std::ranges::inplace_merge
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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template< std::bidirectional_iterator I, std::sentinel_for<I> S,
class Comp = ranges::less, class Proj = std::identity >
requires std::sortable<I, Comp, Proj>
I inplace_merge( I first, I middle, S last,
Comp comp = {}, Proj proj = {} );
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(1) | (since C++20) (constexpr since C++26) |
template< ranges::bidirectional_range R,
class Comp = ranges::less, class Proj = std::identity >
requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
ranges::borrowed_iterator_t<R>
inplace_merge( R&& r, ranges::iterator_t<R> middle,
Comp comp = {}, Proj proj = {} );
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(2) | (since C++20) (constexpr since C++26) |
template< /*execution-policy*/ Ep,
std::random_access_iterator I, std::sized_sentinel_for<I> S,
class Comp = ranges::less, class Proj = std::identity >
requires std::sortable<I, Comp, Proj>
I inplace_merge( Ep&& policy, I first, I middle, S last,
Comp comp = {}, Proj proj = {} );
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(3) | (since C++26) |
template< /*execution-policy*/ Ep, /*sized-random-access-range*/ R,
class Comp = ranges::less, class Proj = std::identity >
requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
ranges::borrowed_iterator_t<R>
inplace_merge( Ep&& policy, R&& r, ranges::iterator_t<R> middle,
Comp comp = {}, Proj proj = {} );
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(4) | (since C++26) |
For the definition of /*execution-policy*/, see this page; for the definition of /*sized-random-access-range*/, see this page.
1) Makes the target range
[first, last) sorted by merging its two consecutive sorted subranges [first, middle) and [middle, last). For each group of equivalent elements in the sorted target range, any pair of elements from the group has the following order:
- If the two elements are from the same subrange, the order between them in the target range follows the original order between them in the subrange.
- Otherwise, the element from the first subrange is always placed before the element from the second subrange.
If any of the two subranges is not a valid range sorted with respect to the comparator
comp and projection proj, the behavior is undefined.2) Same as (1), but uses
ranges::begin(r) as first and ranges::end(r) as last.3) Same as (1), but executed according to
policy.4) Same as (3), but uses
ranges::begin(r) as first and ranges::begin(r) + ranges::distance(r) as last.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.
Parameters
| first | - | the beginning of the first subrange |
| middle | - | the end of the first subrange and the beginning of the second |
| last | - | the sentinel of the second subrange |
| r | - | the target range |
| comp | - | the comparator to be applied to the (projected) elements |
| proj | - | the projection to be applied to the elements |
| policy | - | the execution policy to use |
Return value
The past-the-end iterator of the target range.
Complexity
Given N as ranges::distance(first, last) or ranges::distance(r):
1,2) 𝓞(N⋅log(N)) applications of
comp, proj1 and proj2 (or only exactly N-1 applications if enough additional memory is available).3,4) 𝓞(N⋅log(N)) applications of
comp, proj1 and proj2.Notes
This function attempts to allocate a temporary buffer. If the allocation fails, the less efficient algorithm is chosen.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_constexpr_algorithms |
202306L |
(C++26) | constexpr stable sorting
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Possible implementation
This implementation only shows the slower algorithm used when no additional memory is available. See also the implementation in MSVC STL and libstdc++.
struct inplace_merge_fn
{
template<std::bidirectional_iterator I, std::sentinel_for<I> S,
class Comp = ranges::less, class Proj = std::identity>
requires std::sortable<I, Comp, Proj>
constexpr I operator()(I first, I middle, S last, Comp comp = {}, Proj proj = {}) const
{
I last_it = ranges::next(middle, last);
inplace_merge_slow(first, middle, last_it,
ranges::distance(first, middle),
ranges::distance(middle, last_it),
std::ref(comp), std::ref(proj));
return last_it;
}
template<ranges::bidirectional_range R,
class Comp = ranges::less, class Proj = std::identity>
requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
constexpr ranges::borrowed_iterator_t<R>
operator()(R&& r, ranges::iterator_t<R> middle,
Comp comp = {}, Proj proj = {}) const
{
return (*this)(ranges::begin(r), std::move(middle), ranges::end(r),
std::move(comp), std::move(proj));
}
private:
template<class I, class Comp, class Proj>
static constexpr void inplace_merge_slow(I first, I middle, I last,
std::iter_difference_t<I> n1,
std::iter_difference_t<I> n2,
Comp comp, Proj proj)
{
if (n1 == 0 || n2 == 0)
return;
if (n1 + n2 == 2 && comp(proj(*middle), proj(*first)))
{
ranges::iter_swap(first, middle);
return;
}
I cut1 = first, cut2 = middle;
std::iter_difference_t<I> d1{}, d2{};
if (n1 > n2)
{
d1 = n1 / 2;
ranges::advance(cut1, d1);
cut2 = ranges::lower_bound(middle, last, *cut1,
std::ref(comp), std::ref(proj));
d2 = ranges::distance(middle, cut2);
}
else
{
d2 = n2 / 2;
ranges::advance(cut2, d2);
cut1 = ranges::upper_bound(first, middle, *cut2,
std::ref(comp), std::ref(proj));
d1 = ranges::distance(first, cut1);
}
I new_middle = ranges::rotate(cut1, middle, cut2);
inplace_merge_slow(first, cut1, new_middle, d1, d2,
std::ref(comp), std::ref(proj));
inplace_merge_slow(new_middle, cut2, last, n1 - d1, n2 - d2,
std::ref(comp), std::ref(proj));
}
};
inline constexpr inplace_merge_fn inplace_merge{};
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Example
Run this code
#include <algorithm>
#include <complex>
#include <functional>
#include <iostream>
#include <iterator>
#include <vector>
void print(const auto& v, const auto& rem, int middle = -1)
{
for (int i{}; auto n : v)
std::cout << (i++ == middle ? "│ " : "") << n << ' ';
std::cout << rem << '\n';
}
template<std::random_access_iterator I, std::sentinel_for<I> S>
requires std::sortable<I>
void merge_sort(I first, S last)
{
if (last - first > 1)
{
I middle{first + (last - first) / 2};
merge_sort(first, middle);
merge_sort(middle, last);
std::ranges::inplace_merge(first, middle, last);
}
}
int main()
{
// custom merge-sort demo
std::vector v{8, 2, 0, 4, 9, 8, 1, 7, 3};
print(v, ": before sort");
merge_sort(v.begin(), v.end());
print(v, ": after sort\n");
// merging with comparison function object and projection
using CI = std::complex<int>;
std::vector<CI> r{{0,1}, {0,2}, {0,3}, {1,1}, {1,2}};
const auto middle{std::ranges::next(r.begin(), 3)};
auto comp{std::ranges::less{}};
auto proj{[](CI z) { return z.imag(); }};
print(r, ": before merge", middle - r.begin());
std::ranges::inplace_merge(r, middle, comp, proj);
print(r, ": after merge");
}
Output:
8 2 0 4 9 8 1 7 3 : before sort
0 1 2 3 4 7 8 8 9 : after sort
(0,1) (0,2) (0,3) │ (1,1) (1,2) : before merge
(0,1) (1,1) (0,2) (1,2) (0,3) : after merge