std::ranges::shift_left, std::ranges::shift_right - cppreference.com (original) (raw)

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​.

1. 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 algorithm function objects (informally known as 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.

[edit] Parameters

[edit] Return value

1,2) {first, /*NEW_LAST*/}, where _NEWLAST_ 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 _NEWFIRST_ is the beginning of the resulting range and equivalent to:

• first + n, if n is less than last - first;
• last otherwise.

[edit] Complexity

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

[edit] 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.

[edit] Example

#include #include #include #include #include   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 std::ostream& operator<<(std::ostream& os, std::vector 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 a{1, 2, 3, 4, 5, 6, 7}; std::vector b{1, 2, 3, 4, 5, 6, 7}; std::vector<std::string> c{"α", "β", "γ", "δ", "ε", "ζ", "η"};   std::cout << "vector \tvector \tvector\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 vector vector 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 .

[edit] See also