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<title>Issue 2742: Inconsistent string interface taking string_view</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++17">C++17</a> status.</em></p>
<h3 id="2742"><a href="lwg-defects.html#2742">2742</a>. Inconsistent <code>string</code> interface taking <code>string_view</code></h3>
<p><b>Section:</b> 27.4.3.3 <a href="https://wg21.link/string.cons">[string.cons]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Richard Smith <b>Opened:</b> 2016-07-06 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>1
</p>
<p><b>View all other</b> <a href="lwg-index.html#string.cons">issues</a> in [string.cons].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Generally, <code>basic_string</code> has a constructor matching each <code>assign</code> function and vice versa (except the 
constructor takes an allocator where <code>assign</code> does not). <a href="https://wg21.link/p0254r2">P0254R2</a> violates 
this by adding an <code>assign(basic_string_view, size_type pos, size_type n = npos)</code> but no corresponding constructor.
</p>

<p><i>[2016-08-04 Chicago LWG]</i></p>

<p>
Robert Douglas provides initial wording.
<p/>
We decided against another constructor overload to avoid the semantic confusion between:
</p>
<blockquote><pre>
basic_string(char const*, size_type length, Allocator = Allocator())
</pre></blockquote>
<p>
and
</p>
<blockquote><pre>
template&lt;class T, class Foo = is_convertible_v&lt;T const&amp;, basic_string_view&lt;charT, traits&gt;&gt;
basic_string(T const&amp;, size_type pos, Allocator = Allocator())
</pre></blockquote>
<p>
where someone might call:
</p>
<blockquote><pre>
basic_string("HelloWorld", 5, 5);
</pre></blockquote>
<p>
and get "World", but then call
</p>
<blockquote><pre>
basic_string("HelloWorld", 5);
</pre></blockquote>
<p>
and instead get "Hello". The second parameter changes between length and position. 
</p>

<p><i>[08-2016, Chicago]</i></p>

<p>Fri PM: Move to Tentatively Ready</p>


<p id="res-2742"><b>Proposed resolution:</b></p>
<p>
This wording is relative to N4606.
</p>

<ol>
<li><p>In 27.4.3 <a href="https://wg21.link/basic.string">[basic.string]</a> add the following constructor overload:</p>

<blockquote><pre>
[&hellip;]
basic_string(const basic_string&amp; str, size_type pos,
             const Allocator&amp; a = Allocator());
basic_string(const basic_string&amp; str, size_type pos, size_type n,
             const Allocator&amp; a = Allocator());
<ins>template&lt;class T&gt;
basic_string(const T&amp; t, size_type pos, size_type n, const Allocator&amp; a = Allocator());</ins>
explicit basic_string(basic_string_view&lt;charT, traits&gt; sv,
                      const Allocator&amp; a = Allocator());
[&hellip;]
</pre></blockquote>
</li>

<li><p>In 27.4.3.3 <a href="https://wg21.link/string.cons">[string.cons]</a> add the following ctor definition:</p>

<blockquote>
<pre>
<ins>template&lt;class T&gt;
basic_string(const T&amp; t, size_type pos, size_type n, const Allocator&amp; a = Allocator());</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects:</i> Creates a variable, <code>sv</code>, as if by <code>basic_string_view&lt;charT, traits&gt; sv = t;</code> 
and then behaves the same as:</ins>
<blockquote>
<pre>
<ins>basic_string(sv.substr(pos, n), a)</ins>
</pre>
</blockquote>
<p/>
<ins>-?- <i>Remarks:</i> This constructor shall not participate in overload resolution unless 
<code>is_convertible_v&lt;const T&amp;, basic_string_view&lt;charT, traits&gt;&gt;</code> is <code>true</code>.</ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>





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