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std::make_heap

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

#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