--- title: C++26 Ranges: Parallel Algorithms and Safer Filters description: C++26 finally adds execution policies to range algorithms, closing a gap left since C++20. P3179R9 brings par, par_unseq, seq and unseq to over 50 range... canonical: https://daily.dev/posts/c-26-ranges-parallel-algorithms-and-safer-filters-qbzhzorjm twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: C++26 Ranges: Parallel Algorithms and Safer Filters | daily.dev og:description: C++26 finally adds execution policies to range algorithms, closing a gap left since C++20. P3179R9 brings par, par_unseq, seq and unseq to over 50 range... og:url: https://daily.dev/posts/c-26-ranges-parallel-algorithms-and-safer-filters-qbzhzorjm og:image: https://api.daily.dev/og/posts/qBZHZoRJM.png og:image:alt: C++26 Ranges: Parallel Algorithms and Safer Filters og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # C++26 Ranges: Parallel Algorithms and Safer Filters **[Sandor Dargo](https://daily.dev/sources/sandordargo)** · 6 min read · 4 upvotes · 0 comments ## Summary C++26 finally adds execution policies to range algorithms, closing a gap left since C++20. P3179R9 brings par, par_unseq, seq and unseq to over 50 range algorithms like sort, transform and find, but they require random_access_range and sized_range, so filter_view or unbounded views::iota can't be parallelized directly. Separately, P3725R3 fixes three filter_view foot-guns, including a crash when composing filter with reverse and undefined behavior when modifying elements through a filter iterator. The fix adds const begin()/end() for input ranges and recommends prepending views::to_input to turn runtime UB into compile-time errors. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why can't I use std::execution::par with a filtered range in C++26? Parallel range algorithms in C++26 require both random_access_range and sized_range, not just forward_range, because parallel backends need random access for divide-and-conquer decomposition and a known size to partition work. filter_view is at most bidirectional, so it doesn't qualify; you'd need to materialize it or constrain it first. _Developers planning a parallel ranges migration can track these constraints and gotchas on daily.dev._ ### Why does modifying an element through a C++20 filter_view iterator cause undefined behavior? Writing through a filter_view iterator is undefined behavior if the modification changes whether the predicate still holds for that element, since filter_view relies on the predicate remaining valid, even though the code may appear to work. C++26's P3725R3 relaxes this: it becomes well-defined as long as the predicate isn't re-evaluated on the modified element. _Teams hardening older ranges code against silent UB can follow fixes like this on daily.dev._ ### How do I fix the crash when composing views::filter with views::reverse in C++ ranges? Prepend views::to_input before the filter, as in 'v | views::to_input | views::filter(pred)'. This downgrades the range to input-only, which prevents reverse from compiling since it requires bidirectional access, turning the runtime undefined behavior into a compile-time error, and it also enables const iteration. _Developers debugging ranges UB can keep up with patterns like views::to_input on daily.dev._ ## Similar posts on daily.dev - [C\+\+26 Ranges: Four New Views](https://daily.dev/posts/c-26-ranges-four-new-views-rjvppig9p) · Sandor Dargo · 2 upvotes · 0 comments - [C\+\+26 Ranges: Better Infrastructure](https://daily.dev/posts/c-26-ranges-better-infrastructure-pj46vkcrd) · Sandor Dargo · 0 upvotes · 0 comments - [Structured iteration](https://daily.dev/posts/structured-iteration-hri4jxsmi) · C\+\+ · 3 upvotes · 1 comments - [Is a sized random-access range always common?](https://daily.dev/posts/is-a-sized-random-access-range-always-common--hzkertyrz) · Arthur O'Dwyer · 0 upvotes · 0 comments --- Tags: [#c++](https://daily.dev/tags/c++) [View this post on daily.dev](https://daily.dev/posts/c-26-ranges-parallel-algorithms-and-safer-filters-qbzhzorjm) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"C++26 Ranges: Parallel Algorithms and Safer Filters","url":"https://daily.dev/posts/c-26-ranges-parallel-algorithms-and-safer-filters-qbzhzorjm","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/c-26-ranges-parallel-algorithms-and-safer-filters-qbzhzorjm"},"datePublished":"2026-10-28T20:14:47.383Z","dateModified":"2026-10-28T21:35:56.102Z","description":"C++26 finally adds execution policies to range algorithms, closing a gap left since C++20. 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Developers planning a parallel ranges migration can track these constraints and gotchas on daily.dev."}},{"@type":"Question","name":"Why does modifying an element through a C++20 filter_view iterator cause undefined behavior?","acceptedAnswer":{"@type":"Answer","text":"Writing through a filter_view iterator is undefined behavior if the modification changes whether the predicate still holds for that element, since filter_view relies on the predicate remaining valid, even though the code may appear to work. C++26's P3725R3 relaxes this: it becomes well-defined as long as the predicate isn't re-evaluated on the modified element. Teams hardening older ranges code against silent UB can follow fixes like this on daily.dev."}},{"@type":"Question","name":"How do I fix the crash when composing views::filter with views::reverse in C++ ranges?","acceptedAnswer":{"@type":"Answer","text":"Prepend views::to_input before the filter, as in 'v | views::to_input | views::filter(pred)'. 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