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<tbody><tr>
<td>Document number</td>
<td>P2821R0</td>
</tr>
<tr>
<td>Date</td>
<td>2023-2-20</td>
</tr>
<tr>
<td>Reply-to</td>
<td>
<p><span>Jarrad J. Waterloo &lt;descender76 at gmail dot com&gt;</span></p>
</td>
</tr>
<tr>
<td>Audience</td>
<td>Library Evolution Working Group (LEWG)<br>SG23 Safety and Security</td>
</tr>
</tbody></table><h1 id="spanat" data-id="spanat"><a class="anchor hidden-xs" href="#spanat" title="spanat"><span class="octicon octicon-link"></span></a><a href="http://span.at" rel="noopener" target="_blank"><span>span.at</span></a><span>()</span></h1><style>
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</style><h2 id="Table-of-contents" data-id="Table-of-contents"><a class="anchor hidden-xs" href="#Table-of-contents" title="Table-of-contents"><span class="octicon octicon-link"></span></a><span>Table of contents</span></h2><ul>
<li><a href="#spanat"><span>span.at()</span></a>
<ul>
<li><a href="#Abstract"><span>Abstract</span></a></li>
<li><a href="#Request"><span>Request</span></a></li>
<li><a href="#Motivation"><span>Motivation</span></a>
<ul>
<li><a href="#Safety"><span>Safety</span></a></li>
<li><a href="#Consistency"><span>Consistency</span></a></li>
<li><a href="#Public-Relations"><span>Public Relations</span></a></li>
</ul>
</li>
<li><a href="#Implementation-Experience"><span>Implementation Experience</span></a></li>
<li><a href="#Summary"><span>Summary</span></a></li>
<li><a href="#References"><span>References</span></a></li>
</ul>
</li>
</ul><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 paper proposes the standard adds the </span><code>at</code><span> method to </span><code>std::span</code><span> class in order to address safety, consistency and PR (public relations) concerns.</span></p><h2 id="Request" data-id="Request"><a class="anchor hidden-xs" href="#Request" title="Request"><span class="octicon octicon-link"></span></a><span>Request</span></h2><pre><code class="cpp hljs"><span class="token keyword">constexpr</span> reference <span class="token function">at</span><span class="token punctuation">(</span>size_type idx<span class="token punctuation">)</span> <span class="token keyword">const</span><span class="token punctuation">;</span>
</code></pre><p><span>Returns a reference to the element at specified location idx, with bounds checking.</span></p><p><span>If idx is not within the range of the container, an exception of type std::out_of_range is thrown.</span></p><p><strong><span>Parameters</span></strong>
<span>idx	-	index of the element to return</span></p><p><strong><span>Return value</span></strong>
<span>Reference to the requested element.</span></p><p><strong><span>Exceptions</span></strong>
<span>Throws std::out_of_range if idx &gt;= size().</span></p><p><strong><span>Complexity</span></strong>
<span>Constant.</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><h3 id="Safety" data-id="Safety"><a class="anchor hidden-xs" href="#Safety" title="Safety"><span class="octicon octicon-link"></span></a><span>Safety</span></h3><p><span>This new method is safe in the sense that it has defined behavior instead of undefined behavior. Further, the defined behavior is one that can be caught in the code by catching the exception.</span></p><h3 id="Consistency" data-id="Consistency"><a class="anchor hidden-xs" href="#Consistency" title="Consistency"><span class="octicon octicon-link"></span></a><span>Consistency</span></h3><p><span>The </span><code>std::string</code><span> </span><sup class="footnote-ref"><a href="#fn1" id="fnref1">[1]</a></sup><span>, </span><code>std::string_view</code><span> </span><sup class="footnote-ref"><a href="#fn2" id="fnref2">[2]</a></sup><span>, </span><code>std::deque</code><span> </span><sup class="footnote-ref"><a href="#fn3" id="fnref3">[3]</a></sup><span>, </span><code>std::vector</code><span> </span><sup class="footnote-ref"><a href="#fn4" id="fnref4">[4]</a></sup><span> and </span><code>std::array</code><span> </span><sup class="footnote-ref"><a href="#fn5" id="fnref5">[5]</a></sup><span>, all have both the unsafe </span><code>operator[]</code><span> and the safe </span><code>at</code><span> function.</span></p><table>
<tbody><tr>
<td>
</td>
<td>
<p><code>operator[]</code></p>
</td>
<td>
<p><code>at()</code></p>
</td>
</tr>
<tr>
<td>
<p><code>std::string</code></p>
</td>
<td style="color:green;text-align:center">✓</td>
<td style="color:green;text-align:center">✓</td>
</tr>
<tr>
<td>
<p><code>std::string_view</code></p>
</td>
<td style="color:green;text-align:center">✓</td>
<td style="color:green;text-align:center">✓</td>
</tr>
<tr>
<td>
<p><code>std::deque</code></p>
</td>
<td style="color:green;text-align:center">✓</td>
<td style="color:green;text-align:center">✓</td>
</tr>
<tr>
<td>
<p><code>std::vector</code></p>
</td>
<td style="color:green;text-align:center">✓</td>
<td style="color:green;text-align:center">✓</td>
</tr>
<tr>
<td>
<p><code>std::array</code></p>
</td>
<td style="color:green;text-align:center">✓</td>
<td style="color:green;text-align:center">✓</td>
</tr>
<tr>
<td>
<p><code>std::span</code></p>
</td>
<td style="color:green;text-align:center">✓</td>
<td style="color:red;text-align:center">✗</td>
</tr>
</tbody></table><h3 id="Public-Relations" data-id="Public-Relations"><a class="anchor hidden-xs" href="#Public-Relations" title="Public-Relations"><span class="octicon octicon-link"></span></a><span>Public Relations</span></h3><p><code>C++</code><span> keeps giving easy wins to our rivals even though they like to hit below the belt. It is nice when a supporter of a rival language makes an honest comparison.</span></p><p><em><span>“In both Rust and C++, there is a method for checked array indexing, and a method for unchecked array indexing. The languages actually agree on this issue. They only disagree about which version gets to be spelled with brackets.”</span></em><span> - </span><strong><span>Being Fair about Memory Safety and Performance</span></strong><span> </span><sup class="footnote-ref"><a href="#fn6" id="fnref6">[6]</a></sup></p><p><span>Unfortunately this isn’t totally true.</span></p><p><em><span>“</span><code>std::span&lt;T&gt;</code><span> </span><strong><span>indexing</span></strong><span>”</span></em><span> </span><sup class="footnote-ref"><a href="#fn7" id="fnref7">[7]</a></sup></p><p><span>…</span></p><p><em><span>“</span><code>std::vector</code><span> and </span><code>std::array</code><span> can at least theoretically be used safely because they offer an </span><code>at()</code><span> method which is bounds checked (in practice I’ve never seen this done, but you could imagine a project adopting a static analysis tool which simply banned calls to </span><code>std::vector&lt;T&gt;::operator[]</code><span>). </span><code>span</code><span> does not offer an </span><code>at()</code><span> method, or any other method which performs a bounds checked lookup.”</span></em><span> </span><sup class="footnote-ref"><a href="#fn7" id="fnref7:1">[7:1]</a></sup></p><p><em><span>“Interestingly, both Firefox and Chromium’s backports of std::span do perform bounds checks in operator[], and thus they’ll never be able to safely migrate to std::span.”</span></em><span> - </span><strong><span>Modern C++ Won’t Save Us</span></strong><span> </span><sup class="footnote-ref"><a href="#fn7" id="fnref7:2">[7:2]</a></sup></p><p><span>Consequently, the programming community in general are encouraged to ask some tough questions.</span></p><ol>
<li><span>Why was </span><code>span::at()</code><span> not provided in </span><code>C++20</code><span>?</span></li>
<li><span>Was this an accidental omission or was it deliberate?</span></li>
<li><span>If delibrerate, what was the rationale?</span></li>
<li><span>If delibrerate, are the other </span><code>at</code><span> functions going to be deprecated?</span></li>
<li><span>Why was </span><code>span::at()</code><span> not added in </span><code>C++23</code><span>?</span></li>
<li><span>Is it going to be in </span><code>C++26</code><span>?</span></li>
</ol><p><span>Ultimately, this becomes a stereotypical example of how </span><code>C++</code><span> traditionally handles safety. This example gets to be pointed at for years/decades to come. All of this could have been avoided, along with more effort of adding this function individually had more consideration been given valid safety concerns.</span></p><h2 id="Implementation-Experience" data-id="Implementation-Experience"><a class="anchor hidden-xs" href="#Implementation-Experience" title="Implementation-Experience"><span class="octicon octicon-link"></span></a><span>Implementation Experience</span></h2><p><span>Both of the </span><code>span lite</code><span> and </span><code>Guidelines Support Library</code><span> libraries have this new method implemented for years.</span></p><ul>
<li><code>span lite: A single-file header-only version of a C++20-like span for C++98, C++11 and later</code><span> </span><sup class="footnote-ref"><a href="#fn8" id="fnref8">[8]</a></sup></li>
<li><code>Guidelines Support Library</code><span> </span><sup class="footnote-ref"><a href="#fn9" id="fnref9">[9]</a></sup></li>
</ul><p><span>Likely, there are others too such as </span><em><span>“Firefox and Chromium’s backports of std::span”</span></em><span>. </span><sup class="footnote-ref"><a href="#fn7" id="fnref7:3">[7:3]</a></sup></p><h2 id="Summary" data-id="Summary"><a class="anchor hidden-xs" href="#Summary" title="Summary"><span class="octicon octicon-link"></span></a><span>Summary</span></h2><p><span>Please add the </span><code>at</code><span> method to </span><code>std::span</code><span> class in order to address safety, consistency and PR (public relations) concerns. Also keep ones mind open to an </span><code>std::expected</code><span> version in the future as well as other reasonable safety requests.</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><a href="https://en.cppreference.com/w/cpp/string/basic_string/at" target="_blank" rel="noopener"><span>https://en.cppreference.com/w/cpp/string/basic_string/at</span></a> <a href="#fnref1" class="footnote-backref">↩︎</a></p>
</li>
<li id="fn2" class="footnote-item"><p><a href="https://en.cppreference.com/w/cpp/string/basic_string_view/at" target="_blank" rel="noopener"><span>https://en.cppreference.com/w/cpp/string/basic_string_view/at</span></a> <a href="#fnref2" class="footnote-backref">↩︎</a></p>
</li>
<li id="fn3" class="footnote-item"><p><a href="https://en.cppreference.com/w/cpp/container/deque/at" target="_blank" rel="noopener"><span>https://en.cppreference.com/w/cpp/container/deque/at</span></a> <a href="#fnref3" class="footnote-backref">↩︎</a></p>
</li>
<li id="fn4" class="footnote-item"><p><a href="https://en.cppreference.com/w/cpp/container/vector/at" target="_blank" rel="noopener"><span>https://en.cppreference.com/w/cpp/container/vector/at</span></a> <a href="#fnref4" class="footnote-backref">↩︎</a></p>
</li>
<li id="fn5" class="footnote-item"><p><a href="https://en.cppreference.com/w/cpp/container/array/at" target="_blank" rel="noopener"><span>https://en.cppreference.com/w/cpp/container/array/at</span></a> <a href="#fnref5" class="footnote-backref">↩︎</a></p>
</li>
<li id="fn6" class="footnote-item"><p><a href="https://www.thecodedmessage.com/posts/unsafe/" target="_blank" rel="noopener"><span>https://www.thecodedmessage.com/posts/unsafe/</span></a> <a href="#fnref6" class="footnote-backref">↩︎</a></p>
</li>
<li id="fn7" class="footnote-item"><p><a href="https://alexgaynor.net/2019/apr/21/modern-c++-wont-save-us/" target="_blank" rel="noopener"><span>https://alexgaynor.net/2019/apr/21/modern-c++-wont-save-us/</span></a> <a href="#fnref7" class="footnote-backref">↩︎</a> <a href="#fnref7:1" class="footnote-backref">↩︎</a> <a href="#fnref7:2" class="footnote-backref">↩︎</a> <a href="#fnref7:3" class="footnote-backref">↩︎</a></p>
</li>
<li id="fn8" class="footnote-item"><p><a href="https://github.com/martinmoene/span-lite#at" target="_blank" rel="noopener"><span>https://github.com/martinmoene/span-lite#at</span></a> <a href="#fnref8" class="footnote-backref">↩︎</a></p>
</li>
<li id="fn9" class="footnote-item"><p><a href="https://github.com/microsoft/GSL/blob/main/include/gsl/span_ext#L141" target="_blank" rel="noopener"><span>https://github.com/microsoft/GSL/blob/main/include/gsl/span_ext#L141</span></a> <a href="#fnref9" class="footnote-backref">↩︎</a></p>
</li>
</ol>
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                var hash = element.hash;
                if (hash) {
                    $element.on('click', function (e) {
                        // store hash
                        var hash = this.hash;
                        if ($(hash).length <= 0) return;
                        // prevent default anchor click behavior
                        e.preventDefault();
                        // animate
                        $('body, html').stop(true, true).animate({
                            scrollTop: $(hash).offset().top
                        }, 100, "linear", function () {
                            // when done, add hash to url
                            // (default click behaviour)
                            window.location.hash = hash;
                        });
                    });
                }
            }
        }

        smoothHashScroll();
        var toc = $('.ui-toc');
        var tocAffix = $('.ui-affix-toc');
        var tocDropdown = $('.ui-toc-dropdown');
        //toc
        tocDropdown.click(function (e) {
            e.stopPropagation();
        });

        var enoughForAffixToc = true;

        function generateScrollspy() {
            $(document.body).scrollspy({
                target: ''
            });
            $(document.body).scrollspy('refresh');
            if (enoughForAffixToc) {
                toc.hide();
                tocAffix.show();
            } else {
                tocAffix.hide();
                toc.show();
            }
            $(document.body).scroll();
        }

        function windowResize() {
            //toc right
            var paddingRight = parseFloat(markdown.css('padding-right'));
            var right = ($(window).width() - (markdown.offset().left + markdown.outerWidth() - paddingRight));
            toc.css('right', right + 'px');
            //affix toc left
            var newbool;
            var rightMargin = (markdown.parent().outerWidth() - markdown.outerWidth()) / 2;
            //for ipad or wider device
            if (rightMargin >= 133) {
                newbool = true;
                var affixLeftMargin = (tocAffix.outerWidth() - tocAffix.width()) / 2;
                var left = markdown.offset().left + markdown.outerWidth() - affixLeftMargin;
                tocAffix.css('left', left + 'px');
            } else {
                newbool = false;
            }
            if (newbool != enoughForAffixToc) {
                enoughForAffixToc = newbool;
                generateScrollspy();
            }
        }
        $(window).resize(function () {
            windowResize();
        });
        $(document).ready(function () {
            windowResize();
            generateScrollspy();
        });

        //remove hash
        function removeHash() {
            window.location.hash = '';
        }

        var backtotop = $('.back-to-top');
        var gotobottom = $('.go-to-bottom');

        backtotop.click(function (e) {
            e.preventDefault();
            e.stopPropagation();
            if (scrollToTop)
                scrollToTop();
            removeHash();
        });
        gotobottom.click(function (e) {
            e.preventDefault();
            e.stopPropagation();
            if (scrollToBottom)
                scrollToBottom();
            removeHash();
        });

        var toggle = $('.expand-toggle');
        var tocExpand = false;

        checkExpandToggle();
        toggle.click(function (e) {
            e.preventDefault();
            e.stopPropagation();
            tocExpand = !tocExpand;
            checkExpandToggle();
        })

        function checkExpandToggle () {
            var toc = $('.ui-toc-dropdown .toc');
            var toggle = $('.expand-toggle');
            if (!tocExpand) {
                toc.removeClass('expand');
                toggle.text('Expand all');
            } else {
                toc.addClass('expand');
                toggle.text('Collapse all');
            }
        }

        function scrollToTop() {
            $('body, html').stop(true, true).animate({
                scrollTop: 0
            }, 100, "linear");
        }

        function scrollToBottom() {
            $('body, html').stop(true, true).animate({
                scrollTop: $(document.body)[0].scrollHeight
            }, 100, "linear");
        }
    </script>
</body>

</html>
