std::shift_left() algorithm
- since C++20
// (1)
template< class ForwardIt >
constexpr ForwardIt
shift_left( ForwardIt first, ForwardIt last,
typename std::iterator_traits<ForwardIt>::difference_type n );
// (2)
template< class ExecutionPolicy, class ForwardIt >
ForwardIt
shift_left( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last,
typename std::iterator_traits<ForwardIt>::difference_type n );
-
(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 positionfirst + n + i
to positionfirst + i
. The moves are performed in increasing order ofi
starting from0
. - if
-
(2) Same as (1), but executed according to
policy
.
These overloads participate in overload resolution only if std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
is true
.
Parameters
first last | The original range. |
n | The number of positions to shift. |
policy | The execution policy to use. See execution policy for details. |
Type requirements
ForwardIt | LegacyForwardIterator |
The type of dereferenced ForwardIt
must meet the requirements of MoveAssignable.
Return value
The end of the resulting range:
- If
n
is less thanlast - first
, returnsfirst + (last - first - n)
. - Otherwise, returns
first
.
Complexity
At most std::distance(first, last) - n
assignments.
Exceptions
The overloads with a template parameter named ExecutionPolicy
report errors as follows:
- If execution of a function invoked as part of the algorithm throws an exception and
ExecutionPolicy
is one of the standard policies,std::terminate
is called. For none otherExecutionPolicy
, the behavior is implementation-defined. - If the algorithm fails to allocate memory,
std::bad_alloc
is thrown.
Examples
#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::shift_left(begin(a), end(a), 3);
std::shift_left(begin(b), end(b), 3);
std::shift_left(begin(c), end(c), 3);
std::cout << a << " " << b << " " << c << '\n';
std::shift_right(begin(a), end(a), 2);
std::shift_right(begin(b), end(b), 2);
std::shift_right(begin(c), end(c), 2);
std::cout << a << " " << b << " " << c << '\n';
std::shift_left(begin(a), end(a), 8); // has no effect: n >= last - first
std::shift_left(begin(b), end(b), 8); // ditto
std::shift_left(begin(c), end(c), 8); // ditto
std::cout << a << " " << b << " " << c << '\n';
// std::shift_left(begin(a), end(a), -3); // UB, e.g. segfault.)
}
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 .
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