NAME¶

std::ranges::shift_left,std::ranges::shift_right - std::ranges::shift_left,std::ranges::shift_right

Synopsis¶

Defined in header <algorithm>
Call signature
template< std::permutable I, std::sentinel_for<I> S >

constexpr ranges::subrange<I> (1) (since C++23)

shift_left( I first, S last, std::iter_difference_t<I> n );
template< ranges::forward_range R >

requires std::permutable<ranges::iterator_t<R>> (2) (since C++23)
constexpr ranges::borrowed_subrange_t<R>

shift_left( R&& r, ranges::range_difference_t<R> n );
template< std::permutable I, std::sentinel_for<I> S >

constexpr ranges::subrange<I> (3) (since C++23)

shift_right( I first, S last, std::iter_difference_t<I> n );
template< ranges::forward_range R >

requires std::permutable<ranges::iterator_t<R>> (4) (since C++23)
constexpr ranges::borrowed_subrange_t<R>

shift_right( R&& r, ranges::range_difference_t<R> n );

Shifts the elements in the range [first, last) or r by n positions. The behavior is
undefined if [first, last) is not a valid range.

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.
3) Shifts the elements towards the end 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 + i to position first + n + i. If I models
bidirectional_iterator, then the moves are performed in decreasing order of i
starting from last - first - n - 1.
2,4) Same as (1) or (3) respectively, but uses r as the range, as if using
ranges::begin(r) as first and ranges::end(r) as last.

Elements that are in the original range but not the new range are left in a valid
but unspecified state.

The function-like entities described on this page are niebloids, that is:

* Explicit template argument lists cannot be specified when calling any of them.
* None of them are visible to argument-dependent lookup.
* When any of them are found by normal unqualified lookup as the name to the left
of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler
extensions.

Parameters¶

first - the beginning of the original range
last - the end of the original range
r - the range of elements to shift
n - the number of positions to shift

Return value¶

1,2) {first, /*NEW_LAST*/}, where NEW_LAST is the end of the resulting range and
equivalent to:
* first + (last - first - n), if n is less than last - first;
* first otherwise.
3,4) {/*NEW_FIRST*/, last}, where NEW_FIRST is the beginning of the resulting range
and equivalent to:
* first + n, if n is less than last - first;
* last otherwise.

Complexity¶

1,2) At most ranges::distance(first, last) - n assignments.
3,4) At most ranges::distance(first, last) - n assignment or swaps.

Notes¶

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

Feature-test macro Value Std Feature
__cpp_lib_shift 202202L (C++23) std::ranges::shift_left and
std::ranges::shift_right

Example¶

// Run this code

#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
}

Possible 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 .

See also¶

ranges::move moves a range of elements to a new location
(C++20) (niebloid)
ranges::move_backward moves a range of elements to a new location in backwards order
(C++20) (niebloid)
ranges::rotate rotates the order of elements in a range
(C++20) (niebloid)
shift_left shifts elements in a range
shift_right (function template)
(C++20)