{"id":1788,"date":"2021-04-08T09:05:56","date_gmt":"2021-04-08T14:05:56","guid":{"rendered":"http:\/\/www.jaimerios.com\/?p=1788"},"modified":"2025-12-02T23:31:30","modified_gmt":"2025-12-02T23:31:30","slug":"c-stderase-and-rbegin-rend","status":"publish","type":"post","link":"https:\/\/jaimerios.com\/?p=1788","title":{"rendered":"C++ std::erase and rbegin\/rend"},"content":{"rendered":"\n<p>Once. Only once have I done this.<\/p>\n\n\n\n<p>I had to traverse a <code>std::vector<\/code> in reverse, and at some point, I had to call <code>std::erase<\/code>.<\/p>\n\n\n\n<p>I thought, &#8220;Ok, this should work&#8221; and instead, something bad happened.<\/p>\n\n\n\n<p>I little digging around, and a stackoverflow.com answer later, I saw an answer that left me scratching my head.<\/p>\n\n\n\n<p>The short answer is, if you are using a <code>reverse_iterator<\/code>, you have to supply an iterator that <code>std::erase<\/code> can work with. You do that by calling <code>base<\/code> on the iterator. You also have to move the iterator one position over since reverse_iterators and regular iterators point to different positions\u2026 the nature of each type of iterator.<\/p>\n\n\n\n<p>Here&#8217;s an example of how to do that:<\/p>\n\n\n\n<div class=\"wp-block-kevinbatdorf-code-block-pro\" data-code-block-pro-font-family=\"Code-Pro-JetBrains-Mono\" style=\"font-size:.875rem;font-family:Code-Pro-JetBrains-Mono,ui-monospace,SFMono-Regular,Menlo,Monaco,Consolas,monospace;line-height:1.25rem;--cbp-tab-width:2;tab-size:var(--cbp-tab-width, 2)\"><span style=\"display:flex;align-items:center;padding:10px 0px 0 16px;font-size:0.8em;width:100%;text-align:left;background-color:#1E1E1E;font-style:italic;color:#D4D4D4\"><span style=\"border-bottom:1px solid rgba(234, 191, 191, 0.2)\">C++<\/span><\/span><span role=\"button\" tabindex=\"0\" style=\"color:#D4D4D4;display:none\" aria-label=\"Copy\" class=\"code-block-pro-copy-button\"><pre class=\"code-block-pro-copy-button-pre\" aria-hidden=\"true\"><textarea class=\"code-block-pro-copy-button-textarea\" tabindex=\"-1\" aria-hidden=\"true\" readonly>#include &lt;iostream>\n#include &lt;vector>\n\nvoid print_contents(const std::vector&lt;int>&amp; items);\n\nint main(int, char**)\n{\n  auto some_numbers = std::vector&lt;int>{1, 2, 3, 4, 5, 6};\n  print_contents(some_numbers);\n\n  \/\/ Get an iterator to the last item; then increment that for the second to last item iterator\n  auto last           = some_numbers.rbegin();\n  auto second_to_last = std::next(last);\n\n  \/\/ Now convert the second to last iterator to an iterator erase can use\n  auto pos = std::next(second_to_last).base();\n  some_numbers.erase(pos);\n  print_contents(some_numbers);\n}\n\nvoid print_contents(const std::vector&lt;int>&amp; items)\n{\n  for (const auto&amp; item : items)\n    std::cout &lt;&lt; item &lt;&lt; \"\\n\";\n}<\/textarea><\/pre><svg xmlns=\"http:\/\/www.w3.org\/2000\/svg\" style=\"width:24px;height:24px\" fill=\"none\" viewBox=\"0 0 24 24\" stroke=\"currentColor\" stroke-width=\"2\"><path class=\"with-check\" stroke-linecap=\"round\" stroke-linejoin=\"round\" d=\"M9 5H7a2 2 0 00-2 2v12a2 2 0 002 2h10a2 2 0 002-2V7a2 2 0 00-2-2h-2M9 5a2 2 0 002 2h2a2 2 0 002-2M9 5a2 2 0 012-2h2a2 2 0 012 2m-6 9l2 2 4-4\"><\/path><path class=\"without-check\" stroke-linecap=\"round\" stroke-linejoin=\"round\" d=\"M9 5H7a2 2 0 00-2 2v12a2 2 0 002 2h10a2 2 0 002-2V7a2 2 0 00-2-2h-2M9 5a2 2 0 002 2h2a2 2 0 002-2M9 5a2 2 0 012-2h2a2 2 0 012 2\"><\/path><\/svg><\/span><pre class=\"shiki dark-plus\" style=\"background-color: #1E1E1E\" tabindex=\"0\"><code><span class=\"line\"><span style=\"color: #C586C0\">#include<\/span><span style=\"color: #569CD6\"> <\/span><span style=\"color: #CE9178\">&lt;iostream&gt;<\/span><\/span>\n<span class=\"line\"><span style=\"color: #C586C0\">#include<\/span><span style=\"color: #569CD6\"> <\/span><span style=\"color: #CE9178\">&lt;vector&gt;<\/span><\/span>\n<span class=\"line\"><\/span>\n<span class=\"line\"><span style=\"color: #569CD6\">void<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">print_contents<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #569CD6\">const<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #4EC9B0\">std<\/span><span style=\"color: #D4D4D4\">::<\/span><span style=\"color: #4EC9B0\">vector<\/span><span style=\"color: #D4D4D4\">&lt;<\/span><span style=\"color: #569CD6\">int<\/span><span style=\"color: #D4D4D4\">&gt;<\/span><span style=\"color: #569CD6\">&amp;<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #9CDCFE\">items<\/span><span style=\"color: #D4D4D4\">);<\/span><\/span>\n<span class=\"line\"><\/span>\n<span class=\"line\"><span style=\"color: #569CD6\">int<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">main<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #569CD6\">int<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #569CD6\">char**<\/span><span style=\"color: #D4D4D4\">)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">{<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">  <\/span><span style=\"color: #569CD6\">auto<\/span><span style=\"color: #D4D4D4\"> some_numbers = <\/span><span style=\"color: #4EC9B0\">std<\/span><span style=\"color: #D4D4D4\">::vector&lt;<\/span><span style=\"color: #569CD6\">int<\/span><span style=\"color: #D4D4D4\">&gt;{<\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #B5CEA8\">2<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #B5CEA8\">3<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #B5CEA8\">4<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #B5CEA8\">5<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #B5CEA8\">6<\/span><span style=\"color: #D4D4D4\">};<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">  <\/span><span style=\"color: #DCDCAA\">print_contents<\/span><span style=\"color: #D4D4D4\">(some_numbers);<\/span><\/span>\n<span class=\"line\"><\/span>\n<span class=\"line\"><span style=\"color: #6A9955\">  \/\/ Get an iterator to the last item; then increment that for the second to last item iterator<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">  <\/span><span style=\"color: #569CD6\">auto<\/span><span style=\"color: #D4D4D4\"> last           = <\/span><span style=\"color: #9CDCFE\">some_numbers<\/span><span style=\"color: #D4D4D4\">.<\/span><span style=\"color: #DCDCAA\">rbegin<\/span><span style=\"color: #D4D4D4\">();<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">  <\/span><span style=\"color: #569CD6\">auto<\/span><span style=\"color: #D4D4D4\"> second_to_last = <\/span><span style=\"color: #4EC9B0\">std<\/span><span style=\"color: #D4D4D4\">::<\/span><span style=\"color: #DCDCAA\">next<\/span><span style=\"color: #D4D4D4\">(last);<\/span><\/span>\n<span class=\"line\"><\/span>\n<span class=\"line\"><span style=\"color: #6A9955\">  \/\/ Now convert the second to last iterator to an iterator erase can use<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">  <\/span><span style=\"color: #569CD6\">auto<\/span><span style=\"color: #D4D4D4\"> pos = <\/span><span style=\"color: #4EC9B0\">std<\/span><span style=\"color: #D4D4D4\">::<\/span><span style=\"color: #DCDCAA\">next<\/span><span style=\"color: #D4D4D4\">(second_to_last).<\/span><span style=\"color: #DCDCAA\">base<\/span><span style=\"color: #D4D4D4\">();<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">  <\/span><span style=\"color: #9CDCFE\">some_numbers<\/span><span style=\"color: #D4D4D4\">.<\/span><span style=\"color: #DCDCAA\">erase<\/span><span style=\"color: #D4D4D4\">(pos);<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">  <\/span><span style=\"color: #DCDCAA\">print_contents<\/span><span style=\"color: #D4D4D4\">(some_numbers);<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">}<\/span><\/span>\n<span class=\"line\"><\/span>\n<span class=\"line\"><span style=\"color: #569CD6\">void<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">print_contents<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #569CD6\">const<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #4EC9B0\">std<\/span><span style=\"color: #D4D4D4\">::<\/span><span style=\"color: #4EC9B0\">vector<\/span><span style=\"color: #D4D4D4\">&lt;<\/span><span style=\"color: #569CD6\">int<\/span><span style=\"color: #D4D4D4\">&gt;<\/span><span style=\"color: #569CD6\">&amp;<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #9CDCFE\">items<\/span><span style=\"color: #D4D4D4\">)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">{<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">  <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> (<\/span><span style=\"color: #569CD6\">const<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">auto<\/span><span style=\"color: #D4D4D4\">&amp; item : items)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">    <\/span><span style=\"color: #4EC9B0\">std<\/span><span style=\"color: #D4D4D4\">::cout &lt;&lt; item &lt;&lt; <\/span><span style=\"color: #CE9178\">&quot;<\/span><span style=\"color: #D7BA7D\">\\n<\/span><span style=\"color: #CE9178\">&quot;<\/span><span style=\"color: #D4D4D4\">;<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">}<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n<p><strong>Refs:<\/strong><\/p>\n\n\n\n<p><a href=\"https:\/\/www.cplusplus.com\/reference\/vector\/vector\/rbegin\/\" target=\"_blank\" rel=\"noreferrer noopener\" aria-label=\" (opens in a new tab)\">https:\/\/www.cplusplus.com\/reference\/vector\/vector\/rbegin\/<\/a><\/p>\n\n\n\n<p><a href=\"https:\/\/stackoverflow.com\/a\/1830240\" target=\"_blank\" rel=\"noreferrer noopener\" aria-label=\" (opens in a new tab)\">https:\/\/stackoverflow.com\/a\/1830240<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Once. Only once have I done this. I had to traverse a std::vector in reverse, and at some point, I had to call std::erase. I thought, &#8220;Ok, this should work&#8221; and instead, something bad happened. I little digging around, and a stackoverflow.com answer later, I saw an answer that left me scratching my head. The &#8230; <a title=\"C++ std::erase and rbegin\/rend\" class=\"read-more\" href=\"https:\/\/jaimerios.com\/?p=1788\" aria-label=\"Read more about C++ std::erase and rbegin\/rend\">Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3],"tags":[143],"class_list":["post-1788","post","type-post","status-publish","format-standard","hentry","category-coding","tag-cpp"],"_links":{"self":[{"href":"https:\/\/jaimerios.com\/index.php?rest_route=\/wp\/v2\/posts\/1788","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/jaimerios.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/jaimerios.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/jaimerios.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/jaimerios.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1788"}],"version-history":[{"count":1,"href":"https:\/\/jaimerios.com\/index.php?rest_route=\/wp\/v2\/posts\/1788\/revisions"}],"predecessor-version":[{"id":1885,"href":"https:\/\/jaimerios.com\/index.php?rest_route=\/wp\/v2\/posts\/1788\/revisions\/1885"}],"wp:attachment":[{"href":"https:\/\/jaimerios.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1788"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/jaimerios.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1788"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/jaimerios.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1788"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}