How to make join view in C++ preserving random access?

Question

The code below breaks at last line as joined view v isn't a random access range.

vector<string> v0 = {"word","good","best","good"};
auto it = v0.begin() + 1;
auto r1 = ranges::subrange(v0.begin(), it);
auto r2 = ranges::subrange(it + 1, v0.end());
auto rr = {r1,r2};
auto v = views::join(rr);
auto w1 = v.begin() + 1;

r1 and r2 keep this quality. Is it possible to join two subranges, preserving random access? Is there any workaround? Basically, I need a view of a vector with a specific element excluded, with the random accessible result.

康桓瑋 · Accepted Answer · 2025-04-26 03:35:04Z

9

In c++26 you can use views::concat which supports random access:

auto r1 = std::ranges::subrange(v0.begin(), it);
auto r2 = std::ranges::subrange(it + 1, v0.end());
auto v = std::views::concat(r1, r2);
auto w1 = v.begin() + 1;
std::println("{}", *w1);  // print best
std::println("{}", v[1]); // print best

Demo

edited Apr 26 at 3:35

answered Apr 26 at 3:32

康桓瑋

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3 Comments

ephemerr Apr 26 at 3:34

Is there c++20 solution?

康桓瑋 Apr 26 at 3:38

@ephemerr I don't think join_view can make random access in amortized constant time, so nope. What you want to do is basically what views::concat does, so maybe you could use the range/v3 version of that.

Dominik Kaszewski Apr 26 at 16:07

@ephemerr Should be quite easy to backport it, especially if you only want to support the random access range. Store begin/end of both ranges in the view, and in the iterator having a reference to view just calculate when you switch from the first to the second.

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