std::pop_heap() algorithm

od C++20

template< class RandomIt >
constexpr void pop_heap( RandomIt first, RandomIt last );

// (2)
template< class RandomIt, class Compare >
constexpr void pop_heap( RandomIt first, RandomIt last, Compare comp );

do C++20

template< class RandomIt >
void pop_heap( RandomIt first, RandomIt last );

// (2)
template< class RandomIt, class Compare >
void pop_heap( RandomIt first, RandomIt last, Compare comp );

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

• (1) Elements are compared using operator<.

• (2) Elements are compared using the given binary comparison function comp.

Parameters

first last	The range of elements to make a heap from.
policy	The execution policy to use. See execution policy for details.
comp	Comparison function object (i.e. an object that satisfies the requirements of Compare). The signature of the comparison function should be equivalent to the following: bool cmp(const Type1 &a, const Type2 &b); The signature does not need to have const&, but must not modify arguments. Must accept all values of type (possibly const) Type and Type2, regardless of value category (so Type1& is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy (od C++11)) The types Type1 and Type2 must be such that an object of type RandomIt can be implicitly converted to both of them.

Type requirements

RandomIt	LegacyRandomAccessIterator
The type defereferenced RandomIt	MoveAssignable MoveConstructible

Return value

(none)

Complexity

Given N as std::distance(first, last):

At most 2 * log(N) comparisons.

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 any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Notes

A max heap is a range of elements [f; l) that has the following properties:

• Given N as l - f, for all 0 < i < N, f[(i - 1) / 2] does not compare less than f[i].
• A new element can be added using std::push_heap, in O(log(N)) time.
• The first element can be removed using std::pop_heap, in O(log(N)) time.

Examples

Main.cpp

#include <algorithm>
#include <iostream>
#include <string_view>
#include <type_traits>
#include <vector>

void println(std::string_view rem, auto const& v)
{
    std::cout << rem;
    if constexpr (std::is_scalar_v<std::decay_t<decltype(v)>>)
        std::cout << v;
    else
        for (int e : v)
            std::cout << e << ' ';
    std::cout << '\n';
}

int main()
{
    std::vector<int> v {3, 1, 4, 1, 5, 9};

    std::make_heap(v.begin(), v.end());
    println("after make_heap: ", v);

    std::pop_heap(v.begin(), v.end()); // moves the largest to the end
    println("after pop_heap:  ", v);

    int largest = v.back();
    println("largest element: ", largest);

    v.pop_back(); // actually removes the largest element
    println("after pop_back:  ", v);
}

Output

after make_heap: 9 5 4 1 1 3
after pop_heap:  5 3 4 1 1 9
largest element: 9
after pop_back:  5 3 4 1 1