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<title>Issue 3156: ForwardIterator should only mean forward iterator</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#Resolved">Resolved</a> status.</em></p>
<h3 id="3156"><a href="lwg-defects.html#3156">3156</a>. <code>ForwardIterator</code> should only mean forward iterator</h3>
<p><b>Section:</b> 26.11 <a href="https://wg21.link/specialized.algorithms">[specialized.algorithms]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2018-09-06 <b>Last modified:</b> 2020-05-02</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#specialized.algorithms">active issues</a> in [specialized.algorithms].</p>
<p><b>View all other</b> <a href="lwg-index.html#specialized.algorithms">issues</a> in [specialized.algorithms].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
26.11 <a href="https://wg21.link/specialized.algorithms">[specialized.algorithms]</a> para 1.2 describes how the specialized
algorithms with a template parameter named <code>ForwardIterator</code> impose
requirements on the type passed as argument for that parameter: it must meet
the <code><i>Cpp17ForwardIterator</i></code> requirements, which is consistent with
how the rest of the Library uses the template parameter name
<code>ForwardIterator</code>, and many of the required operations on that type must
not throw exceptions, which is <em>not</em> consistent with how the rest of the
Library uses that name.
</p><p>
To avoid confusion and keep the meaning of requirements imposed by template
parameter names crisp, the specialized memory algorithms should use a different
template parameter name for this different set of requirements.
</p><p>
Note that the proposed change has no normative effect; it's simply a
clarification of the existing wording.
</p>

<p><i>[2018-09 Reflector prioritization]</i></p>

<p>Set Priority to 3</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>This wording is relative to <a href="https://wg21.link/n4762">N4762</a>.</p>

<ol>
<li><p>Modify 20.2.2 <a href="https://wg21.link/memory.syn">[memory.syn]</a> as indicated:</p>
<blockquote>
<pre>
[&hellip;]

// 26.11 <a href="https://wg21.link/specialized.algorithms">[specialized.algorithms]</a>, specialized algorithms
template&lt;class T&gt;
  constexpr T* addressof(T&amp; r) noexcept;
template&lt;class T&gt;
  const T* addressof(const T&amp;&amp;) = delete;
template&lt;class <ins>NoThrow</ins>ForwardIterator&gt;
  void uninitialized_default_construct(<ins>NoThrow</ins>ForwardIterator first, <ins>NoThrow</ins>ForwardIterator last);
template&lt;class ExecutionPolicy, class <ins>NoThrow</ins>ForwardIterator&gt;
  void uninitialized_default_construct(ExecutionPolicy&amp;&amp; exec, // see 26.3.5 <a href="https://wg21.link/algorithms.parallel.overloads">[algorithms.parallel.overloads]</a>
                                       <ins>NoThrow</ins>ForwardIterator first, <ins>NoThrow</ins>ForwardIterator last);
template&lt;class <ins>NoThrow</ins>ForwardIterator, class Size&gt;
  <ins>NoThrow</ins>ForwardIterator uninitialized_default_construct_n(<ins>NoThrow</ins>ForwardIterator first, Size n);
template&lt;class ExecutionPolicy, class <ins>NoThrow</ins>ForwardIterator, class Size&gt;
  <ins>NoThrow</ins>ForwardIterator uninitialized_default_construct_n(ExecutionPolicy&amp;&amp; exec, // see 26.3.5 <a href="https://wg21.link/algorithms.parallel.overloads">[algorithms.parallel.overloads]</a>
                                                    <ins>NoThrow</ins>ForwardIterator first, Size n);
template&lt;class <ins>NoThrow</ins>ForwardIterator&gt;
  void uninitialized_value_construct(<ins>NoThrow</ins>ForwardIterator first, <ins>NoThrow</ins>ForwardIterator last);
template&lt;class ExecutionPolicy, class <ins>NoThrow</ins>ForwardIterator&gt;
  void uninitialized_value_construct(ExecutionPolicy&amp;&amp; exec, // see 26.3.5 <a href="https://wg21.link/algorithms.parallel.overloads">[algorithms.parallel.overloads]</a>
                                     <ins>NoThrow</ins>ForwardIterator first, <ins>NoThrow</ins>ForwardIterator last);
template&lt;class <ins>NoThrow</ins>ForwardIterator, class Size&gt;
  <ins>NoThrow</ins>ForwardIterator uninitialized_value_construct_n(<ins>NoThrow</ins>ForwardIterator first, Size n);
template&lt;class ExecutionPolicy, class <ins>NoThrow</ins>ForwardIterator, class Size&gt;
  <ins>NoThrow</ins>ForwardIterator uninitialized_value_construct_n(ExecutionPolicy&amp;&amp; exec, // see 26.3.5 <a href="https://wg21.link/algorithms.parallel.overloads">[algorithms.parallel.overloads]</a>
                                                  <ins>NoThrow</ins>ForwardIterator first, Size n);
template&lt;class InputIterator, class <ins>NoThrow</ins>ForwardIterator&gt;
  <ins>NoThrow</ins>ForwardIterator uninitialized_copy(InputIterator first, InputIterator last,
                                     <ins>NoThrow</ins>ForwardIterator result);
template&lt;class ExecutionPolicy, class InputIterator, class <ins>NoThrow</ins>ForwardIterator&gt;
  <ins>NoThrow</ins>ForwardIterator uninitialized_copy(ExecutionPolicy&amp;&amp; exec, // see 26.3.5 <a href="https://wg21.link/algorithms.parallel.overloads">[algorithms.parallel.overloads]</a>
                                     InputIterator first, InputIterator last,
                                     <ins>NoThrow</ins>ForwardIterator result);
template&lt;class InputIterator, class Size, class <ins>NoThrow</ins>ForwardIterator&gt;
  <ins>NoThrow</ins>ForwardIterator uninitialized_copy_n(InputIterator first, Size n,
                                       <ins>NoThrow</ins>ForwardIterator result);
template&lt;class ExecutionPolicy, class InputIterator, class Size, class <ins>NoThrow</ins>ForwardIterator&gt;
  <ins>NoThrow</ins>ForwardIterator uninitialized_copy_n(ExecutionPolicy&amp;&amp; exec, // see 26.3.5 <a href="https://wg21.link/algorithms.parallel.overloads">[algorithms.parallel.overloads]</a>
                                       InputIterator first, Size n,
                                       <ins>NoThrow</ins>ForwardIterator result);
template&lt;class InputIterator, class <ins>NoThrow</ins>ForwardIterator&gt;
  <ins>NoThrow</ins>ForwardIterator uninitialized_move(InputIterator first, InputIterator last,
                                     <ins>NoThrow</ins>ForwardIterator result);
template&lt;class ExecutionPolicy, class InputIterator, class <ins>NoThrow</ins>ForwardIterator&gt;
  <ins>NoThrow</ins>ForwardIterator uninitialized_move(ExecutionPolicy&amp;&amp; exec, // see 26.3.5 <a href="https://wg21.link/algorithms.parallel.overloads">[algorithms.parallel.overloads]</a>
                                     InputIterator first, InputIterator last,
                                     <ins>NoThrow</ins>ForwardIterator result);
template&lt;class InputIterator, class Size, class <ins>NoThrow</ins>ForwardIterator&gt;
  pair&lt;InputIterator, <ins>NoThrow</ins>ForwardIterator&gt; uninitialized_move_n(InputIterator first, Size n,
                                                            <ins>NoThrow</ins>ForwardIterator result);
template&lt;class ExecutionPolicy, class InputIterator, class Size, class <ins>NoThrow</ins>ForwardIterator&gt;
  pair&lt;InputIterator, <ins>NoThrow</ins>ForwardIterator&gt; uninitialized_move_n(ExecutionPolicy&amp;&amp; exec, // see 26.3.5 <a href="https://wg21.link/algorithms.parallel.overloads">[algorithms.parallel.overloads]</a>
                                                            InputIterator first, Size n,
                                                            <ins>NoThrow</ins>ForwardIterator result);
template&lt;class <ins>NoThrow</ins>ForwardIterator, class T&gt;
  void uninitialized_fill(<ins>NoThrow</ins>ForwardIterator first, <ins>NoThrow</ins>ForwardIterator last, const T&amp; x);
template&lt;class ExecutionPolicy, class <ins>NoThrow</ins>ForwardIterator, class T&gt;
  void uninitialized_fill(ExecutionPolicy&amp;&amp; exec, // see 26.3.5 <a href="https://wg21.link/algorithms.parallel.overloads">[algorithms.parallel.overloads]</a>
                          <ins>NoThrow</ins>ForwardIterator first, <ins>NoThrow</ins>ForwardIterator last, const T&amp; x);
template&lt;class <ins>NoThrow</ins>ForwardIterator, class Size, class T&gt;
  <ins>NoThrow</ins>ForwardIterator uninitialized_fill_n(<ins>NoThrow</ins>ForwardIterator first, Size n, const T&amp; x);
template&lt;class ExecutionPolicy, class <ins>NoThrow</ins>ForwardIterator, class Size, class T&gt;
  <ins>NoThrow</ins>ForwardIterator uninitialized_fill_n(ExecutionPolicy&amp;&amp; exec, // see 26.3.5 <a href="https://wg21.link/algorithms.parallel.overloads">[algorithms.parallel.overloads]</a>
                                       <ins>NoThrow</ins>ForwardIterator first, Size n, const T&amp; x);
template&lt;class T&gt;
  void destroy_at(T* location);
template&lt;class <ins>NoThrow</ins>ForwardIterator&gt;
  void destroy(<ins>NoThrow</ins>ForwardIterator first, <ins>NoThrow</ins>ForwardIterator last);
template&lt;class ExecutionPolicy, class <ins>NoThrow</ins>ForwardIterator&gt;
  void destroy(ExecutionPolicy&amp;&amp; exec, // see 26.3.5 <a href="https://wg21.link/algorithms.parallel.overloads">[algorithms.parallel.overloads]</a>
               <ins>NoThrow</ins>ForwardIterator first, <ins>NoThrow</ins>ForwardIterator last);
template&lt;class <ins>NoThrow</ins>ForwardIterator, class Size&gt;
  <ins>NoThrow</ins>ForwardIterator destroy_n(<ins>NoThrow</ins>ForwardIterator first, Size n);
template&lt;class ExecutionPolicy, class <ins>NoThrow</ins>ForwardIterator, class Size&gt;
  <ins>NoThrow</ins>ForwardIterator destroy_n(ExecutionPolicy&amp;&amp; exec, // see 26.3.5 <a href="https://wg21.link/algorithms.parallel.overloads">[algorithms.parallel.overloads]</a>
                            <ins>NoThrow</ins>ForwardIterator first, Size n);

// 20.3.1 <a href="https://wg21.link/unique.ptr">[unique.ptr]</a>, class template unique_ptr
[&hellip;]
</pre>
</blockquote>
</li>
<li><p>Modify 26.11 <a href="https://wg21.link/specialized.algorithms">[specialized.algorithms]</a> as indicated:</p>
<blockquote>
<p>
[&hellip;]
</p><p>
(1.2) &mdash; If an algorithm's template parameter is named
<code><ins>NoThrow</ins>ForwardIterator</code>, the template argument shall satisfy
the <code><i>Cpp17ForwardIterator</i></code> requirements
(24.3.5.5 <a href="https://wg21.link/forward.iterators">[forward.iterators]</a>), and is required to have the property that
no exceptions are thrown from increment, assignment, comparison, or indirection
through valid iterators.
</p><p>
[&hellip;]
</p>
</blockquote>
</li>
<li><p>Modify the declarations of the specialized algorithms in the remainder of
26.11 <a href="https://wg21.link/specialized.algorithms">[specialized.algorithms]</a> to agree with the proposed changes to
20.2.2 <a href="https://wg21.link/memory.syn">[memory.syn]</a> above.</p>
</li>
</ol>
</blockquote>

<p><i>[2020-05-02; Reflector discussions]</i></p>

<p>
The issue has been resolved by accepting <a href="https://wg21.link/p1963r0">P1963R0</a>
in Prague 2020.
</p>


<p id="res-3156"><b>Proposed resolution:</b></p>
<p>
Resolved by <a href="https://wg21.link/p1963r0">P1963R0</a>.
</p>





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