NAME¶

std::pop_heap - std::pop_heap

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 );

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 [first, last) not a non-empty heap, the behavior is undefined.

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.

Parameters¶

first, last - the non-empty heap to modify
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);
comp -
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.

Complexity¶

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

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

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

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 the behavior is undefined
[first, last) is empty in this case

See also¶

push_heap adds an element to a max heap
(function template)
is_heap checks if the given range is a max heap
(C++11) (function template)
is_heap_until finds the largest subrange that is a max heap
(C++11) (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 removes the largest element from a max heap
(C++20) (niebloid)