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<title>Issue 3662: basic_string::append/assign(NTBS, pos, n) suboptimal</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#New">New</a> status.</em></p>
<h3 id="3662"><a href="lwg-active.html#3662">3662</a>. <code>basic_string::append/assign(NTBS, pos, n)</code> suboptimal</h3>
<p><b>Section:</b> 27.4.3.7.2 <a href="https://wg21.link/string.append">[string.append]</a>, 27.4.3.7.3 <a href="https://wg21.link/string.assign">[string.assign]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2022-01-21 <b>Last modified:</b> 2022-01-30</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#string.append">issues</a> in [string.append].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
This will allocate temporary string, then copy one byte from it:
</p>
<blockquote>
<pre>
std::string s;
s.append("a very very long string that doesn't fit in SSO buffer", 13, 1);
</pre>
</blockquote>
<p>
The problem is that there is no overload accepting an NTBS there, so it converts to a temporary string using:
</p>
<blockquote><pre>
append(const basic_string&amp;, size_t, size_t = npos);
</pre></blockquote>
<p>
C++17 added a new overload for a <code>string_view</code>:
</p>
<blockquote><pre>
append(const T&amp;, size_t, size_t = npos);
</pre></blockquote>
<p>
But that overload is constrained to not allow <code>const char*</code> arguments, so we still create a temporary string.
<p/>
If we're going to accept those arguments, we should do so efficiently, without forcing an unnecessary allocation.
<p/>
We could do better if we split the <code>append(const T&amp;, ...)</code> overload into:
</p>
<blockquote><pre>
append(const T&amp;, size_t);
append(const T&amp;, size_t, size_t);
</pre></blockquote>
<p>
and then remove the <code>!is_convertible_v&lt;const T&amp;, const charT*&gt;</code> constraint from the second 
overload (<b>Option A</b> in the proposed resolution).
<p/>
Alternatively, we can leave the <code>append(const T&amp;, size_t, size_t)</code> overload alone and just add one 
taking exactly the arguments used above. That seems simpler to me, and I prefer that (<b>Option B</b> in the Proposed Resolution).
<p/>
The same problem applies to <code>assign("...", pos, n)</code>.
</p>

<p><i>[2022-01-30; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>



<p id="res-3662"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4901" title=" Working Draft, Standard for Programming Language C++">N4901</a>. 
</p>

<blockquote class="note">
<p>
[<i>Drafting Note:</i> Two mutually exclusive options are prepared, depicted below by <b>Option A</b> and 
<b>Option B</b>, respectively.] 
</p>
</blockquote>

<p>
<b>Option A:</b>
</p>

<ol>
<li><p>Modify 27.4.3.1 <a href="https://wg21.link/basic.string.general">[basic.string.general]</a>, class template <code>basic_string</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<i>// 27.4.3.7 <a href="https://wg21.link/string.modifiers">[string.modifiers]</a>, modifiers</i>
[&hellip;]
template&lt;class T&gt;
  constexpr basic_string&amp; append(const T&amp; t);
<ins>template&lt;class T&gt;
  constexpr basic_string&amp; append(const T&amp; t, size_type pos);</ins>
template&lt;class T&gt;
  constexpr basic_string&amp; append(const T&amp; t, size_type pos, size_type n <del>= npos</del>);
constexpr basic_string&amp; append(const charT* s, size_type n);
[&hellip;]
template&lt;class T&gt;
  constexpr basic_string&amp; assign(const T&amp; t);
<ins>template&lt;class T&gt;
  constexpr basic_string&amp; assign(const T&amp; t, size_type pos);</ins>
template&lt;class T&gt;
  constexpr basic_string&amp; assign(const T&amp; t, size_type pos, size_type n <del>= npos</del>);
constexpr basic_string&amp; assign(const charT* s, size_type n);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 27.4.3.7.2 <a href="https://wg21.link/string.append">[string.append]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class T&gt;
  constexpr basic_string&amp; append(const T&amp; t);
</pre>
<blockquote>
<p>
-3- <i>Constraints:</i>
</p>
<ol style="list-style-type:none">
<li><p>(3.1) &mdash; <code>is_convertible_v&lt;const T&amp;, basic_string_view&lt;charT, traits&gt;&gt;</code> 
is <code>true</code> and</p></li>
<li><p>(3.2) &mdash; <code>is_convertible_v&lt;const T&amp;, const charT*&gt;</code> is <code>false</code>.</p></li>
</ol>
<p>
-4- <i>Effects:</i> Equivalent to:
</p>
<blockquote><pre>
basic_string_view&lt;charT, traits&gt; sv = t;
return append(sv.data(), sv.size());
</pre></blockquote>
</blockquote>
<pre>
<ins>template&lt;class T&gt;
  constexpr basic_string&amp; append(const T&amp; t, size_type pos);</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Constraints:</i></ins>
</p>
<ol style="list-style-type:none">
<li><p><ins>(?.1) &mdash; <code>is_convertible_v&lt;const T&amp;, basic_string_view&lt;charT, traits&gt;&gt;</code> 
is <code>true</code> and</ins></p></li>
<li><p><ins>(?.2) &mdash; <code>is_convertible_v&lt;const T&amp;, const charT*&gt;</code> is <code>false</code>.</ins></p></li>
</ol>
<p>
<ins>-?- <i>Effects:</i> Equivalent to:</ins>
</p>
<blockquote><pre>
<ins>basic_string_view&lt;charT, traits&gt; sv = t;
return append(sv.substr(pos));</ins>
</pre></blockquote>
</blockquote>
<pre>
template&lt;class T&gt;
  constexpr basic_string&amp; append(const T&amp; t, size_type pos, size_type n <del>= npos</del>);
</pre>
<blockquote>
<p>
-5- <i>Constraints:</i>
</p>
<ol style="list-style-type:none">
<li><p><del>(5.1) &mdash;</del> <code>is_convertible_v&lt;const T&amp;, basic_string_view&lt;charT, traits&gt;&gt;</code> 
is <code>true</code> <del>and</del></p></li>
<li><p><del>(5.2) &mdash; <code>is_convertible_v&lt;const T&amp;, const charT*&gt;</code> is <code>false</code></del>.</p></li>
</ol>
<p>
-6- <i>Effects:</i> Equivalent to:
</p>
<blockquote><pre>
basic_string_view&lt;charT, traits&gt; sv = t;
return append(sv.substr(pos, n));
</pre></blockquote>
</blockquote>
</blockquote>
</li>

<li><p>Modify 27.4.3.7.3 <a href="https://wg21.link/string.assign">[string.assign]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class T&gt;
  constexpr basic_string&amp; assign(const T&amp; t);
</pre>
<blockquote>
<p>
-4- <i>Constraints:</i>
</p>
<ol style="list-style-type:none">
<li><p>(4.1) &mdash; <code>is_convertible_v&lt;const T&amp;, basic_string_view&lt;charT, traits&gt;&gt;</code> 
is <code>true</code> and</p></li>
<li><p>(4.2) &mdash; <code>is_convertible_v&lt;const T&amp;, const charT*&gt;</code> is <code>false</code>.</p></li>
</ol>
<p>
-5- <i>Effects:</i> Equivalent to:
</p>
<blockquote><pre>
basic_string_view&lt;charT, traits&gt; sv = t;
return assign(sv.data(), sv.size());
</pre></blockquote>
</blockquote>
<pre>
<ins>template&lt;class T&gt;
  constexpr basic_string&amp; assign(const T&amp; t, size_type pos);</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Constraints:</i></ins>
</p>
<ol style="list-style-type:none">
<li><p><ins>(?.1) &mdash; <code>is_convertible_v&lt;const T&amp;, basic_string_view&lt;charT, traits&gt;&gt;</code> 
is <code>true</code> and</ins></p></li>
<li><p><ins>(?.2) &mdash; <code>is_convertible_v&lt;const T&amp;, const charT*&gt;</code> is <code>false</code>.</ins></p></li>
</ol>
<p>
<ins>-?- <i>Effects:</i> Equivalent to:</ins>
</p>
<blockquote><pre>
<ins>basic_string_view&lt;charT, traits&gt; sv = t;
return assign(sv.substr(pos));</ins>
</pre></blockquote>
</blockquote>
<pre>
template&lt;class T&gt;
  constexpr basic_string&amp; assign(const T&amp; t, size_type pos, size_type n <del>= npos</del>);
</pre>
<blockquote>
<p>
-6- <i>Constraints:</i>
</p>
<ol style="list-style-type:none">
<li><p><del>(6.1) &mdash;</del> <code>is_convertible_v&lt;const T&amp;, basic_string_view&lt;charT, traits&gt;&gt;</code> 
is <code>true</code> <del>and</del></p></li>
<li><p><del>(6.2) &mdash; <code>is_convertible_v&lt;const T&amp;, const charT*&gt;</code> is <code>false</code></del>.</p></li>
</ol>
<p>
-7- <i>Effects:</i> Equivalent to:
</p>
<blockquote><pre>
basic_string_view&lt;charT, traits&gt; sv = t;
return assign(sv.substr(pos, n));
</pre></blockquote>
</blockquote>
</blockquote>
</li>
</ol>

<p>
<b>Option B:</b>
</p>

<ol>
<li><p>Modify 27.4.3.1 <a href="https://wg21.link/basic.string.general">[basic.string.general]</a>, class template <code>basic_string</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<i>// 27.4.3.7 <a href="https://wg21.link/string.modifiers">[string.modifiers]</a>, modifiers</i>
[&hellip;]
constexpr basic_string&amp; append(const charT* s, size_type n);
constexpr basic_string&amp; append(const charT* s);
<ins>constexpr basic_string&amp; append(const charT* s, size_type pos, size_type n);</ins>
constexpr basic_string&amp; append(size_type n, charT c);
[&hellip;]
constexpr basic_string&amp; assign(const charT* s, size_type n);
constexpr basic_string&amp; assign(const charT* s);
<ins>constexpr basic_string&amp; assign(const charT* s, size_type pos, size_type n);</ins>
constexpr basic_string&amp; assign(size_type n, charT c);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 27.4.3.7.2 <a href="https://wg21.link/string.append">[string.append]</a> as indicated:</p>

<blockquote>
<pre>
constexpr basic_string&amp; append(const charT* s, size_type n);
</pre>
<blockquote>
<p>
-7- <i>Preconditions:</i> <code>[s, s + n)</code> is a valid range.
<p/>
-8- <i>Effects:</i> Appends a copy of the range <code>[s, s + n)</code> to the string.
<p/>
-9- <i>Returns:</i> <code>*this</code>.
</p>
</blockquote>
<pre>
constexpr basic_string&amp; append(const charT* s);
</pre>
<blockquote>
<p>
-10- <i>Effects:</i> Equivalent to: <code>return append(s, traits::length(s));</code>
</p>
</blockquote>
<pre>
<ins>constexpr basic_string&amp; append(const charT* s, size_type pos, size_type n);</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects:</i> Equivalent to: <code>return append(basic_string_view&lt;charT, traits&gt;(s).substr(pos, n));</code></ins>
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 27.4.3.7.3 <a href="https://wg21.link/string.assign">[string.assign]</a> as indicated:</p>

<blockquote>
<pre>
constexpr basic_string&amp; assign(const charT* s, size_type n);
</pre>
<blockquote>
<p>
-8- <i>Preconditions:</i> <code>[s, s + n)</code> is a valid range.
<p/>
-9- <i>Effects:</i> Replaces the string controlled by <code>*this</code> with a copy of the range <code>[s, s + n)</code>.
<p/>
-10- <i>Returns:</i> <code>*this</code>.
</p>
</blockquote>
<pre>
constexpr basic_string&amp; assign(const charT* s);
</pre>
<blockquote>
<p>
-11- <i>Effects:</i> Equivalent to: <code>return assign(s, traits::length(s));</code>
</p>
</blockquote>
<pre>
<ins>constexpr basic_string&amp; assign(const charT* s, size_type pos, size_type n);</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects:</i> Equivalent to: <code>return assign(basic_string_view&lt;charT, traits&gt;(s).substr(pos, n));</code></ins>
</p>
</blockquote>
</blockquote>
</li>

</ol>





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