std::make_heap

 

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

Constructs a heap in the range [first, last).

1) The constructed heap is with respect to operator<(until C++20)std::less{}(since C++20).

2) The constructed heap is with respect to comp.

If any of the following conditions is satisfied, the behavior is undefined:

The type of *first is not Swappable.	(until C++11)
RandomIt is not ValueSwappable. The type of *first is not MoveConstructible. The type of *first is not MoveAssignable.	(since C++11)

Parameters

first, last	-	the range 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.
- Compare must meet the requirements of Compare.

Complexity

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

1) At most \(\scriptsize 3N\)3N comparisons using operator<(until C++20)std::less{}(since C++20).

2) At most \(\scriptsize 3N\)3N applications of the comparison function comp.

Example

Run this code

#include <algorithm>
#include <functional>
#include <iostream>
#include <string_view>
#include <vector>
 
void print(std::string_view text, const std::vector<int>& v = {})
{
    std::cout << text << ": ";
    for (const auto& e : v)
        std::cout << e << ' ';
    std::cout << '\n';
}
 
int main()
{
    print("Max heap");
 
    std::vector<int> v{3, 2, 4, 1, 5, 9};
    print("initially, v", v);
 
    std::make_heap(v.begin(), v.end());
    print("after make_heap, v", v);
 
    std::pop_heap(v.begin(), v.end());
    print("after pop_heap, v", v);
 
    auto top = v.back();
    v.pop_back();
    print("former top element", {top});
    print("after removing the former top element, v", v);
 
    print("\nMin heap");
 
    std::vector<int> v1{3, 2, 4, 1, 5, 9};
    print("initially, v1", v1);
 
    std::make_heap(v1.begin(), v1.end(), std::greater<>{});
    print("after make_heap, v1", v1);
 
    std::pop_heap(v1.begin(), v1.end(), std::greater<>{});
    print("after pop_heap, v1", v1);
 
    auto top1 = v1.back();
    v1.pop_back();
    print("former top element", {top1});
    print("after removing the former top element, v1", v1);
}

Output:

Max heap:
initially, v: 3 2 4 1 5 9
after make_heap, v: 9 5 4 1 2 3
after pop_heap, v: 5 3 4 1 2 9
former top element: 9
after removing the former top element, v: 5 3 4 1 2
 
Min heap:
initially, v1: 3 2 4 1 5 9
after make_heap, v1: 1 2 4 3 5 9
after pop_heap, v1: 2 3 4 9 5 1
former top element: 1
after removing the former top element, v1: 2 3 4 9 5

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 3032	C++98	the elements of [first, last) was not required to be swappable	required

See also

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)
push_heap	adds an element to a max heap (function template)
pop_heap	removes the largest element from a max heap (function template)
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)
ranges::make_heap (C++20)	creates a max heap out of a range of elements (niebloid)