# std::make_heap

From cppreference.com
< cpp‎ | algorithm

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

Constructs a max heap in the range `[first, last)`. The first version of the function uses operator< to compare the elements, the second uses the given comparison function `comp`.

### Parameters

first, last | - | the range of elements to make the heap from

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.
- The type of dereferenced `RandomIt` must meet the requirements of MoveAssignable and MoveConstructible.

### Return value

(none)

### Complexity

At most 3*std::distance(first, last) comparisons.

### Notes

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

  - With `N = l - f`, for all `0 < i < N`, `f[floor(

    i-1
    2

    )]` does not compare less than `f[i]`.
  - a new element can be added using std::push_heap()
  - the first element can be removed using std::pop_heap()

### Example

Run this code

    #include <iostream>
    #include <algorithm>
    #include <vector>

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

        std::cout << "initially, v: ";
        for (auto i : v) std::cout << i << ' ';
        std::cout << '\n';

        std::make_heap(v.begin(), v.end());

        std::cout << "after make_heap, v: ";
        for (auto i : v) std::cout << i << ' ';
        std::cout << '\n';

        std::pop_heap(v.begin(), v.end());
        auto largest = v.back();
        v.pop_back();
        std::cout << "largest element: " << largest << '\n';

        std::cout << "after removing the largest element, v: ";
        for (auto i : v) std::cout << i << ' ';
        std::cout << '\n';
    }

Output:

    initially, v: 3 1 4 1 5 9
    after make_heap, v: 9 5 4 1 1 3
    largest element: 9
    after removing the largest element, v: 5 3 4 1 1

### See also

sort_heap | turns a max heap into a range of elements sorted in ascending order (function template)
priority_queue | adapts a container to provide priority queue (class template)
greater | function object implementing x > y (class template)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/algorithm/make_heap&oldid=103638"