std::qsort(3)

NAME
std::qsort - std::qsort

Synopsis
   Defined in header <cstdlib>
   void qsort( void *ptr, std::size_t count,
               std::size_t size, /* c-compare-pred */* comp ); (1)
   void qsort( void *ptr, std::size_t count,
               std::size_t size, /* compare-pred */* comp );
   extern "C" using /* c-compare-pred */ = int(const void*,
   const void*); (2) (exposition only*)
   extern "C++" using /* compare-pred */ = int(const void*,
   const void*);
   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.
   If the type of the elements of the array is not a
   PODType
   (until C++11)
   TriviallyCopyable type
   (since C++11), the behavior is undefined.

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
           equivalent.
           The signature of the comparison function should be equivalent to the
           following:
   comp -
            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 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 <array>
 #include <climits>
 #include <compare>
 #include <cstdlib>
 #include <iostream>
 int main()
 {
     std::array a{-2, 99, 0, -743, INT_MAX, 2, INT_MIN, 4};
     std::qsort
     (
         a.data(),
         a.size(),
         sizeof(decltype(a)::value_type),
         [](const void* x, const void* y)
         {
             const int arg1 = *static_cast<const int*>(x);
             const int arg2 = *static_cast<const int*>(y);
             const auto cmp = arg1 <=> arg2;
             if (cmp < 0)
                 return -1;
             if (cmp > 0)
                 return 1;
             return 0;
         }
     );
     for (int ai : a)
         std::cout << ai << ' ';
     std::cout << '\n';
 }

Output:
 -2147483648 -743 -2 0 2 4 99 2147483647
   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 405 C++98 the elements of the array could have any type limited to PODType

See also
   bsearch searches an array for an element of unspecified type
              (function)
   sort sorts a range into ascending order
              (function template)
   is_trivial checks if a type is trivial
   (C++11) (class template)
   C documentation for
   qsort