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        std::ranges::upto(n) - HackMD
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<tr><th>Doc. no.:</th>    <td>P3060R0</td></tr>
<tr><th>Date:</th>        <td>2023-11-22</td></tr>
<tr><th>Audience:</th>    <td>LEWG</td></tr>
<tr><th>Reply-to:</th>    <td>Weile Wei &lt;weilewei09@gmail.com&gt;</td></tr>
</tbody></table><h1 id="Add-stdrangesupton" data-id="Add-stdrangesupton"><a class="anchor hidden-xs" href="#Add-stdrangesupton" title="Add-stdrangesupton"><span class="octicon octicon-link"></span></a><span class="ui-comment-inline-span">Add std::ranges::upto(n)</span></h1><p><span class="toc"><ul>
<li><a href="#Add-stdrangesupton" title="Add std::ranges::upto(n)">Add std::ranges::upto(n)</a><ul>
<li><a href="#Abstract" title="Abstract">Abstract</a></li>
<li><a href="#Motivation" title="Motivation">Motivation</a></li>
<li><a href="#Implementation" title="Implementation">Implementation</a></li>
<li><a href="#Test-cases-verified-on-various-platforms" title="Test cases verified on various platforms">Test cases verified on various platforms</a></li>
<li><a href="#Background" title="Background">Background</a></li>
<li><a href="#Technical-Decisions" title="Technical Decisions">Technical Decisions</a></li>
<li><a href="#Wording" title="Wording">Wording</a><ul>
<li><a href="#Header-ltrangesgt-synopsis-rangessyn" title="Header <ranges> synopsis [ranges.syn]">Header &lt;ranges&gt; synopsis [ranges.syn]</a></li>
<li><a href="#New-subsection-under-rangeiotaview" title="New subsection under [range.iota.view]">New subsection under [range.iota.view]</a></li>
</ul>
</li>
<li><a href="#References" title="References">References</a></li>
</ul>
</li>
</ul>
</span></p><h2 id="Abstract" data-id="Abstract"><a class="anchor hidden-xs" href="#Abstract" title="Abstract"><span class="octicon octicon-link"></span></a><span>Abstract</span></h2><p><span>This proposal suggests adding </span><code>std::ranges::upto(n)</code><span> to the C++ Standard Library as a range adaptor that generates a sequence of integers from </span><code>0</code><span> to </span><code>n-1</code><span>.</span></p><h2 id="Motivation" data-id="Motivation"><a class="anchor hidden-xs" href="#Motivation" title="Motivation"><span class="octicon octicon-link"></span></a><span>Motivation</span></h2><p><span>Typically, generating such a sequence of integers involves </span><code>std::views::iota</code><span> combined with </span><code>std::views::take</code><span>, or manual iteration, which can be cumbersome and not straightforward, particularly with unsigned types like container sizes. Consider the following typical usage:</span></p><table class="no-alt"><tbody>
<tr><th>
<p><span>before</span></p>
</th><td>
<pre><code class="cpp hljs"><span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">:</span> std<span class="token double-colon punctuation">::</span>views<span class="token double-colon punctuation">::</span><span class="token function">iota</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token operator">|</span> std<span class="token double-colon punctuation">::</span>views<span class="token double-colon punctuation">::</span><span class="token function">take</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    std<span class="token double-colon punctuation">::</span>cout <span class="token operator">&lt;&lt;</span> i <span class="token operator">&lt;&lt;</span> <span class="token char">' '</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator">&lt;&lt;</span> <span class="token char">'\n'</span><span class="token punctuation">;</span>
</code></pre>
</td></tr><tr><th>
<p><mark><span>after</span></mark></p>
</th>
<td>
<pre><code class="cpp hljs"><span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">:</span> std<span class="token double-colon punctuation">::</span>views<span class="token double-colon punctuation">::</span><span class="token function">upto</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    std<span class="token double-colon punctuation">::</span>cout <span class="token operator">&lt;&lt;</span> i <span class="token operator">&lt;&lt;</span> <span class="token char">' '</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator">&lt;&lt;</span> <span class="token char">'\n'</span><span class="token punctuation">;</span>
</code></pre>
</td></tr>
</tbody></table><p><code>std::ranges::upto(n)</code><span> eases this pattern by providing a straightforward and type-safe method to generate integer sequences, improving code readability and minimizing repetitive code.</span></p><h2 id="Implementation" data-id="Implementation"><a class="anchor hidden-xs" href="#Implementation" title="Implementation"><span class="octicon octicon-link"></span></a><span>Implementation</span></h2><pre><code class="cpp hljs"><span class="token keyword">namespace</span> std<span class="token double-colon punctuation">::</span>ranges <span class="token punctuation">{</span>
    <span class="token keyword">inline</span> <span class="token keyword">constexpr</span> <span class="token keyword">auto</span> upto <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token operator">&lt;</span>std<span class="token double-colon punctuation">::</span>integral I<span class="token operator">&gt;</span> <span class="token punctuation">(</span>I n<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> std<span class="token double-colon punctuation">::</span>views<span class="token double-colon punctuation">::</span><span class="token function">iota</span><span class="token punctuation">(</span>I<span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> n<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><h2 id="Test-cases-verified-on-various-platforms" data-id="Test-cases-verified-on-various-platforms"><a class="anchor hidden-xs" href="#Test-cases-verified-on-various-platforms" title="Test-cases-verified-on-various-platforms"><span class="octicon octicon-link"></span></a><span>Test cases verified on various platforms</span></h2><p><span>Test case:</span></p><pre><code class="cpp hljs"><span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">constexpr</span> <span class="token keyword">auto</span> up_to_ten <span class="token operator">=</span> std<span class="token double-colon punctuation">::</span>ranges<span class="token double-colon punctuation">::</span><span class="token function">upto</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">auto</span> i <span class="token operator">:</span> up_to_ten<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        std<span class="token double-colon punctuation">::</span>cout <span class="token operator">&lt;&lt;</span> i <span class="token operator">&lt;&lt;</span> <span class="token char">' '</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    std<span class="token double-colon punctuation">::</span>cout <span class="token operator">&lt;&lt;</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><p><span>Expected output:</span></p><pre><code>0 1 2 3 4 5 6 7 8 9
</code></pre><p><span>The implementation is confirmed to work with clang version 16.0.0+, gcc version 11.1+, and msvc v19.32+: </span><ruby><a href="https://godbolt.org/z/T7YoMzsdG" target="_blank" rel="noopener"><span class="godbolt"><span> </span></span><span>T7YoMzsdG</span></a><rt><span>Compiler Explorer</span></rt></ruby></p><h2 id="Background" data-id="Background"><a class="anchor hidden-xs" href="#Background" title="Background"><span class="octicon octicon-link"></span></a><span>Background</span></h2><p><span>Two preceding proposals have provided fundation for </span><code>std::ranges::upto(n)</code><span>:</span></p><ol>
<li><em><span>P2214R2: A Plan for C++26 Ranges</span></em><sup class="footnote-ref"><a href="#fn1" id="fnref1">[1]</a></sup><span> highlights the issue with </span><code>views::iota(0, r.size())</code><span> not compiling due to mismatched types. </span><code>std::ranges::views::iota</code><span> requires both arguments to be of the same type, or at least commonly comparable. This becomes problematic when comparing </span><code>int</code><span> (often 32-bit) with </span><code>std::size_t</code><span> (often 64-bit), which is usually wider on 64-bit systems. An example illustrating this issue is available at </span><ruby><a href="https://godbolt.org/z/TGzP4be7K" target="_blank" rel="noopener"><span class="godbolt"><span> </span></span><span>TGzP4be7K</span></a><rt><span>Compiler Explorer</span></rt></ruby><span>.</span></li>
</ol><pre><code class="cpp hljs">std<span class="token double-colon punctuation">::</span>vector <span class="token function">vec</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">auto</span> res <span class="token operator">=</span> std<span class="token double-colon punctuation">::</span>ranges<span class="token double-colon punctuation">::</span>views<span class="token double-colon punctuation">::</span><span class="token function">iota</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> vec<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre><ol start="2">
<li><em><span>P1894R0: Proposal of std::upto, std::indices and std::enumerate</span></em><sup class="footnote-ref"><a href="#fn2" id="fnref2">[2]</a></sup><span> proposed an implementation (see below) that our proposal refines. Our approach offers a more intuitive and consistent interface, fitting seamlessly with existing standard library components without adding complexity.</span></li>
</ol><pre><code class="cpp hljs"><span class="token comment">// std::upto implementation example</span>
<span class="token comment">// P1894R0: Proposal of std::upto, std::indices and std::enumerate</span>
<span class="token keyword">namespace</span> std <span class="token punctuation">{</span>
    <span class="token keyword">template</span><span class="token operator">&lt;</span><span class="token keyword">typename</span> <span class="token class-name">Int</span><span class="token operator">&gt;</span>
    <span class="token keyword">constexpr</span> <span class="token keyword">auto</span> <span class="token function">upto</span><span class="token punctuation">(</span>Int<span class="token operator">&amp;&amp;</span> i<span class="token punctuation">)</span> <span class="token keyword">noexcept</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> std<span class="token double-colon punctuation">::</span>ranges<span class="token double-colon punctuation">::</span>iota_view<span class="token punctuation">{</span><span class="token function">Int</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span>std<span class="token double-colon punctuation">::</span><span class="token generic-function"><span class="token function">forward</span><span class="token generic class-name"><span class="token operator">&lt;</span>Int<span class="token operator">&gt;</span></span></span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre><h2 id="Technical-Decisions" data-id="Technical-Decisions"><a class="anchor hidden-xs" href="#Technical-Decisions" title="Technical-Decisions"><span class="octicon octicon-link"></span></a><span>Technical Decisions</span></h2><ol>
<li><span>Limiting to </span><code>std::integral</code><span>: This constraint ensure</span><span class="ui-comment-inline-span">s functio</span><span>nality is restricted to integral types, yielding predictable behavior and avoiding the pitfalls of generic types. A more relaxed constraint to </span><code>std::default_initializable</code><span> and </span><code>std::incrementable</code><span> would allow iterators to compile but might introduce undefined behavior. An example illustrating this issue is available at </span><ruby><a href="https://godbolt.org/z/hGx1T9sxG" target="_blank" rel="noopener"><span class="godbolt"><span> </span></span><span>hGx1T9sxG</span></a><rt><span>Compiler Explorer</span></rt></ruby><span>:</span></li>
</ol><pre><code class="cpp hljs"><span class="token keyword">namespace</span> std<span class="token double-colon punctuation">::</span>ranges <span class="token punctuation">{</span>
    <span class="token comment">// a more relaxed design pattern, compiled but UB</span>
    <span class="token keyword">inline</span> <span class="token keyword">constexpr</span> <span class="token keyword">auto</span> upto2 <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span>
    <span class="token operator">&lt;</span><span class="token keyword">typename</span> <span class="token class-name">T</span><span class="token operator">&gt;</span>
    <span class="token keyword">requires</span> std<span class="token double-colon punctuation">::</span>default_initializable<span class="token operator">&lt;</span>T<span class="token operator">&gt;</span> <span class="token operator">&amp;&amp;</span> std<span class="token double-colon punctuation">::</span><span class="token generic-function"><span class="token function">incrementable</span><span class="token generic class-name"><span class="token operator">&lt;</span>T<span class="token operator">&gt;</span></span></span>
        <span class="token punctuation">(</span>T n<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> std<span class="token double-colon punctuation">::</span>views<span class="token double-colon punctuation">::</span><span class="token function">iota</span><span class="token punctuation">(</span>T<span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> n<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">int</span> myint <span class="token operator">=</span> <span class="token number">10</span><span class="token punctuation">;</span>
    <span class="token keyword">int</span><span class="token operator">*</span> ptr <span class="token operator">=</span> <span class="token operator">&amp;</span>myint<span class="token punctuation">;</span>

    <span class="token comment">// !!!compiled but undefined behavior!!!</span>
    <span class="token keyword">auto</span> up_to_ten2 <span class="token operator">=</span> std<span class="token double-colon punctuation">::</span>ranges<span class="token double-colon punctuation">::</span><span class="token function">upto2</span><span class="token punctuation">(</span>CustomIterator<span class="token punctuation">{</span>ptr<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">auto</span> i <span class="token operator">:</span> up_to_ten2<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        std<span class="token double-colon punctuation">::</span>cout <span class="token operator">&lt;&lt;</span> <span class="token operator">*</span>i <span class="token operator">&lt;&lt;</span> <span class="token char">' '</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><ol start="2">
<li>
<p><span>Lambda-based Approach: Employing a lambda is consistent with the established range adaptor patterns. Moreoever, the </span><code>constexpr</code><span> specifier on the lambda ensures that the lambda object is a constant expression.</span></p>
</li>
<li>
<p><span>Leveraging Existing </span><code>iota_view</code><span> Instead of Creating a New </span><code>upto_view</code><span>: Introducing </span><code>upto_view</code><span> would mean adding a component similar to what already exists, causing confusion and maintainability issues for users. By leveraging </span><code>iota_view</code><span>, we simplify the implementation and reuse of what the current C++ Standard Library offers. Additionally, any future optimizations to </span><code>iota_view</code><span> will automatically benefit </span><code>std::ranges::upto</code><span>. Therefore, by extending </span><code>iota_view</code><span>, </span><code>std::ranges::upto</code><span> becomes more maintainable, efficient, and simple.</span></p>
</li>
</ol><h2 id="Wording" data-id="Wording"><a class="anchor hidden-xs" href="#Wording" title="Wording"><span class="octicon octicon-link"></span></a><span>Wording</span></h2><p><span>This wording is relative to </span><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2023/n4964.pdf" target="_blank" rel="noopener"><span>N4964</span></a><span>.</span></p><h3 id="Header-ltrangesgt-synopsis-rangessyn" data-id="Header-ltrangesgt-synopsis-rangessyn"><a class="anchor hidden-xs" href="#Header-ltrangesgt-synopsis-rangessyn" title="Header-ltrangesgt-synopsis-rangessyn"><span class="octicon octicon-link"></span></a><span>Header </span>&lt;ranges&gt;<span> synopsis </span><a href="http://www.eelis.net/c++draft/ranges.syn" target="_blank" rel="noopener"><span>[ranges.syn]</span></a></h3><p><span>Insert a new item in the </span><code>std::ranges</code><span> namespace:</span></p><pre><code class="cpp hljs"><span class="token keyword">namespace</span> std<span class="token double-colon punctuation">::</span>ranges <span class="token punctuation">{</span>
    <span class="token keyword">inline</span> <span class="token keyword">constexpr</span> <span class="token keyword">auto</span> upto <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token operator">&lt;</span>std<span class="token double-colon punctuation">::</span>integral I<span class="token operator">&gt;</span> <span class="token punctuation">(</span>I n<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> std<span class="token double-colon punctuation">::</span>views<span class="token double-colon punctuation">::</span><span class="token function">iota</span><span class="token punctuation">(</span>I<span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> n<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><h3 id="New-subsection-under-rangeiotaview" data-id="New-subsection-under-rangeiotaview"><a class="anchor hidden-xs" href="#New-subsection-under-rangeiotaview" title="New-subsection-under-rangeiotaview"><span class="octicon octicon-link"></span></a><span>New subsection under </span><a href="http://www.eelis.net/c++draft/range.iota.view" target="_blank" rel="noopener"><span>[range.iota.view]</span></a></h3><p><span>Add a new subsection to describe </span><code>std::ranges::upto</code><span>:</span></p><p><span>26.6.4.2.1 upto function [range.upto]</span></p><p><span>-1- The name </span><code>upto</code><span> denotes a range adaptor object (</span><a href="http://www.eelis.net/c++draft/range.adaptor.object" target="_blank" rel="noopener"><span>[range.adaptor.object]</span></a><span>). The expression </span><code>std::ranges::upto(n)</code><span> is expression-equivalent to </span><code>std::views::iota(I{}, n)</code><span> where </span><code>I</code><span> is the type of </span><code>n</code><span>.</span></p><p><span>-2- Constraints: The type </span><code>I</code><span> shall satisfy the </span><code>std::integral</code><span> concept.</span></p><p><span>-3- Returns: A view of the integers in the range [0, n).</span></p><p><span>-4- Complexity: Constant time.</span></p><p><span>-5- Remarks: This function template shall not participate in overload resolution unless </span><code>std::integral&lt;I&gt;</code><span> is true.</span></p><h2 id="References" data-id="References"><a class="anchor hidden-xs" href="#References" title="References"><span class="octicon octicon-link"></span></a><span>References</span></h2><hr class="footnotes-sep"><section class="footnotes">
<ol class="footnotes-list">
<li id="fn1" class="footnote-item"><p><span>P2214R2 </span><em><span>A Plan for C++26 Ranges.</span></em>
<a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2760r0.html" target="_blank" rel="noopener"><span>https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2760r0.html</span></a> <a href="#fnref1" class="footnote-backref">↩︎</a></p>
</li>
<li id="fn2" class="footnote-item"><p><span>P1894R0 </span><em><span>Proposal of std::upto, std::indices and std::enumerate.</span></em>
<a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1894r0.pdf" target="_blank" rel="noopener"><span>https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1894r0.pdf</span></a> <a href="#fnref2" class="footnote-backref">↩︎</a></p>
</li>
</ol>
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