std::sentinel_for(3)

std::sentinel_for 💅

okay bestie, this is the C++ Standard Libary page for std::sentinel_for, and it is literally giving “concept that defines the relationship between an iterator and a sentinel” energy. no cap, she matters for ranges. ✨

👑 NAME 👑

std::sentinel_for — the concept herself, fr fr.

🌸 Synopsis 🌸

Defined in header <iterator>.

template< class S, class I >

    concept sentinel_for =
        std::semiregular<S> &&                 (since C++20)
        std::input_or_output_iterator<I> &&

        __WeaklyEqualityComparableWith<S, I>;

So here’s the tea, bestie: the sentinel_for concept specifies the relationship between an input_or_output_iterator type and a semiregular type whose values denote a range. 🫶

The exposition-only concept __WeaklyEqualityComparableWith is described in equality_comparable. iykyk.

🎀 Semantic requirements 🎀

Let s and i be values of type S and I, respectively, such that [i, s) denotes a range. sentinel_for<S, I> is modeled only if:

• i == s is well-defined.
• If bool(i != s) then i is dereferenceable and [++i, s) denotes a range.
• std::assignable_from<I&, S> is either modeled or not satisfied.

The domain of == can change over time. Given an iterator i and sentinel s such that [i, s) denotes a range and i != s, [i, s) is not required to continue to denote a range after incrementing any iterator equal to i (and so i == s is no longer required to be well-defined after such an increment). 🤯 that’s the plot twist, bestie.

💖 Notes 💖

• A sentinel type and its corresponding iterator type are not required to model equality_comparable_with, because the sentinel type may not be comparable with itself, and they are not required to have a common reference type. periodt.
• It has been permitted to use a sentinel type different from the iterator type in the range-based for loop since C++17. (iykyk)

🦋 Defect reports 🦋

The following behavior-changing defect reports were applied retroactively to previously published C++ standards. not me keeping receipts 📝

• DR LWG 3453
  • Applied to: C++20
  • Behavior as published: semantic requirements for sentinel_for were too loose for ranges::advance
  • Correct behavior: strengthened

source/version and date, bestie: http://cppreference.com, 2024.06.10. byeee, go model some ranges responsibly 🧚💜