NAME

std::move_backward - std::move_backward

Synopsis

Defined in header <algorithm>
template< class BidirIt1, class BidirIt2 > (since C++11)
BidirIt2 move_backward( BidirIt1 first, BidirIt1 last, BidirIt2 (until C++20)
d_last );
template< class BidirIt1, class BidirIt2 >
constexpr BidirIt2 move_backward( BidirIt1 first, BidirIt1 last, (since C++20)
BidirIt2 d_last );

Moves the elements from the range [first, last), to another range ending at d_last.
The elements are moved in reverse order (the last element is moved first), but their
relative order is preserved.

The behavior is undefined if d_last is within (first, last]. std::move must be used
instead of std::move_backward in that case.

Parameters

first, last - the range of the elements to move
d_last - end of the destination range

Type requirements

-
BidirIt1, BidirIt2 must meet the requirements of LegacyBidirectionalIterator.

Return value

Iterator in the destination range, pointing at the last element moved.

Complexity

Exactly last - first move assignments.

Possible implementation

template< class BidirIt1, class BidirIt2 >
BidirIt2 move_backward(BidirIt1 first, BidirIt1 last,
BidirIt2 d_last)
{
while (first != last) {
*(--d_last) = std::move(*(--last));
}
return d_last;
}

Notes

When moving overlapping ranges, std::move is appropriate when moving to the left
(beginning of the destination range is outside the source range) while
std::move_backward is appropriate when moving to the right (end of the destination
range is outside the source range).

Example

// Run this code

#include <algorithm>
#include <iostream>
#include <iterator>
#include <string>
#include <string_view>
#include <vector>

using container = std::vector<std::string>;

void print(std::string_view comment, const container& src, const container& dst = {})
{
auto prn = [](std::string_view name, const container& cont) {
std::cout << name;
for (const auto &s: cont) { std::cout << (s.empty() ? "∙" : s.data()) << ' '; }
std::cout << '\n';
};
std::cout << comment << '\n';
prn("src: ", src);
if (dst.empty()) return;
prn("dst: ", dst);
}

int main()
{
container src{"foo", "bar", "baz"};
container dst{"qux", "quux", "quuz", "corge"};
print("Non-overlapping case; before move_backward:", src, dst);
std::move_backward(src.begin(), src.end(), dst.end());
print("After:", src, dst);

src = {"snap", "crackle", "pop", "lock", "drop"};
print("Overlapping case; before move_backward:", src);
std::move_backward(src.begin(), std::next(src.begin(), 3), src.end());
print("After:", src);
}

Output:

Non-overlapping case; before move_backward:
src: foo bar baz
dst: qux quux quuz corge
After:
src: ∙ ∙ ∙
dst: qux foo bar baz
Overlapping case; before move_backward:
src: snap crackle pop lock drop
After:
src: ∙ ∙ snap crackle pop