std::ranges::shift_left() algorithm

od C++20

Simplified

// (1)
constexpr ranges::subrange<I>
    shift_left( I first, S last, std::iter_difference_t<I> n );

// (2)
constexpr ranges::borrowed_subrange_t<R>
    shift_left( R&& r, ranges::range_difference_t<R> n );

The type of arguments are generic and have the following constraints:

• I - std::permutable
• S - std::sentinel_for<I>
• R - std::ranges::forward_range

Additionally, each overload has the following constraints:

• (2) - std::permutable<ranges::iterator_t<R>>

(The std:: namespace was ommitted here for readability)

Detailed

// (1)
template<
  std::permutable I,
  std::sentinel_for<I> S
>
constexpr ranges::subrange<I>
    shift_left( I first, S last, std::iter_difference_t<I> n );

// (2)
template<
  ranges::forward_range R
>
  requires std::permutable<ranges::iterator_t<R>>
constexpr ranges::borrowed_subrange_t<R>
    shift_left( R&& r, ranges::range_difference_t<R> n );

Shifts the elements in the range [first; last) or r by n positions.

• (1) Shifts the elements towards the beginning of the range:

  • If n == 0 || n >= last - first, there are no effects.
  • If n < 0, the behavior is undefined.
  • Otherwise, for every integer i in [0; last - first - n), moves the element originally at position first + n + i to position first + i. The moves are performed in increasing order of i starting from 0.

• ** (2)** Same as (1), but uses r as the range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are niebloids.

Undefined Behaviour

The behavior is undefined

if [first; last) is not a valid range.

Parameters

first last	The original range.
r	The range of elements to shift.
n	The number of positions to shift.

Return value

{
  first,
  /*NEW_LAST*/
}

Where NEW_LAST is the end of the resulting range and equivalent to:

• If n is less than last - first, first + (last - first - n).
• Otherwise, first.

Complexity

At most ranges::distance(first, last) - n assignments.

Exceptions

(none)

Possible implementation

shift_left(1) and shift_left(2)

struct shift_left_fn
{
    template<std::bidirectional_iterator I, std::sentinel_for<I> S>
		requires std::permutable<I>
    constexpr I operator()(I first, S last) const
    {
        auto last2 {ranges::next(first, last)};
        for (auto tail {last2}; !(first == tail or first == --tail); ++first)
            ranges::iter_swap(first, tail);
        return last2;
    }

    template<ranges::bidirectional_range R>
		requires std::permutable<ranges::iterator_t<R>>
    constexpr ranges::borrowed_iterator_t<R>
        operator()(R&& r) const
    {
        return (*this)(ranges::begin(r), ranges::end(r));
    }
};

inline constexpr shift_left_fn shift_left {};

Notes

ranges::shift_left / ranges::shift_right has better efficiency on common implementations if I models bidirectional_iterator or random_access_iterator.

Implementations (e.g. MSVC STL) may enable vectorization when the iterator type models contiguous_iterator and swapping its value type calls neither non-trivial special member function nor ADL-found swap.

Examples

Main.cpp

#include <algorithm>
#include <iostream>
#include <string>
#include <type_traits>
#include <vector>

struct S
{
    int value {0};
    bool specified_state {true};

    S(int v = 0) : value {v} {}
    S(S const& rhs) = default;
    S(S&& rhs) { *this = std::move(rhs); }
    S& operator=(S const& rhs) = default;
    S& operator=(S&& rhs)
    {
        if (this != &rhs)
        {
            value = rhs.value;
            specified_state = rhs.specified_state;
            rhs.specified_state = false;
        }
        return *this;
    }
};

template<typename T>
std::ostream& operator<<(std::ostream& os, std::vector<T> const& v)
{
    for (const auto& s : v)
    {
        if constexpr (std::is_same_v<T, S>)
            s.specified_state ? os << s.value << ' ' : os << ". ";
        else if constexpr (std::is_same_v<T, std::string>)
            os << (s.empty() ? "." : s) << ' ';
        else
            os << s << ' ';
    }
    return os;
}

int main()
{
    std::cout << std::left;

    std::vector<S> a {1, 2, 3, 4, 5, 6, 7};
    std::vector<int> b {1, 2, 3, 4, 5, 6, 7};
    std::vector<std::string> c {"α", "β", "γ", "δ", "ε", "ζ", "η"};

    std::cout << "vector<S> \tvector<int> \tvector<string>\n";
    std::cout << a << "  " << b << "  " << c << '\n';

    std::ranges::shift_left(a, 3);
    std::ranges::shift_left(b, 3);
    std::ranges::shift_left(c, 3);
    std::cout << a << "  " << b << "  " << c << '\n';

    std::ranges::shift_right(a, 2);
    std::ranges::shift_right(b, 2);
    std::ranges::shift_right(c, 2);
    std::cout << a << "  " << b << "  " << c << '\n';

    std::ranges::shift_left(a, 8);  // has no effect: n >= last - first
    std::ranges::shift_left(b, 8);  // ditto
    std::ranges::shift_left(c, 8);  // ditto
    std::cout << a << "  " << b << "  " << c << '\n';

//  std::ranges::shift_left(a, -3);  // UB
}

Output

vector<S>       vector<int>     vector<string>
1 2 3 4 5 6 7   1 2 3 4 5 6 7   α β γ δ ε ζ η
4 5 6 7 . . .   4 5 6 7 5 6 7   δ ε ζ η . . .
. . 4 5 6 7 .   4 5 4 5 6 7 5   . . δ ε ζ η .
. . 4 5 6 7 .   4 5 4 5 6 7 5   . . δ ε ζ η .