# std::make_heap

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class RandomIt > void make_heap( RandomIt first, RandomIt last ); | (1) |
template< class RandomIt, class Compare > void make_heap( RandomIt first, RandomIt last, Compare comp ); | (2) |

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);

The signature does not need to have const &, but the function object must not modify the objects passed to it.
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 `RandomAccessIterator`.
- 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)

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