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std::qsort

Defined in header <cstdlib>

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

void qsort( void *ptr, std::size_t count, std::size_t size, /* compare-pred */* comp );
void qsort( void *ptr, std::size_t count, std::size_t size, /* c-compare-pred */* comp );

(1)

extern "C++" using /* compare-pred */ = int(const void*, const void*); // exposition-only
extern "C" using /* c-compare-pred */ = int(const void*, const void*); // exposition-only

(2)

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
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 and 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 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():

#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