std::sort_heap

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Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
sort_heap
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
Operations on uninitialized storage
(C++17)
(C++17)
(C++17)
C library
 
Defined in header <algorithm>
(1)
template< class RandomIt >
void sort_heap( RandomIt first, RandomIt last );
(until C++20)
template< class RandomIt >
constexpr void sort_heap( RandomIt first, RandomIt last );
(since C++20)
(2)
template< class RandomIt, class Compare >
void sort_heap( RandomIt first, RandomIt last, Compare comp );
(until C++20)
template< class RandomIt, class Compare >
constexpr void sort_heap( RandomIt first, RandomIt last, Compare comp );
(since C++20)

Converts the heap [firstlast) into a sorted range. The heap property is no longer maintained.

1) [firstlast) is a heap with respect to operator<, and will be sorted with respect to operator<.
2) [firstlast) is a heap with respect to comp, and will be sorted with respect to comp.

If [firstlast) is not a heap, the behavior is undefined.

Parameters

first, last - the heap to be sorted
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 ValueSwappable and LegacyRandomAccessIterator.
-
The type of dereferenced RandomIt must meet the requirements of MoveAssignable and MoveConstructible.
-
Compare must meet the requirements of Compare.

Return value

(none)

Complexity

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

1) At most 2N·log(N) comparisons using operator<.
2) At most 2N·log(N) applications of the comparison function comp.

Notes

A heap with respect to comp (max heap) is a random access range [firstlast) that has the following properties:

  • Given N as last - first, for all integer i where 0 < i < N, bool(comp(first[(i - 1) / 2], first[i])) is false.
  • A new element can be added using std::push_heap, in 𝓞(log N) time.
  • *first can be removed using std::pop_heap, in 𝓞(log N) time.

Possible implementation

sort_heap (1)
template<class RandomIt>
void sort_heap(RandomIt first, RandomIt last)
{
    while (first != last)
        std::pop_heap(first, last--);
}
sort_heap (2)
template<class RandomIt, class Compare>
void sort_heap(RandomIt first, RandomIt last, Compare comp)
{
    while (first != last)
        std::pop_heap(first, last--, comp);
}

Example

#include <algorithm>
#include <iostream>
#include <string_view>
#include <vector>
 
void println(std::string_view fmt, auto const& v)
{
    for (std::cout << fmt; const auto &i : v)
        std::cout << i << ' ';
    std::cout << '\n';
}
 
int main()
{
    std::vector<int> v{3, 1, 4, 1, 5, 9};
 
    std::make_heap(v.begin(), v.end());
    println("after make_heap, v: ", v);
 
    std::sort_heap(v.begin(), v.end());
    println("after sort_heap, v: ", v);
}

Output:

after make_heap, v: 9 4 5 1 1 3
after sort_heap, v: 1 1 3 4 5 9

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 193 C++98 heap required *first to be the largest element there can be elements equal to *first
LWG 2166 C++98 the heap reqirement did not match the
definition of max heap closely enough
requirement imporved
LWG 2444 C++98 at most N·log(N) comparisons were allowed increased to 2N·log(N)

See also

(C++11)
checks if the given range is a max heap
(function template)
finds the largest subrange that is a max heap
(function template)
creates a max heap out of a range of elements
(function template)
removes the largest element from a max heap
(function template)
adds an element to a max heap
(function template)
turns a max heap into a range of elements sorted in ascending order
(niebloid)