{"id":102595,"date":"2019-06-17T07:00:00","date_gmt":"2019-06-17T14:00:00","guid":{"rendered":"http:\/\/devblogs.microsoft.com\/oldnewthing\/?p=102595"},"modified":"2019-06-16T21:28:07","modified_gmt":"2019-06-17T04:28:07","slug":"20190617-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20190617-00\/?p=102595","title":{"rendered":"A simple workaround for the fact that std::equal<\/CODE> takes its predicate by value"},"content":{"rendered":"

The versions of the std::equal<\/code> function that takes a binary predicate accepts the predicate by value, which means that if you are using a functor, it will be copied, which may be unnecessary or unwanted.<\/p>\n

In my case, the functor had a lot of state, and I didn’t want to copy it.<\/p>\n

class comparer\r\n{\r\n  ...\r\n\r\n  template<typename R>\r\n  bool ranges_equiv(R const& left, R const& right)\r\n  {\r\n    using T = typename std::decay_t<decltype(*begin(left))>;\r\n    return std::equal(\r\n      begin(left), end(left),\r\n      begin(right), end(right),\r\n      equiv<T>);\r\n  }\r\n\r\n  template<typename T>\r\n  bool equiv(T const& left, T const& right) = delete;\r\n\r\n  template<>\r\n  bool equiv(Doodad const& left, Doodad const& right)\r\n  {\r\n    return (!check_names || equiv(left.Name(), right.Name())) &&\r\n           (!check_children || ranges_equiv(left.Children(), right.Children()));\r\n  }\r\n\r\n  ... other overloads omitted ...\r\n};\r\n<\/pre>\n
The idea behind the comparer<\/code> is that you configure it with information about what you care about and what you don’t, and then you call equiv<\/code> and let it walk the object hierarchy comparing the things you asked for according to the rules you specified.<\/p>\n
This works great, except that std::equal<\/code> copies its predicate, and our comparer<\/code> is somewhat expensive to copy, since it may have lots of configuration std::string<\/code>s and stuff. What we’re looking for is a version that takes the predicate by reference, so that we can use the same comparer<\/code> all the way down.<\/p>\n
The workaround is to replace the predicate with something that is cheap to copy.<\/p>\n
  template<typename R>\r\n  bool ranges_equiv(R const& left, R const& right)\r\n  {\r\n    return std::equal(\r\n      begin(left), end(left),\r\n      begin(right), end(right),\r\n      [this](auto&& l, auto&& r) { return equiv(l, r); }<\/span>);\r\n  }\r\n<\/pre>\n
Instead of passing a full comparer<\/code> object, we pass a lambda that captures the comparer<\/code>‘s this<\/code> pointer. This lambda is cheap to copy, and it allows us to reuse the same comparer<\/code> all the way down the object hierarchy.<\/p>\n
This solution looks obvious in retrospect, but I got all hung up trying to create a cheap copyable object, like a nested type called compare_forwarder<\/code> that kept a std::reference_wrapper<\/code> to the comparer<\/code>, before realizing that I was just writing a verbose version of a lambda.<\/p>\n
 <\/p>\n","protected":false},"excerpt":{"rendered":"
Pass something else that is easy to copy.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-102595","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Pass something else that is easy to copy.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/102595","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=102595"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/102595\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=102595"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=102595"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=102595"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}