std::sort
| Defined in header <algorithm>
|
||
template< class RandomIt >
void sort( RandomIt first, RandomIt last );
|
(1) | (constexpr since C++20) |
template< class RandomIt, class Compare >
void sort( RandomIt first, RandomIt last, Compare comp );
|
(2) | (constexpr since C++20) |
template< class ExecutionPolicy, class RandomIt >
void sort( ExecutionPolicy&& policy,
RandomIt first, RandomIt last );
|
(3) | (since C++17) |
template< class ExecutionPolicy, class RandomIt, class Compare >
void sort( ExecutionPolicy&& policy,
RandomIt first, RandomIt last, Compare comp );
|
(4) | (since C++17) |
Sorts the elements in the target range [first, last). The order of equivalent elements is not guaranteed to be preserved.
comp.policy.true:
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|
(until C++20) |
|
|
(since C++20) |
If any of the following conditions is satisfied, the behavior is undefined:
|
(until C++11) |
|
(since C++11) |
Parameters
| first, last | - | the pair of iterators defining the target range |
| comp | - | comparison function object (i.e. an object that satisfies the requirements of Compare) which returns true if the first argument is less than (i.e. is ordered before) the second. The signature of the comparison function should be equivalent to the following:
While the signature does not need to have |
| policy | - | the execution policy to use |
| Type requirements | ||
-RandomIt must meet the requirements of LegacyRandomAccessIterator.
| ||
-Compare must meet the requirements of Compare.
| ||
Complexity
Given N as last - first:
operator<(until C++20)std::less{}(since C++20).comp.Exceptions
- If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
- If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).
Notes
Before LWG713, the complexity requirement allowed sort() to be implemented using only Quicksort, which may need 𝓞(N2
) comparisons in the worst case.
Introsort can handle all cases with 𝓞(N·log(N)) comparisons (without incurring additional overhead in the average case), and thus is usually used for implementing sort().
libc++ has not implemented the corrected time complexity requirement until LLVM 14.
Possible implementation
See also the implementations in libstdc++ and libc++.
Example
#include <algorithm>
#include <array>
#include <functional>
#include <iostream>
#include <string_view>
int main()
{
std::array<int, 10> s{5, 7, 4, 2, 8, 6, 1, 9, 0, 3};
auto print = [&s](const std::string_view rem)
{
for (auto a : s)
std::cout << a << ' ';
std::cout << ": " << rem << '\n';
};
std::sort(s.begin(), s.end());
print("sorted with the default operator<");
std::sort(s.begin(), s.end(), std::greater<int>());
print("sorted with the standard library compare function object");
struct
{
bool operator()(int a, int b) const { return a < b; }
}
customLess;
std::sort(s.begin(), s.end(), customLess);
print("sorted with a custom function object");
std::sort(s.begin(), s.end(), [](int a, int b)
{
return a > b;
});
print("sorted with a lambda expression");
}
Output:
0 1 2 3 4 5 6 7 8 9 : sorted with the default operator<
9 8 7 6 5 4 3 2 1 0 : sorted with the standard library compare function object
0 1 2 3 4 5 6 7 8 9 : sorted with a custom function object
9 8 7 6 5 4 3 2 1 0 : sorted with a lambda expression
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 713 | C++98 | the 𝓞(N·log(N)) time complexity was only required on the average | it is required for the worst case |