# std::pop_heap

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class RandomIt > void pop_heap( RandomIt first, RandomIt last ); |  | (until C++20)
template< class RandomIt > constexpr void pop_heap( RandomIt first, RandomIt last ); |  | (since C++20)
| (2) |
template< class RandomIt, class Compare > void pop_heap( RandomIt first, RandomIt last, Compare comp ); |  | (until C++20)
template< class RandomIt, class Compare > constexpr void pop_heap( RandomIt first, RandomIt last, Compare comp ); |  | (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<.
2) `[`first`,` last`)` is a heap with respect to comp.

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

### Parameters

first, last | - | the non-empty heap to modify
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 ValueSwappable and LegacyRandomAccessIterator.
- The type of dereferenced `RandomIt` must meet the requirements of MoveAssignable and MoveConstructible.
- `Compare` must meet the requirements of Compare.

### Return value

(none)

### Complexity

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

1) At most \(\scriptsize 2 \cdot log(N)\)2·log(N) comparisons using operator<.
2) At most \(\scriptsize 2 \cdot log(N)\)2·log(N) applications of the comparison function comp.

### Notes

A heap with respect to comp (max heap) is a random access range `[`first`,` last`)` that has the following properties:

- Given \(\scriptsize N\)N as last - first, for all integer i where \(\scriptsize 0 < i < N\)0 < i < N, bool(comp(first[(i - 1) / 2], first[i])) is false.
- A new element can be added using std::push_heap, in \(\scriptsize \mathcal{O}(\log N)\)𝓞(log N) time.
- *first can be removed using std::pop_heap, in \(\scriptsize \mathcal{O}(\log N)\)𝓞(log N) time.

### Example

Run this code

    #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

### 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 193 | C++98 | heap required *first to be the largest element | there can be elements equal to *first
LWG 1205 | C++98 | the behavior was unclear if `[`first`,` last`)` is empty | the behavior is undefined in this case
LWG 2166 | C++98 | the heap reqirement did not match the definition of max heap closely enough | requirement imporved

### 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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