< cpp‎ | algorithm
Revision as of 10:22, 13 August 2013 by Cubbi (Talk | contribs)

Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations

Numeric operations
C library
Defined in header <cstdlib>
void qsort( void *ptr, std::size_t count, std::size_t size,
            int (*comp)(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 undefined.



ptr - pointer to the array to sort
count - number of element 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



The type of the elements of the array must be a trivial type, otherwise the behavior is undefined.


The following code sorts an array of integers using qsort().

#include <iostream>
#include <cstdlib>
int compare_ints(const void* a, const void* b)   // comparison function
    int arg1 = *reinterpret_cast<const int*>(a);
    int arg2 = *reinterpret_cast<const int*>(b);
    if(arg1 < arg2) return -1;
    if(arg1 > arg2) return 1;
    return 0;
int main()
    int a[] = { -2, 99, 0, -743, 2, 3, 4 };
    constexpr std::size_t size = sizeof(a)/sizeof(int);
    std::qsort(a, size, sizeof(int), compare_ints);
    for (int ai : a) std::cout << ai << ' ';


-743 -2 0 2 3 4 99

See also

searches an array for an element of unspecified type
(function) [edit]
sorts a range into ascending order
(function template) [edit]
checks if a type is trivial
(class template) [edit]
C documentation for qsort