Difference between revisions of "cpp/algorithm/qsort"
m (→Example: add INT_MAX to the sample) |
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If {{tt|comp}} indicates two elements as equivalent, their order is unspecified. | If {{tt|comp}} indicates two elements as equivalent, their order is unspecified. | ||
| − | If the type of the elements of the array is not a {{rev inl|until=c++11|{{named req|PODType}}}}{{rev inl|since=c++11|{{named req| | + | If the type of the elements of the array is not a {{rev inl|until=c++11|{{named req|PODType}}}}{{rev inl|since=c++11|{{named req|TriviallyCopyable}} type}}, the behavior is undefined. |
===Parameters=== | ===Parameters=== | ||
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-2147483648 -743 -2 0 2 4 99 2147483647 | -2147483648 -743 -2 0 2 4 99 2147483647 | ||
}} | }} | ||
| + | |||
| + | ===Defect reports=== | ||
| + | {{dr list begin}} | ||
| + | {{dr list item|wg=lwg|dr=3521|std=c++11|before=element type was required to be {{named req|TrivialType}}|after=a {{named req|TriviallyCopyable}} type is OK}} | ||
| + | {{dr list end}} | ||
===See also=== | ===See also=== | ||
Revision as of 02:33, 8 June 2021
| Defined in header <cstdlib>
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| 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.
Contents |
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 <iostream> #include <cstdlib> #include <climits> #include <compare> #include <array> 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* a, const void* b) -> int { const int arg1 = *(static_cast<const int*>(a)); const int arg2 = *(static_cast<const int*>(b)); const auto cmp = arg1 <=> arg2; if (cmp < 0) return -1; if (cmp > 0) return 1; return 0; }); for (int ai : a) std::cout << ai << ' '; }
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 3521 | C++11 | element type was required to be TrivialType | a TriviallyCopyable type is OK |
See also
| searches an array for an element of unspecified type (function) | |
| sorts a range into ascending order (function template) | |
| (C++11) |
checks if a type is trivial (class template) |
| C documentation for qsort
| |
