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Showing content from https://cplusplus.com/reference/algorithm/partial_sort/ below:

function template

std::partial_sort default (1)

template <class RandomAccessIterator>  void partial_sort (RandomAccessIterator first, RandomAccessIterator middle,                     RandomAccessIterator last);

custom (2)

template <class RandomAccessIterator, class Compare>  void partial_sort (RandomAccessIterator first, RandomAccessIterator middle,                     RandomAccessIterator last, Compare comp);

Partially sort elements in range

Rearranges the elements in the range [first,last), in such a way that the elements before middle are the smallest elements in the entire range and are sorted in ascending order, while the remaining elements are left without any specific order.

The elements are compared using operator< for the first version, and comp for the second.

Parameters

first, last: Random-access iterators to the initial and final positions of the sequence to be partially sorted. The range used is [first,last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.
Notice that in this function these are not consecutive parameters, but the first and the third.
middle: Random-access iterator pointing to the element within the range [first,last) that is used as the upper boundary of the elements that are fully sorted.
comp: Binary function that accepts two elements in the range as arguments, and returns a value convertible to bool. The value returned indicates whether the element passed as first argument is considered to go before the second in the specific strict weak ordering it defines.
The function shall not modify any of its arguments.
This can either be a function pointer or a function object.

RandomAccessIterator shall point to a type for which swap is properly defined and which is both move-constructible and move-assignable.

Return value none

Example

// partial_sort example
#include <iostream>     // std::cout
#include <algorithm>    // std::partial_sort
#include <vector>       // std::vector

bool myfunction (int i,int j) { return (i<j); }

int main () {
  int myints[] = {9,8,7,6,5,4,3,2,1};
  std::vector<int> myvector (myints, myints+9);

  // using default comparison (operator <):
  std::partial_sort (myvector.begin(), myvector.begin()+5, myvector.end());

  // using function as comp
  std::partial_sort (myvector.begin(), myvector.begin()+5, myvector.end(),myfunction);

  // print out content:
  std::cout << "myvector contains:";
  for (std::vector<int>::iterator it=myvector.begin(); it!=myvector.end(); ++it)
    std::cout << ' ' << *it;
  std::cout << '\n';

  return 0;
}

Possible output:

myvector contains: 1 2 3 4 5 9 8 7 6

Complexity On average, less than linearithmic in the distance between first and last: Performs approximately N*log(M) comparisons of elements (where N is this distance, and M is the distance between first and middle). It also performs up to that many element swaps (or moves).

Data races The objects in the range [first,last) are modified.

Exceptions Throws if any of the element comparisons, the element swaps (or moves) or the operations on iterators throws.
Note that invalid arguments cause undefined behavior.

See also

sort: Sort elements in range (function template)

stable_sort: Sort elements preserving order of equivalents (function template)

search: Search range for subsequence (function template)

reverse: Reverse range (function template)

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