std::qsort() algorithm

since C++98

// (1)
void qsort( void *ptr, std::size_t count, std::size_t size, cpp_compare_pred comp );

// (2)
void qsort( void *ptr, std::size_t count, std::size_t size, c_compare_pred comp );

With the exposition-only types defined as follows:

extern "C++" using cpp_compare_pred = int(const void*, const void*);
extern "C" using c_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.

caution

If comp indicates two elements as equivalent, their order is unspecified.

Undefined Behaviour

If the type of the elements of the array is not a PODType (until C++11)TriviallyCopyable (since C++11) type, the behavior is undefined

.

Parameters

ptr	Pointer to the array to sort.
count	The number of elements in the array.
size	The size of each element of the array in bytes.
comp	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. Zero if the arguments are equivalent. 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 Must return consistent results when called for the same objects, regardless of their positions in the array

Return value

(none)

Complexity

Not specified.

Exceptions

(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).

Examples

Main.cpp

#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