std::qsort (3) - Linux Manuals

std::qsort: std::qsort

NAME

std::qsort - std::qsort

Synopsis

Defined in header <cstdlib>
void qsort( void *ptr, std::size_t count, std::size_t size, /*compare-pred*/* comp ); (1)
void qsort( void *ptr, std::size_t count, std::size_t size, /*c-compare-pred*/* comp );
extern "C++" using /*compare-pred*/ = int(const void*, const void*); // exposition-only (2)
extern "C" using /*c-compare-pred*/ = int(const void*, const void*); // exposition-only

Sorts the given array pointed to by ptr in ascending order. The array contains count elements of size bytes. Function pointed to by comp is used for object comparison.
If comp indicates two elements as equivalent, their order is unspecified.

Parameters

ptr - pointer to the array to sort
count - number of elements in the array
size - size of each element in the array in bytes
        comparison function which returns a negative integer value if the first argument is less than the second,
        a positive integer value if the first argument is greater than the second and zero if the arguments are equal.
comp - The signature of the comparison function should be equivalent to the following:
        int cmp(const void *a, const void *b);
        The function must not modify the objects passed to it and must return consistent results when called for the same objects, regardless of their positions in the array.
        

Return value

(none)

Notes

Despite the name, C++, C, and POSIX standards do not require this function to be implemented using quicksort or make any complexity or stability guarantees.
The type of the elements of the array must be a TrivialType, otherwise the behavior is undefined.
The two overloads provided by the C++ standard library are distinct because the types of the parameter comp are distinct (language_linkage is part of its type)

Example

The following code sorts an array of integers using qsort().
// Run this code

  #include <iostream>
  #include <cstdlib>
  #include <climits>

  int main()
  {
      int a[] = {-2, 99, 0, -743, 2, INT_MIN, 4};
      constexpr std::size_t size = sizeof a / sizeof *a;

      std::qsort(a, size, sizeof *a, [](const void* a, const void* b)
      {
          int arg1 = *static_cast<const int*>(a);
          int arg2 = *static_cast<const int*>(b);

          if(arg1 < arg2) return -1;
          if(arg1 > arg2) return 1;
          return 0;

      // return (arg1 > arg2) - (arg1 < arg2); // possible shortcut
      // return arg1 - arg2; // erroneous shortcut (fails if INT_MIN is present)
      });

      for(int ai : a)
          std::cout << ai << ' ';
  }

Output:

  -2147483648 -743 -2 0 2 4 99