Projections and Custom Views

Projections: transform for algorithms

Most range algorithms accept a final projection — a callable applied to elements before the algorithm sees them:

std::vector<Student> roster = ...;
std::ranges::sort(roster, std::ranges::less{}, &Student::score);   // sort by score
auto oldest = std::ranges::max_element(roster, {}, &Student::age);

No copy, no pre-transform pass, no separate key vector. {} is the default comparator; the projection goes last.

The view interface — what "custom" means

A minimal view needs: begin/end (iterators), and empty/size when cheaply available. The practical recipe: store a std::ranges::range reference + your parameters, expose an iterator whose operator++ and operator* implement your transformation, and mark the wrapper std::ranges::view-compatible by inheriting view semantics (or simply deriving from std::ranges::view_interface).

template <std::ranges::view V>
class SquaredView : public std::ranges::view_interface<SquaredView<V>> {
    V base_;
public:
    SquaredView() = default;
    explicit SquaredView(V v) : base_(std::move(v)) {}
    auto begin() const { return std::ranges::begin(base_); }   // simplified: transform iterator
    auto end() const   { return std::ranges::end(base_); }
};

The real custom view uses std::views::transform-style iterator adaptation; the graded exercise instead has you build a stride view (every K-th element) with a hand-written iterator — small enough to finish, complete enough to teach the machinery.