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std::partial_sort_copy - cppreference.com

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

At most d_last - d_first of the elements are placed sorted to the range [d_first, d_first + n). n is the number of elements to sort (std::min(std::distance(first, last), d_last - d_first)). The order of equal elements is not guaranteed to be preserved.

Elements are

with respect to

.

3) Elements are sorted with respect to comp.

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

These overloads participate in overload resolution only if all following conditions are satisfied:

If *first is not writable to d_first, the program is ill-formed.

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

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. ​

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

Given \(\scriptsize N\)N as std::distance(first, last), \(\scriptsize D\)D as d_last - d_first:

Approximately

comparisons using

.

3,4) Approximately \(\scriptsize N \cdot \log(\min(N,D))\)N·log(min(N,D)) applications of the comparator comp.

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.

See also the implementations in libstdc++ and libc++.

The following code sorts a vector of integers and copies them into a smaller and a larger vector.

#include <algorithm>
#include <functional>
#include <iostream>
#include <string_view>
#include <type_traits>
#include <vector>
 
void println(std::string_view rem, const auto& 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

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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