std::partial_sort() algorithm

since C++20

// (1)
template< class RandomIt >
constexpr void partial_sort( RandomIt first, RandomIt middle, RandomIt last );

// (2)
template< class RandomIt, class Compare >
constexpr void partial_sort( RandomIt first, RandomIt middle, RandomIt last, Compare comp );

// (3)
template< class ExecutionPolicy, class RandomIt >
void partial_sort( ExecutionPolicy&& policy, RandomIt first, RandomIt middle, RandomIt last );

// (4)
template< class ExecutionPolicy, class RandomIt, class Compare >
void partial_sort( ExecutionPolicy&& policy, RandomIt first, RandomIt middle, RandomIt last, Compare comp );

since C++17

// (1)
template< class RandomIt >
void partial_sort( RandomIt first, RandomIt middle, RandomIt last );

// (2)
template< class RandomIt, class Compare >
void partial_sort( RandomIt first, RandomIt middle, RandomIt last, Compare comp );

// (3)
template< class ExecutionPolicy, class RandomIt >
void partial_sort( ExecutionPolicy&& policy, RandomIt first, RandomIt middle, RandomIt last );

// (4)
template< class ExecutionPolicy, class RandomIt, class Compare >
void partial_sort( ExecutionPolicy&& policy, RandomIt first, RandomIt middle, RandomIt last, Compare comp );

until C++17

// (1)
template< class RandomIt >
void partial_sort( RandomIt first, RandomIt middle, RandomIt last );

// (2)
template< class RandomIt, class Compare >
void partial_sort( RandomIt first, RandomIt middle, RandomIt last,
                   Compare comp );

Sorts some of the elements in the range [first; last) in ascending order, storing the result in the range [d_first; d_last).

Given N as min(last - first, d_last - d_first) (the number of elements to sort):

At most d_last - d_first of the elements are placed sorted to the range [d_first; d_first + n).

caution

The order of equal elements is not guaranteed to be preserved.

• (1) Elements are compared using operator<.

• (2) Elements are compared using the given binary comparison function comp.

• (3, 4) Same as (1) and (2), but executed according to policy.

  Overload Resolution

  These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.  (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.  (since C++20)

Parameters

first last	The range of elements to sort partially.
d_first d_last	The destination range.
policy	The execution policy to use. See execution policy for details.
cmp	Comparison function object (i.e. an object that satisfies the requirements of Compare). 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 must not modify arguments. Must accept all values of type (possibly const) Type and Type2, regardless of value category (so 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 implicitly converted to both of them.

Type requirements

InputIt	LegacyInputIterator
ForwardIt	LegacyForwardIterator
RandomIt	ValueSwappable LegacyRandomAccessIterator
Type of dereferenced RandomIt	MoveAssignable MoveConstructible
Compare	Compare

Return value

An iterator to the element defining the upper boundary of the sorted range, i.e. d_first + min(last - first, d_last - d_first).

Complexity

O(N * log(min(D, N)) applications of comp.

Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Examples

Main.cpp

#include <algorithm>
#include <functional>
#include <iostream>
#include <string_view>
#include <type_traits>
#include <vector>

void println(std::string_view rem, auto const& v)
{
    std::cout << rem;
    if constexpr (std::is_scalar_v<std::decay_t<decltype(v)>>)
        std::cout << v;
    else
        for (int e : v)
            std::cout << e << ' ';
    std::cout << '\n';
}

int main()
{
    const auto v0 = {4, 2, 5, 1, 3};
    std::vector<int> v1 {10, 11, 12};
    std::vector<int> v2 {10, 11, 12, 13, 14, 15, 16};
    std::vector<int>::iterator it;

    it = std::partial_sort_copy(v0.begin(), v0.end(), v1.begin(), v1.end());
    println("Writing to the smaller vector in ascending order gives: ", v1);

    if (it == v1.end())
        println("The return value is the end iterator", ' ');

    it = std::partial_sort_copy(v0.begin(), v0.end(), v2.begin(), v2.end(),
                                std::greater<int>());

    println("Writing to the larger vector in descending order gives: ", v2);
    println("The return value is the iterator to ", *it);
}

Output

Writing to the smaller vector in ascending order gives: 1 2 3
The return value is the end iterator
Writing to the larger vector in descending order gives: 5 4 3 2 1 15 16
The return value is the iterator to 15