{"id":104325,"date":"2020-10-01T07:00:00","date_gmt":"2020-10-01T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=104325"},"modified":"2020-10-01T07:22:15","modified_gmt":"2020-10-01T14:22:15","slug":"20201001-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20201001-00\/?p=104325","title":{"rendered":"Bonus operations for C++\/WinRT iterators: The IIterable<T>, and C++\/WinRT collections"},"content":{"rendered":"

Last time, we saw that C++\/WinRT provides a few bonus operators to the IIterator<T><\/code><\/a> in order to make it work a little bit more smoothly with the C++ standard library.<\/p>\n

Today we’re going to look at IIterable<T><\/code>, which is an interface that says “You can get an iterator from me.”<\/p>\n

These Windows Runtime interfaces correspond to analogous concepts in many programming languages.<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n
C++\/WinRT<\/td>\nIIterable<T><\/tt><\/td>\nIIterator<T><\/tt><\/td>\n<\/tr>\n
iterator = o.First();<\/tt>
\nfor (auto&& v : o) ...<\/tt><\/td>\n<\/tr>\n
C++<\/td>\nbegin<\/tt>\/end<\/tt><\/td>\niterator<\/tt><\/td>\n<\/tr>\n
iterator = begin(o);<\/tt>
\nfor (auto&& v : o) ...<\/tt><\/td>\n<\/tr>\n
C#<\/td>\nIEnumerable<T><\/tt><\/td>\nIEnumerator<T><\/tt><\/td>\n<\/tr>\n
enumerator = o.GetEnumerator();<\/tt>
\nforeach (var v in o) ...<\/tt><\/td>\n<\/tr>\n
Java<\/td>\nIterable<T><\/tt><\/td>\nIterator<T><\/tt><\/td>\n<\/tr>\n
iterator = enumerable.iterator();<\/tt>
\nfor (var v : o) ...<\/tt><\/td>\n<\/tr>\n
JavaScript<\/td>\n@@iterator<\/tt><\/td>\n(unnamed)<\/td>\n<\/tr>\n
iterator = o[Symbol.iterator]();<\/tt>
\nfor (v in o) ...<\/tt><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

As I noted in the table above, these iterators are designed primarily for use by ranged for statements.<\/p>\n

for (auto&& value : collection)\r\n{\r\n    \/* do something with value *\/\r\n}\r\n<\/pre>\n
They can also be used in more general algorithms:<\/p>\n
std::vector<int> to_vector(IIterable<int> const& collection)\r\n{\r\n  std::vector<int> v;\r\n  std::copy(begin(collection), end(collection), std::back_inserter(v));\r\n  return v;\r\n}\r\n<\/pre>\n
Here’s a peek behind the scenes: For collections which support a GetAt<\/code> method (such as IVector<\/code>, IVectorView<\/code>, and IBindableVector<\/code>), this is implemented by an internal fast_iterator<\/code>, and the expansion of the ranged for loop comes out like this:<\/p>\n
auto&& range = collection;\r\nauto size = range.Size();\r\nfor (uint32_t index = 0; index < size; ++index)\r\n{\r\n  auto&& value = range.GetAt(index);\r\n  \/* do something with value *\/\r\n}\r\n<\/pre>\n
The temporary range<\/code> is part of the ranged for statement. There are some pre-existing subtleties here,  which I leave you to learn about<\/a>.<\/p>\n
For collections which are not indexable, but which are nevertheless iterable, the code falls back to the traditional Iterator<\/code>-based loop:<\/p>\n
for (auto iterator = as_cpp_iterator(collection.First()); iterator; ++iterator)\r\n{\r\n  auto&& value = *iterator;\r\n  \/* do something with value *\/\r\n}\r\n<\/pre>\n
That version takes advantage of iterator overloads we saw last time.<\/p>\n
But wait, we’re not done yet. There’s a little gotcha here that we’ll look at next time.<\/p>\n","protected":false},"excerpt":{"rendered":"
Stepping through a collection.<\/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-104325","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Stepping through a collection.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/104325","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=104325"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/104325\/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=104325"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=104325"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=104325"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}