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std::pop_heap

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

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 [first, last).

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

  • [first, last) is empty.
  • [first, last) is not a heap with respect to the corresponding comparator.
(until C++11)
(since C++11)

Contents

[edit] Parameters

first, last - the pair of iterators defining the non-empty binary heap range of elements to modify (extract root item)
comp - 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.

Type requirements
-
RandomIt must meet the requirements of LegacyRandomAccessIterator.
-
Compare must meet the requirements of Compare.

[edit] 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.

[edit] 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

[edit] Defect reports

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

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

[edit] See also

 adds an element to a max heap
(function template) [edit]
(C++11)
 checks if the given range is a max heap
(function template) [edit]
(C++11)
 finds the largest subrange that is a max heap
(function template) [edit]
creates a max heap out of a range of elements
(function template) [edit]
turns a max heap into a range of elements sorted in ascending order
(function template) [edit]
(C++20)
 removes the largest element from a max heap
(algorithm function object)[edit]
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