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<\/p>\nThe algorithms of the ranges library are lazy, can work directly on the container, and can easily be composed. But they have more to offer: projections. A projection is a mapping of a set into a subset. Let me show you in this post what that means:<\/p>\n
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I ended my last post, “The Ranges Libary in C++20: More Details<\/a>” with a comparison of <\/p>\n In contrast, the second overload does not return an Iterator I, but a A projection is a mapping of a set into a subset. What does this mean?<\/p>\n std::sort<\/code><\/a> and std::ranges::sort<\/code>. Here are the two overloads of std::ranges::sort<\/code>:<\/p>\n\n \ntemplate<\/span> <<\/span>std::<\/span>random_access_iterator I, std::<\/span>sentinel_for<<\/span>I><\/span> S,\n class<\/span> Comp<\/span> =<\/span> ranges::<\/span>less, class<\/span> Proj<\/span> =<\/span> std::<\/span>identity><\/span>\nrequires std::<\/span>sortable<<\/span>I, Comp, Proj><\/span>\nconstexpr I sort(I first, S last, Comp comp =<\/span> {}, Proj proj =<\/span> {});\n\ntemplate<\/span> <<\/span>ranges::<\/span>random_access_range R, class<\/span> Comp<\/span> =<\/span> ranges::<\/span>less, \n class<\/span> Proj<\/span> =<\/span> std::<\/span>identity><\/span>\nrequires std::<\/span>sortable<<\/span>ranges::<\/span>iterator_t<\/span><<\/span>R><\/span>, Comp, Proj><\/span>\nconstexpr ranges::<\/span>borrowed_iterator_t<\/span><<\/span>R><\/span> sort(R&&<\/span> r, Comp comp =<\/span> {}, Proj proj =<\/span> {});\n<\/pre>\n<\/div>\n\nWhen you study the first overload, you notice that it takes a sortable range R<\/code>, a predicate Comp<\/code>, and a projection Proj<\/code>. The predicate Comp<\/code> uses for default ranges::less<\/a>, and the projection Proj<\/code> the identity std::identity<\/a> that does return its arguments unchanged. std::identity<\/code> have been added with C++20, a function object defined in the header <functional<\/a>>.<\/div>\n<\/div>\nIn short, here are the components:<\/div>\n<\/div>\n\n
Comp <\/code>(binary functions that return a boolean)
\n<\/code><\/li>\nProj <\/code>(mapping of a set into a subset)
\n<\/code><\/li>\n std::sentinel_for<I><\/code> (a unique value that indicates\u00a0 the end of a sequence)
\n<\/code><\/li>\nstd::random_access_iterator, std::sortable<I, Comp, Proj><\/code>, and std::sentinel_for<I><\/code><\/li>\n<\/ul>\n ranges::borrowed_iterator_t<\/code>. Of course, this is also a concept and guarantees that the returned iterator is safe to use afterward. Consequentially, we call this iterator a safe iterator. I will write more about std::ranges::borrowed_iterator_t<\/code> in an upcoming post.<\/p>\nProjection<\/h2>\n
![\"rangeProjection\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2022\/05\/rangeProjection.png\")
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