std::pop_heap(3)

💅 std::pop_heap(3) — C++ Standard Library, bestie edition

okay so welcome to std::pop_heap, a genuine main-character function from the C++ Standard Library (that's the manual name, keep it in your notes 📓). She lives in the <algorithm> header and she's here to yeet the top dog out of your heap. Sit down, grab a matcha, we're doing this 🍵✨

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🌸 NAME 🌸

std::pop_heap — literally std::pop_heap, that's it, that's the name, no cap 💅

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✨ SYNOPSIS ✨

Defined in header <algorithm>:

template< class RandomIt >                               (1)
(constexpr since C++20)
void pop_heap( RandomIt first, RandomIt last );
template< class RandomIt, class Compare >
void pop_heap( RandomIt first, RandomIt last, Compare    (2)
(constexpr since C++20)
comp );

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🎀 DESCRIPTION 🎀

So basically 💖 she swaps the value chillin' at first with the value at last - 1, and then makes the subrange [first, last - 1) into a heap. The vibe of all that? It has the effect of removing the first element from the heap [first, last). Iconic behavior, no notes 🫶

1. [first, last) is a heap with respect to operator< (until C++20) std::less{} (since C++20).
2. [first, last) is a heap with respect to comp.

🚨 If [first, last) is not a non-empty heap, the behavior is undefined. Not "kinda weird", not "slightly off" — undefined. We do not go there 🙈

If any of the following conditions is satisfied, the behavior is undefined:

• [first, last) is empty.
• [first, last) is not a heap.
• The type of *first is not Swappable. (until C++11)
• RandomIt is not ValueSwappable.
• The type of *first is not MoveConstructible. (since C++11)
• The type of *first is not MoveAssignable.

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🌈 PARAMETERS 🌈

• first, last — the non-empty heap to modify 💫
• comp — a comparison function object (i.e. an object that satisfies the requirements of Compare) which returns true if the first argument is less than the second.

The signature of the comparison function should be equivalent to the following:

bool cmp(const Type1& a, const Type2& b);

While the signature does not need to have const&, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1& is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy (since C++11)).

The types Type1 and Type2 must be such that an object of type RandomIt can be dereferenced and then implicitly converted to both of them. periodt 💅

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👑 TYPE REQUIREMENTS 👑

• RandomIt must meet the requirements of LegacyRandomAccessIterator.
• Compare must meet the requirements of Compare.

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🔥 COMPLEXITY 🔥

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

1. At most 2log(N) comparisons using operator< (until C++20) std::less{} (since C++20).
2. At most 2log(N) applications of the comparison function comp.

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🦋 EXAMPLE 🦋

// Run this code

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

void println(std::string_view rem, const auto& 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

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💀 DEFECT REPORTS 💀

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• LWG 1205 — Applied to: C++98. Behavior as published: the behavior was unclear if [first, last) is empty. Correct behavior: the behavior is undefined in this case.

(they really said "we forgot to say what happens when it's empty" and then decided the answer was chaos 😭)

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🌟 SEE ALSO 🌟

• push_heap — adds an element to a max heap (function template)
• is_heap (C++11) — checks if the given range is a max heap (function template)
• is_heap_until (C++11) — finds the largest subrange that is a max heap (function template)
• make_heap — creates a max heap out of a range of elements (function template)
• sort_heap — turns a max heap into a range of elements sorted in ascending order (function template)
• ranges::pop_heap (C++20) — removes the largest element from a max heap (niebloid)

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k bye bestie, go pop some heaps responsibly 💜 this page came from http://cppreference.com, dated 2024.06.10, and it's page std::pop_heap(3) — mwah 🎀✨