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Category: algorithms Component type: function


Sort is an overloaded name; there are actually two sort functions.

template <class RandomAccessIterator>
void sort(RandomAccessIterator first, RandomAccessIterator last);

template <class RandomAccessIterator, class StrictWeakOrdering>
void sort(RandomAccessIterator first, RandomAccessIterator last,
          StrictWeakOrdering comp);


Sort sorts the elements in [first, last) into ascending order, meaning that if i and j are any two valid iterators in [first, last) such that i precedes j, then *j is not less than *i. Note: sort is not guaranteed to be stable. That is, suppose that *i and *j are equivalent: neither one is less than the other. It is not guaranteed that the relative order of these two elements will be preserved by sort. [1]

The two versions of sort differ in how they define whether one element is less than another. The first version compares objects using operator<, and the second compares objects using a functors comp.


Defined in the standard header algorithm, and in the nonstandard backward-compatibility header algo.h.

Requirements on types

For the first version, the one that takes two arguments:

  • RandomAccessIterator is a model of RandomAccessIterator.
  • RandomAccessIterator is mutable.
  • RandomAccessIterator's value type is LessThanComparable.
  • The ordering relation on RandomAccessIterator's value type is a strict weak ordering, as defined in the LessThanComparable requirements.

For the second version, the one that takes three arguments:

  • RandomAccessIterator is a model of RandomAccessIterator.
  • RandomAccessIterator is mutable.
  • StrictWeakOrdering is a model of StrictWeakOrdering.
  • RandomAccessIterator's value type is convertible to StrictWeakOrdering's argument type.


  • [first, last) is a valid range.


O(N log(N)) comparisons (both average and worst-case), where N is last - first. [2]


int A[] = {1, 4, 2, 8, 5, 7};
const int N = sizeof(A) / sizeof(int);
sort(A, A + N);
copy(A, A + N, ostream_iterator<int>(cout, " "));
// The output is " 1 2 4 5 7 8".


[1] Stable sorting is sometimes important if you are sorting records that have multiple fields: you might, for example, want to sort a list of people by first name and then by last name. The algorithm stable_sort does guarantee to preserve the relative ordering of equivalent elements.

[2] Earlier versions of sort used the quicksort algorithm (C. A. R. Hoare, Comp. J. 5, 1962), using a pivot chosen by median of three (R. C. Singleton, CACM 12, 1969). Quicksort has O(N log(N)) average complexity, but quadratic worst-case complexity. See section 5.2.2 of Knuth for a discussion. (D. E. Knuth, The Art of Computer Programming. Volume 3: Sorting and Searching. Addison-Wesley, 1975.) The current implementation of sort, however, uses the introsort algorithm (D. R. Musser, "Introspective Sorting and Selection Algorithms", Software Practice and Experience 27(8):983, 1997.) whose worst case complexity is O(N log(N)). Introsort is very similar to median-of-three quicksort, and is at least as fast as quicksort on average.

See also

stable_sort, partial_sort, partial_sort_copy, sort_heap, is_sorted, binary_search, lower_bound, upper_bound, less<T>, StrictWeakOrdering, LessThanComparable

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