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