
  class range_formatter {
    formatter&lt;T, charT&gt; <i>underlying_</i>;                                          // <i>exposition only</i>
    basic_string_view&lt;charT> <i>separator_</i> = <i>STATICALLY-WIDEN</i>&lt;charT&gt;(", ");      // <i>exposition only</i>
    basic_string_view&lt;charT> <i>opening-bracket_</i> = <i>STATICALLY-WIDEN</i>&lt;charT&gt;("["); // <i>exposition only</i>
    basic_string_view&lt;charT> <i>closing-bracket_</i> = <i>STATICALLY-WIDEN</i>&lt;charT&gt;("]"); // <i>exposition only</i>

  public:
    constexpr void set_separator(basic_string_view&lt;charT&gt; sep) <ins>noexcept</ins>;
    constexpr void set_brackets(basic_string_view&lt;charT&gt; opening,
                                basic_string_view&lt;charT&gt; closing) <ins>noexcept</ins>;
    constexpr formatter&lt;T, charT&gt;&amp; underlying() <ins>noexcept</ins> { return <i>underlying_</i>; }
    constexpr const formatter&lt;T, charT&gt;&amp; underlying() const <ins>noexcept</ins> { return <i>underlying_</i>; }

    [&hellip;]
  };
}
</pre>
</blockquote>
[&hellip;]
<pre>
constexpr void set_separator(basic_string_view&lt;charT&gt; sep) <ins>noexcept</ins>;
</pre>
<blockquote>
<p>
-7- <i>Effects:</i> Equivalent to: <code><i>separator_</i> = sep;</code>
</p>
</blockquote>
<pre>
constexpr void set_brackets(basic_string_view&lt;charT&gt; opening, basic_string_view&lt;charT&gt; closing) <ins>noexcept</ins>;
</pre>
<blockquote>
<p>
-8- <i>Effects:</i> Equivalent to:
<pre>
<i>opening-bracket_</i> = opening;
<i>closing-bracket_</i> = closing;
</pre>
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 28.5.7.3 <a href="https://wg21.link/format.range.fmtdef">[format.range.fmtdef]</a> as indicated:</p>
  
<blockquote>
<blockquote>
<pre>
namespace std {
  template&lt;ranges::input_range R, class charT&gt;
  struct <i>range-default-formatter</i>&lt;range_format::sequence, R, charT&gt; {    // <i>exposition only</i>
  private:
    using <i>maybe-const-r</i> = <i>fmt-maybe-const</i>&lt;R, charT&gt;;                    // <i>exposition only</i>
    range_formatter&lt;remove_cvref_t&lt;ranges::range_reference_t&lt;<i>maybe-const-r</i>&gt;&gt;,
                    charT&gt; <i>underlying_</i>;                                 // <i>exposition only</i>

  public:
    constexpr void set_separator(basic_string_view&lt;charT&gt; sep) <ins>noexcept</ins>;
    constexpr void set_brackets(basic_string_view&lt;charT&gt; opening,
                                basic_string_view&lt;charT&gt; closing) <ins>noexcept</ins>;
  
    [&hellip;]
  };
}
</pre>
</blockquote>
<pre>
constexpr void set_separator(basic_string_view&lt;charT&gt; sep) <ins>noexcept</ins>;
</pre>
<blockquote>
<p>
-1- <i>Effects:</i> Equivalent to: <code><i>underlying_</i>.set_separator(sep);</code>.
</p>
</blockquote>
<pre>
constexpr void set_brackets(basic_string_view&lt;charT&gt; opening, basic_string_view&lt;charT&gt; closing) <ins>noexcept</ins>;
</pre>
<blockquote>
<p>
-2- <i>Effects:</i> Equivalent to: <code><i>underlying_</i>.set_brackets(opening, closing);</code>.
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 28.5.9 <a href="https://wg21.link/format.tuple">[format.tuple]</a> as indicated:</p>
<blockquote>
<blockquote>
<pre>
namespace std {
  template&lt;class charT, formattable&lt;charT&gt;... Ts&gt;
  struct formatter&lt;<i>pair-or-tuple</i>&lt;Ts...&gt;, charT&gt; {
  private:
    tuple&lt;formatter&lt;remove_cvref_t&lt;Ts&gt;, charT&gt;...&gt; <i>underlying_</i>;               // <i>exposition only</i>
    basic_string_view&lt;charT> <i>separator_</i> = <i>STATICALLY-WIDEN</i>&lt;charT&gt;(", ");      // <i>exposition only</i>
    basic_string_view&lt;charT> <i>opening-bracket_</i> = <i>STATICALLY-WIDEN</i>&lt;charT&gt;("("); // <i>exposition only</i>
    basic_string_view&lt;charT> <i>closing-bracket_</i> = <i>STATICALLY-WIDEN</i>&lt;charT&gt;(")"); // <i>exposition only</i>

  public:
    constexpr void set_separator(basic_string_view&lt;charT&gt; sep) <ins>noexcept</ins>;
    constexpr void set_brackets(basic_string_view&lt;charT&gt; opening,
                                basic_string_view&lt;charT&gt; closing) <ins>noexcept</ins>;

    [&hellip;]
  };
}
</pre>
</blockquote>
[&hellip;]
<pre>
constexpr void set_separator(basic_string_view&lt;charT&gt; sep) <ins>noexcept</ins>;
</pre>
<blockquote>
<p>
-5- <i>Effects:</i> Equivalent to: <code><i>separator_</i> = sep;</code>
</p>
</blockquote>
<pre>
constexpr void set_brackets(basic_string_view&lt;charT&gt; opening, basic_string_view&lt;charT&gt; closing) <ins>noexcept</ins>;
</pre>
<blockquote>
<p>
-6- <i>Effects:</i> Equivalent to:
<pre>
<i>opening-bracket_</i> = opening;
<i>closing-bracket_</i> = closing;
</pre>
</p>
</blockquote>
</blockquote>
</li>
</ol>






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