std::qsort

 

C++

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Technical Specifications

 

Algorithm library

Execution policies (C++17)
is_execution_policy	execution::seqexecution::parexecution::par_unseq	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policy
Non-modifying sequence operations
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each for_each_n (C++17)	countcount_if mismatch equal lexicographical_compare	findfind_iffind_if_not (C++11) find_end find_first_of	adjacent_find search search_n
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	removeremove_if replacereplace_if reverse rotate unique random_shuffle (until C++17)	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
Operations on uninitialized storage
uninitialized_copy uninitialized_move (C++17)	uninitialized_copy_n (C++11) uninitialized_move_n (C++17)	uninitialized_fill	uninitialized_fill_n
Partitioning operations
is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
Sorting operations
is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
Binary search operations
lower_bound	upper_bound	binary_search	equal_range
Set operations (on sorted ranges)
merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
Heap operations
is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
Minimum/maximum operations
max max_element	min min_element	minmax (C++11) minmax_element (C++11)	clamp (C++17)
Permutations
is_permutation (C++11)	next_permutation	prev_permutation
Numeric operations
iota (C++11) inner_product adjacent_difference	accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17) exclusive_scan (C++17)	transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
C library
qsort	bsearch

 

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)

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 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 equal. 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

See also

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