std::ranges::pop_heap() algorithm

od C++20

Simplified

// (1)
constexpr I
    pop_heap( I first, S last, Comp comp = {}, Proj proj = {} );

// (2)
constexpr ranges::borrowed_iterator_t<R>
    pop_heap( R&& r, Comp comp = {}, Proj proj = {} );

The type of arguments are generic and have the following constraints:

• I - std::random_access_iterator
• S - std::sentinel_for<I>
• R - std::ranges::random_access_range
• Comp - (none)
• Proj - (none)

The Proj and Comp template arguments have the following default types: std::identity, ranges::less for all overloads.

Additionally, each overload has the following constraints:

• (1) - std::sortable<I, Comp, Proj>
• (2) - std::sortable<ranges::iterator_t<R>, Comp, Proj>

Detailed

// (1)
template<
  std::random_access_iterator I,
  std::sentinel_for<I> S,
  class Comp = ranges::less,
  class Proj = std::identity
>
  requires std::sortable<I, Comp, Proj>
constexpr I
    pop_heap( I first, S last, Comp comp = {}, Proj proj = {} );

// (2)
template<
  ranges::random_access_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>
    pop_heap( R&& r, Comp comp = {}, Proj proj = {} );

Swaps the value in the position first and the value in the position last - 1 and makes the subrange [first; last - 1) into a max heap.
This has the effect of removing the first element from the heap defined by the range [first; last).

• (1) Elements are compared using the given binary comparison function comp and projection object proj.

• (2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are niebloids.

Parameters

first last	The range of elements defining the valid nonempty heap to modify.
r	The range of elements defining the valid nonempty heap to modify.
pred	Predicate to apply to the projected elements.
proj	The projection to apply to the elements.

Return value

An iterator equal to last.

Complexity

Given N ranges::distance(first, last):

At most 2 * log(N) comparisons and 4 * log(N) projections.

Exceptions

(none)

Notes

A max heap is a range of elements [f; l), arranged with respect to comparator comp and projection proj, that has the following properties:

• Given N as l - f, p = f[(i - 1) / 2], and q = f[i], for all 0 < i < N, the expression std::invoke(comp, std::invoke(proj, p), std::invoke(proj, q)) evaluates to false.
• A new element can be added using ranges::push_heap, in O(log(N)) time.
• The first element can be removed using ranges::pop_heap, in O(log(N)) time.

Examples

Main.cpp

#include <algorithm>
#include <array>
#include <iostream>
#include <iterator>
#include <string_view>

template<class I = int*>
void print(std::string_view rem, I first = {}, I last = {},
           std::string_view term = "\n")
{
    for (std::cout << rem; first != last; ++first)
        std::cout << *first << ' ';
    std::cout << term;
}

int main()
{
    std::array v {3, 1, 4, 1, 5, 9, 2, 6, 5, 3};
    print("initially, v: ", v.cbegin(), v.cend());

    std::ranges::make_heap(v);
    print("make_heap, v: ", v.cbegin(), v.cend());

    print("convert heap into sorted array:");
    for (auto n {std::ssize(v)}; n >= 0; --n)
    {
        std::ranges::pop_heap(v.begin(), v.begin() + n);
        print("[ ", v.cbegin(), v.cbegin() + n, "]  ");
        print("[ ", v.cbegin() + n, v.cend(), "]\n");
    }
}

Possible Output

initially, v: 3 1 4 1 5 9 2 6 5 3
make_heap, v: 9 6 4 5 5 3 2 1 1 3
convert heap into sorted array:
[ 6 5 4 3 5 3 2 1 1 9 ]  [ ]
[ 5 5 4 3 1 3 2 1 6 ]  [ 9 ]
[ 5 3 4 1 1 3 2 5 ]  [ 6 9 ]
[ 4 3 3 1 1 2 5 ]  [ 5 6 9 ]
[ 3 2 3 1 1 4 ]  [ 5 5 6 9 ]
[ 3 2 1 1 3 ]  [ 4 5 5 6 9 ]
[ 2 1 1 3 ]  [ 3 4 5 5 6 9 ]
[ 1 1 2 ]  [ 3 3 4 5 5 6 9 ]
[ 1 1 ]  [ 2 3 3 4 5 5 6 9 ]
[ 1 ]  [ 1 2 3 3 4 5 5 6 9 ]
[ ]  [ 1 1 2 3 3 4 5 5 6 9 ]