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    std::make_heap

    From cppreference.com
     
    C++
     
    Algorithm library
    Constrained algorithms and algorithms on ranges (C++20)
    Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
    Non-modifying sequence operations    
    Batch operations
    (C++17)
    Search operations
    Modifying sequence operations
    Copy operations
    (C++11)
    (C++11)
    (C++11)
    Swap operations
    Transformation operations
    Generation operations
    Removing operations
    Order-changing operations
    (until C++17)(C++11)
    (C++20)(C++20)
    Sampling operations
    (C++17)

    Sorting and related operations
    Partitioning operations
    (C++11)    
    (C++11)

    Sorting operations
    Binary search operations
    (on partitioned ranges)
    Set operations (on sorted ranges)
    Merge operations (on sorted ranges)
    Heap operations
    Minimum/maximum operations
    (C++11)
    (C++17)
    Lexicographical comparison operations
    (C++20)
    Permutation operations


     
    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 out of the target 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:

    (until C++11)
    (since C++11)

    Parameters

    first, last - the pair of iterators defining the target range
    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 N as std::distance(first, last):

    1) At most 3N comparisons using operator<(until C++20)std::less{}(since C++20).
    2) At most 3N applications of the comparison function comp.

    Example

    Run this code
    #include <algorithm>
#include <functional>
#include <print>
#include <vector>

int main()
{
    std::println("Max heap");

    std::vector<int> v{3, 2, 4, 1, 5, 9};
    std::println("initially: {}", v);

    std::make_heap(v.begin(), v.end());
    std::println("after make_heap: {}", v);

    std::pop_heap(v.begin(), v.end());
    std::println("after pop_heap: {}", v);

    auto top = v.back();
    v.pop_back();
    std::println("former top element: {}", top);
    std::println("after removing the former top element: {}\n", v);

    std::println("Min heap");

    std::vector<int> v1{3, 2, 4, 1, 5, 9};
    std::println("initially: {}", v1);

    std::make_heap(v1.begin(), v1.end(), std::greater<>{});
    std::println("after make_heap: {}", v1);

    std::pop_heap(v1.begin(), v1.end(), std::greater<>{});
    std::println("after pop_heap: {}", v1);

    auto top1 = v1.back();
    v1.pop_back();
    std::println("former top element: {}", top1);
    std::println("after removing the former top element: {}", v1);
}

    Output: 

    Max heap
initially: [3, 2, 4, 1, 5, 9]
after make_heap: [9, 5, 4, 1, 2, 3]
after pop_heap: [5, 3, 4, 1, 2, 9]
former top element: 9
after removing the former top element: [5, 3, 4, 1, 2]

Min heap
initially: [3, 2, 4, 1, 5, 9]
after make_heap: [1, 2, 4, 3, 5, 9]
after pop_heap: [2, 3, 4, 9, 5, 1]
former top element: 1
after removing the former top element: [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 the target range was not required to be swappable required

    See also

    (C++20)
    		 creates a max heap out of a range of elements
    (algorithm function object)[edit]
    (C++11)
    		 checks if the given range is a max heap
    (function template & algorithm function object)[edit]
    (C++20)
    (C++11)
    		 finds the largest subrange that is a max heap
    (function template & algorithm function object)[edit]
    (C++20)
    		adds an element to a max heap
    (function template & algorithm function object)[edit]
    (C++20)
    		removes the largest element from a max heap
    (function template & algorithm function object)[edit]
    (C++20)
    		turns a max heap into a range of elements sorted in ascending order
    (function template & algorithm function object)[edit]
    (C++20)
    		adapts a container to provide priority queue
    (class template) [edit]