<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 2485: get() should be overloaded for const tuple&amp;&amp;</title>
<meta property="og:title" content="Issue 2485: get() should be overloaded for const tuple&amp;&amp;">
<meta property="og:description" content="C++ library issue. Status: C++17">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue2485.html">
<meta property="og:type" content="website">
<meta property="og:image" content="http://cplusplus.github.io/LWG/images/cpp_logo.png">
<meta property="og:image:alt" content="C++ logo">
<style>
  p {text-align:justify}
  li {text-align:justify}
  pre code.backtick::before { content: "`" }
  pre code.backtick::after { content: "`" }
  blockquote.note
  {
    background-color:#E0E0E0;
    padding-left: 15px;
    padding-right: 15px;
    padding-top: 1px;
    padding-bottom: 1px;
  }
  ins {background-color:#A0FFA0}
  del {background-color:#FFA0A0}
  table.issues-index { border: 1px solid; border-collapse: collapse; }
  table.issues-index th { text-align: center; padding: 4px; border: 1px solid; }
  table.issues-index td { padding: 4px; border: 1px solid; }
  table.issues-index td:nth-child(1) { text-align: right; }
  table.issues-index td:nth-child(2) { text-align: left; }
  table.issues-index td:nth-child(3) { text-align: left; }
  table.issues-index td:nth-child(4) { text-align: left; }
  table.issues-index td:nth-child(5) { text-align: center; }
  table.issues-index td:nth-child(6) { text-align: center; }
  table.issues-index td:nth-child(7) { text-align: left; }
  table.issues-index td:nth-child(5) span.no-pr { color: red; }
  @media (prefers-color-scheme: dark) {
     html {
        color: #ddd;
        background-color: black;
     }
     ins {
        background-color: #225522
     }
     del {
        background-color: #662222
     }
     a {
        color: #6af
     }
     a:visited {
        color: #6af
     }
     blockquote.note
     {
        background-color: rgba(255, 255, 255, .10)
     }
  }
</style>
</head>
<body>
<hr>
<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="2485"><a href="lwg-defects.html#2485">2485</a>. <code>get()</code> should be overloaded for <code>const tuple&amp;&amp;</code></h3>
<p><b>Section:</b> 22.4.8 <a href="https://wg21.link/tuple.elem">[tuple.elem]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Stephan T. Lavavej <b>Opened:</b> 2015-03-27 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>1
</p>
<p><b>View all other</b> <a href="lwg-index.html#tuple.elem">issues</a> in [tuple.elem].</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>
<code>const</code> rvalues are weird, but they're part of the type system. Consider the following code:
</p>
<blockquote>
<pre>
#include &lt;functional&gt;
#include &lt;string&gt;
#include &lt;tuple&gt;

using namespace std; 

string str1() { return "one"; }
const string str2() { return "two"; }
tuple&lt;string&gt; tup3() { return make_tuple("three"); }
const tuple&lt;string&gt; tup4() { return make_tuple("four"); }

int main() {
  // cref(str1()); // BAD, properly rejected
  // cref(str2()); // BAD, properly rejected
  // cref(get&lt;0&gt;(tup3())); // BAD, properly rejected
  cref(get&lt;0&gt;(tup4())); // BAD, but improperly accepted!
}
</pre>
</blockquote>
<p>
As <code>tuple</code> is a fundamental building block (and the only convenient way to have variadic data members), it should 
not open a hole in the type system. <code>get()</code> should imitate 7.6.1.5 <a href="https://wg21.link/expr.ref">[expr.ref]</a>'s rules for accessing data members.  
(This is especially true for <code>pair</code>, where both <code>get&lt;0&gt;()</code> and <code>.first</code> are available.)
<p/>
While we're in the neighborhood, we can dramatically simplify the wording here. All we need to do is follow 
22.4.8 <a href="https://wg21.link/tuple.elem">[tuple.elem]</a>/9's example of saying "Returns: A reference to STUFF", and allow implementers to figure out how 
to achieve that with the given return types.
</p>

<p><i>[2015-05, Lenexa]</i></p>

<p>
TP: for the existing overloads there's no change to the code, just descriptions?<br/>
STL: right.<br/>
JW: I love it<br/>
MC: in favor of moving to Ready and bringing up for vote on Friday<br/>
7 in favor, none opposed 
</p>


<p id="res-2485"><b>Proposed resolution:</b></p>
<p>This wording is relative to N4296.</p>

<ol>
<li><p>Change 22.2 <a href="https://wg21.link/utility">[utility]</a>/2 "Header <code>&lt;utility&gt;</code> synopsis" as depicted:</p>

<blockquote>
<pre>
[&hellip;]
template&lt;size_t I, class T1, class T2&gt;
  constexpr tuple_element_t&lt;I, pair&lt;T1, T2&gt;&gt;&amp;
    get(pair&lt;T1, T2&gt;&amp;) noexcept;
template&lt;size_t I, class T1, class T2&gt;
  constexpr tuple_element_t&lt;I, pair&lt;T1, T2&gt;&gt;&amp;&amp;
    get(pair&lt;T1, T2&gt;&amp;&amp;) noexcept;
template&lt;size_t I, class T1, class T2&gt;
  constexpr const tuple_element_t&lt;I, pair&lt;T1, T2&gt;&gt;&amp;
    get(const pair&lt;T1, T2&gt;&amp;) noexcept;
<ins>template&lt;size_t I, class T1, class T2&gt;
  constexpr const tuple_element_t&lt;I, pair&lt;T1, T2&gt;&gt;&amp;&amp;
    get(const pair&lt;T1, T2&gt;&amp;&amp;) noexcept;</ins>
template &lt;class T, class U&gt;
  constexpr T&amp; get(pair&lt;T, U&gt;&amp; p) noexcept;
template &lt;class T, class U&gt;
  constexpr const T&amp; get(const pair&lt;T, U&gt;&amp; p) noexcept;
template &lt;class T, class U&gt;
  constexpr T&amp;&amp; get(pair&lt;T, U&gt;&amp;&amp; p) noexcept;
<ins>template &lt;class T, class U&gt;
  constexpr const T&amp;&amp; get(const pair&lt;T, U&gt;&amp;&amp; p) noexcept;</ins>
template &lt;class T, class U&gt;
  constexpr T&amp; get(pair&lt;U, T&gt;&amp; p) noexcept;
template &lt;class T, class U&gt;
  constexpr const T&amp; get(const pair&lt;U, T&gt;&amp; p) noexcept;
template &lt;class T, class U&gt;
  constexpr T&amp;&amp; get(pair&lt;U, T&gt;&amp;&amp; p) noexcept;
<ins>template &lt;class T, class U&gt;
  constexpr const T&amp;&amp; get(const pair&lt;U, T&gt;&amp;&amp; p) noexcept;</ins>
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Change 22.4.1 <a href="https://wg21.link/tuple.general">[tuple.general]</a>/2 "Header <code>&lt;tuple&gt;</code> synopsis" as depicted:</p>

<blockquote>
<pre>
[&hellip;]
<i>// 20.4.2.6, element access:</i>
template &lt;size_t I, class... Types&gt;
constexpr tuple_element_t&lt;I, tuple&lt;Types...&gt;&gt;&amp;
get(tuple&lt;Types...&gt;&amp;) noexcept;
template &lt;size_t I, class... Types&gt;
constexpr tuple_element_t&lt;I, tuple&lt;Types...&gt;&gt;&amp;&amp;
get(tuple&lt;Types...&gt;&amp;&amp;) noexcept;
template &lt;size_t I, class... Types&gt;
constexpr const tuple_element_t&lt;I, tuple&lt;Types...&gt;&gt;&amp;
get(const tuple&lt;Types...&gt;&amp;) noexcept;
<ins>template &lt;size_t I, class... Types&gt;
constexpr const tuple_element_t&lt;I, tuple&lt;Types...&gt;&gt;&amp;&amp;
get(const tuple&lt;Types...&gt;&amp;&amp;) noexcept;</ins>
template &lt;class T, class... Types&gt;
constexpr T&amp; get(tuple&lt;Types...&gt;&amp; t) noexcept;
template &lt;class T, class... Types&gt;
constexpr T&amp;&amp; get(tuple&lt;Types...>&amp;&amp; t) noexcept;
template &lt;class T, class... Types&gt;
constexpr const T&amp; get(const tuple&lt;Types...&gt;&amp; t) noexcept;
<ins>template &lt;class T, class... Types&gt;
constexpr const T&amp;&amp; get(const tuple&lt;Types...&gt;&amp;&amp; t) noexcept;</ins>
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Change 23.3.1 <a href="https://wg21.link/sequences.general">[sequences.general]</a>/2 "Header <code>&lt;array&gt;</code> synopsis" as depicted:</p>

<blockquote>
<pre>
[&hellip;]
template &lt;size_t I, class T, size_t N&gt;
constexpr T&amp; get(array&lt;T, N&gt;&amp;) noexcept;
template &lt;size_t I, class T, size_t N&gt;
constexpr T&amp;&amp; get(array&lt;T, N&gt;&amp;&amp;) noexcept;
template &lt;size_t I, class T, size_t N&gt;
constexpr const T&amp; get(const array&lt;T, N&gt;&amp;) noexcept;
<ins>template &lt;size_t I, class T, size_t N&gt;
constexpr const T&amp;&amp; get(const array&lt;T, N&gt;&amp;&amp;) noexcept;</ins>
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Change 22.3.4 <a href="https://wg21.link/pair.astuple">[pair.astuple]</a> as depicted:</p>

<blockquote>
<pre>
template&lt;size_t I, class T1, class T2&gt;
constexpr tuple_element_t&lt;I, pair&lt;T1, T2&gt;&gt;&amp;
get(pair&lt;T1, T2&gt;&amp; p) noexcept;
template&lt;size_t I, class T1, class T2&gt;
constexpr const tuple_element_t&lt;I, pair&lt;T1, T2&gt;&gt;&amp;
get(const pair&lt;T1, T2&gt;&amp; p) noexcept;
</pre>
<blockquote>
<p>
<del>-3- <i>Returns</i>: If <code>I == 0</code> returns <code>p.first</code>; if <code>I == 1</code> returns <code>p.second</code>; 
otherwise the program is ill-formed.</del>
</p>
</blockquote>
<pre>
template&lt;size_t I, class T1, class T2&gt;
constexpr tuple_element_t&lt;I, pair&lt;T1, T2&gt;&gt;&amp;&amp;
get(pair&lt;T1, T2&gt;&amp;&amp; p) noexcept;
<ins>template&lt;size_t I, class T1, class T2&gt;
constexpr const tuple_element_t&lt;I, pair&lt;T1, T2&gt;&gt;&amp;&amp;
get(const pair&lt;T1, T2&gt;&amp;&amp; p) noexcept;</ins>
</pre>
<blockquote>
<p>
-4- <i>Returns</i>: If <code>I == 0</code> returns <ins>a reference to</ins> 
<code><del>std::forward&lt;T1&amp;&amp;&gt;(</del>p.first<del>)</del></code>; if <code>I == 1</code> 
returns <ins>a reference to</ins> <code><del>std::forward&lt;T2&amp;&amp;&gt;(</del>p.second<del>)</del></code>; 
otherwise the program is ill-formed.
</p>
</blockquote>
<pre>
template &lt;class T, class U&gt;
constexpr T&amp; get(pair&lt;T, U&gt;&amp; p) noexcept;
template &lt;class T, class U&gt;
constexpr const T&amp; get(const pair&lt;T, U&gt;&amp; p) noexcept;
</pre>
<blockquote>
<p>
<del>-5- Requires: <code>T</code> and <code>U</code> are distinct types. Otherwise, the program is ill-formed.</del>
<p/>
<del>-6- Returns: <code>get&lt;0&gt;(p)</code>;</del>
</p>
</blockquote>
<pre>
template &lt;class T, class U&gt;
constexpr T&amp;&amp; get(pair&lt;T, U&gt;&amp;&amp; p) noexcept;
<ins>template &lt;class T, class U&gt;
constexpr const T&amp;&amp; get(const pair&lt;T, U&gt;&amp;&amp; p) noexcept;</ins>
</pre>
<blockquote>
<p>
-7- Requires: <code>T</code> and <code>U</code> are distinct types. Otherwise, the program is ill-formed.
<p/>
-8- Returns: <ins>A reference to <code>p.first</code>.</ins><del><code>get&lt;0&gt;(std::move(p))</code>;</del>
</p>
</blockquote>
<pre>
template &lt;class T, class U&gt;
constexpr T&amp; get(pair&lt;U, T&gt;&amp; p) noexcept;
template &lt;class T, class U&gt;
constexpr const T&amp; get(const pair&lt;U, T&gt;&amp; p) noexcept;
</pre>
<blockquote>
<p>
<del>-9- Requires: <code>T</code> and <code>U</code> are distinct types. Otherwise, the program is ill-formed.</del>
<p/>
<del>-10- Returns: <code>get&lt;1&gt;(p)</code>;</del>
</p>
</blockquote>
<pre>
template &lt;class T, class U&gt;
constexpr T&amp;&amp; get(pair&lt;U, T&gt;&amp;&amp; p) noexcept;
<ins>template &lt;class T, class U&gt;
constexpr const T&amp;&amp; get(const pair&lt;U, T&gt;&amp;&amp; p) noexcept;</ins>
</pre>
<blockquote>
<p>
-11- Requires: <code>T</code> and <code>U</code> are distinct types. Otherwise, the program is ill-formed.
<p/>
-12- Returns: <ins>A reference to <code>p.second</code>.</ins><del><code>get&lt;1&gt;(std::move(p))</code>;</del>
</p>
</blockquote>
</blockquote>
</li>

<li><p>Change 22.4.8 <a href="https://wg21.link/tuple.elem">[tuple.elem]</a> as depicted:</p>

<blockquote>
<pre>
template &lt;size_t I, class... Types&gt;
  constexpr tuple_element_t&lt;I, tuple&lt;Types...&gt; &gt;&amp; get(tuple&lt;Types...&gt;&amp; t) noexcept;
</pre>
<blockquote>
<p>
<del>-1- <i>Requires</i>: <code>I &lt; sizeof...(Types)</code>. The program is ill-formed if <code>I</code> is out of bounds.</del>
<p/>
<del>-2- <i>Returns</i>: A reference to the <code>I</code>th element of <code>t</code>, where indexing is zero-based.</del>
</p>
</blockquote>
<pre>
template &lt;size_t I, class... Types&gt;
  constexpr tuple_element_t&lt;I, tuple&lt;Types...&gt; &gt;&amp;&amp; get(tuple&lt;Types...&gt;&amp;&amp; t) noexcept; <ins><i>// Note A</i></ins>
</pre>
<blockquote>
<p>
<del>-3- <i>Effects</i>: Equivalent to return std::forward&lt;typename tuple_element&lt;I, tuple&lt;Types...&gt; &gt;::type&amp;&amp;&gt;(get&lt;I&gt;(t));</del>
<p/>
<del>-4- <i>Note</i>: if a <code>T</code> in <code>Types</code> is some reference type <code>X&amp;</code>, the return type is <code>X&amp;</code>, not 
<code>X&amp;&amp;</code>. However, if the element type is a non-reference type <code>T</code>, the return type is <code>T&amp;&amp;</code>.</del>
</p>
</blockquote>
<pre>
template &lt;size_t I, class... Types&gt;
  constexpr tuple_element_t&lt;I, tuple&lt;Types...&gt; &gt; const&amp; get(const tuple&lt;Types...&gt;&amp; t) noexcept; <ins><i>// Note B</i></ins>
<ins>template &lt;size_t I, class... Types&gt;
  constexpr const tuple_element_t&lt;I, tuple&lt;Types...&gt; &gt;&amp;&amp; get(const tuple&lt;Types...&gt;&amp;&amp; t) noexcept;</ins>  
</pre>
<blockquote>
<p>
-5- <i>Requires</i>: <code>I &lt; sizeof...(Types)</code>. The program is ill-formed if <code>I</code> is out of bounds.
<p/>
-6- <i>Returns</i>: A <del>const</del> reference to the <code>I</code>th element of <code>t</code>, where indexing is zero-based.
<p/>
<ins>-?- [<i>Note A</i>: if a <code>T</code> in <code>Types</code> is some reference type <code>X&amp;</code>, the return type is <code>X&amp;</code>, 
not <code>X&amp;&amp;</code>. However, if the element type is a non-reference type <code>T</code>, the return type is <code>T&amp;&amp;</code>. 
&mdash; <i>end note</i>]</ins>
<p/>
-7- [<i>Note <ins>B</ins></i>: Constness is shallow. If a <code>T</code> in <code>Types</code> is some reference type <code>X&amp;</code>, 
the return type is <code>X&amp;</code>, not <code>const X&amp;</code>. However, if the element type is non-reference type <code>T</code>, 
the return type is <code>const T&amp;</code>. This is consistent with how constness is defined to work for member variables of 
reference type. &mdash; <i>end note</i>]
</p>
</blockquote>
<pre>
template &lt;class T, class... Types&gt;
  constexpr T&amp; get(tuple&lt;Types...&gt;&amp; t) noexcept;
template &lt;class T, class... Types&gt;
  constexpr T&amp;&amp; get(tuple&lt;Types...&gt;&amp;&amp; t) noexcept;
template &lt;class T, class... Types&gt;
  constexpr const T&amp; get(const tuple&lt;Types...&gt;&amp; t) noexcept;
<ins>template &lt;class T, class... Types&gt;
  constexpr const T&amp;&amp; get(const tuple&lt;Types...&gt;&amp;&amp; t) noexcept;</ins>
</pre>
<blockquote>
<p>
-8- <i>Requires</i>: The type <code>T</code> occurs exactly once in <code>Types...</code>. Otherwise, the program is ill-formed.
<p/>
-9- <i>Returns</i>: A reference to the element of <code>t</code> corresponding to the type <code>T</code> in <code>Types...</code>.
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>

<li><p>Change 23.3.3.7 <a href="https://wg21.link/array.tuple">[array.tuple]</a> as depicted:</p>

<blockquote>
<pre>
template &lt;size_t I, class T, size_t N&gt;
<ins>  </ins>constexpr T&amp; get(array&lt;T, N&gt;&amp; a) noexcept;
</pre>
<blockquote>
<p>
<del>-3- <i>Requires</i>: <code>I &lt; N</code>. The program is ill-formed if <code>I</code> is out of bounds.</del>
<p/>
<del>-4- <i>Returns</i>: A reference to the <code>I</code>th element of <code>a</code>, where indexing is zero-based.</del>
</p>
</blockquote>
<pre>
template &lt;size_t I, class T, size_t N&gt;
<ins>  </ins>constexpr T&amp;&amp; get(array&lt;T, N&gt;&amp;&amp; a) noexcept;
</pre>
<blockquote>
<p>
<del>-5- <i>Effects</i>: Equivalent to <code>return std::move(get&lt;I&gt;(a));</code></del>
</p>
</blockquote>
<pre>
template &lt;size_t I, class T, size_t N&gt;
<ins>  </ins>constexpr const T&amp; get(const array&lt;T, N&gt;&amp; a) noexcept;
<ins>template &lt;size_t I, class T, size_t N&gt;
  constexpr const T&amp;&amp; get(const array&lt;T, N&gt;&amp;&amp; a) noexcept;</ins>
</pre>
<blockquote>
<p>
-6- <i>Requires</i>: <code>I &lt; N</code>. The program is ill-formed if <code>I</code> is out of bounds.
<p/>
-7- <i>Returns</i>: A <del>const</del> reference to the <code>I</code>th element of <code>a</code>, where indexing is zero-based.
</p>
</blockquote>
</blockquote>
</li>


</ol>






</body>
</html>
