# 27 Algorithms library [algorithms]

## 27.2 Algorithms requirements [algorithms.requirements]

All of the algorithms are separated from the particular implementations of data structures and are parameterized by iterator types.
Because of this, they can work with program-defined data structures, as long as these data structures have iterator types satisfying the assumptions on the algorithms.
The entities defined in the std​::​ranges namespace in this Clause are not found by argument-dependent name lookup ([basic.lookup.argdep]).
When found by unqualified ([basic.lookup.unqual]) name lookup for the postfix-expression in a function call ([expr.call]), they inhibit argument-dependent name lookup.
[Example 1: void foo() { using namespace std::ranges; std::vector<int> vec{1,2,3}; find(begin(vec), end(vec), 2); // #1 }
The function call expression at #1 invokes std​::​ranges​::​find, not std​::​find, despite that (a) the iterator type returned from begin(vec) and end(vec) may be associated with namespace std and (b) std​::​find is more specialized ([temp.func.order]) than std​::​ranges​::​find since the former requires its first two parameters to have the same type.
— end example]
For purposes of determining the existence of data races, algorithms shall not modify objects referenced through an iterator argument unless the specification requires such modification.
Throughout this Clause, where the template parameters are not constrained, the names of template parameters are used to express type requirements.
• If an algorithm's Effects element specifies that a value pointed to by any iterator passed as an argument is modified, then the type of that argument shall meet the requirements of a mutable iterator ([iterator.requirements]).
• If an algorithm's template parameter is named InputIterator, InputIterator1, or InputIterator2, the template argument shall meet the Cpp17InputIterator requirements ([input.iterators]).
• If an algorithm's template parameter is named OutputIterator, OutputIterator1, or OutputIterator2, the template argument shall meet the Cpp17OutputIterator requirements ([output.iterators]).
• If an algorithm's template parameter is named ForwardIterator, ForwardIterator1, ForwardIterator2, or NoThrowForwardIterator, the template argument shall meet the Cpp17ForwardIterator requirements ([forward.iterators]) if it is required to be a mutable iterator, or model forward_iterator ([iterator.concept.forward]) otherwise.
• If an algorithm's template parameter is named NoThrowForwardIterator, the template argument is also required to have the property that no exceptions are thrown from increment, assignment, or comparison of, or indirection through, valid iterators.
• If an algorithm's template parameter is named BidirectionalIterator, BidirectionalIterator1, or BidirectionalIterator2, the template argument shall meet the Cpp17BidirectionalIterator requirements ([bidirectional.iterators]) if it is required to be a mutable iterator, or model bidirectional_iterator ([iterator.concept.bidir]) otherwise.
• If an algorithm's template parameter is named RandomAccessIterator, RandomAccessIterator1, or RandomAccessIterator2, the template argument shall meet the Cpp17RandomAccessIterator requirements ([random.access.iterators]) if it is required to be a mutable iterator, or model random_access_iterator ([iterator.concept.random.access]) otherwise.
[Note 1:
These requirements do not affect iterator arguments that are constrained, for which iterator category and mutability requirements are expressed explicitly.
— end note]
Both in-place and copying versions are provided for certain algorithms.214
When such a version is provided for algorithm it is called algorithm_copy.
Algorithms that take predicates end with the suffix _if (which follows the suffix _copy).
When not otherwise constrained, the Predicate parameter is used whenever an algorithm expects a function object ([function.objects]) that, when applied to the result of dereferencing the corresponding iterator, returns a value testable as true.
If an algorithm takes Predicate pred as its argument and first as its iterator argument with value type T, the expression pred(*first) shall be well-formed and the type decltype(pred(*first)) shall model boolean-testable ([concept.booleantestable]).
The function object pred shall not apply any non-constant function through its argument.
Given a glvalue u of type (possibly const) T that designates the same object as *first, pred(u) shall be a valid expression that is equal to pred(*first).
When not otherwise constrained, the BinaryPredicate parameter is used whenever an algorithm expects a function object that, when applied to the result of dereferencing two corresponding iterators or to dereferencing an iterator and type T when T is part of the signature, returns a value testable as true.
If an algorithm takes BinaryPredicate binary_pred as its argument and first1 and first2 as its iterator arguments with respective value types T1 and T2, the expression binary_pred(*first1, *first2) shall be well-formed and the type decltype(binary_pred(*first1, *first2)) shall model boolean-testable.
Unless otherwise specified, BinaryPredicate always takes the first iterator's value_type as its first argument, that is, in those cases when T value is part of the signature, the expression binary_pred(*first1, value) shall be well-formed and the type decltype(binary_pred(*first1, value)) shall model boolean-testable.
binary_pred shall not apply any non-constant function through any of its arguments.
Given a glvalue u of type (possibly const) T1 that designates the same object as *first1, and a glvalue v of type (possibly const) T2 that designates the same object as *first2, binary_pred(u, *first2), binary_pred(*first1, v), and binary_pred(u, v) shall each be a valid expression that is equal to binary_pred(*first1, *first2), and binary_pred(u, value) shall be a valid expression that is equal to binary_pred(*first1, value).
The parameters UnaryOperation, BinaryOperation, BinaryOperation1, and BinaryOperation2 are used whenever an algorithm expects a function object ([function.objects]).
[Note 2:
Unless otherwise specified, algorithms that take function objects as arguments can copy those function objects freely.
If object identity is important, a wrapper class that points to a non-copied implementation object such as reference_wrapper<T> ([refwrap]), or some equivalent solution, can be used.
— end note]
When the description of an algorithm gives an expression such as *first == value for a condition, the expression shall evaluate to either true or false in boolean contexts.
In the description of the algorithms, operator + is used for some of the iterator categories for which it does not have to be defined.
In these cases the semantics of a + n are the same as those of auto tmp = a; for (; n < 0; ++n) --tmp; for (; n > 0; --n) ++tmp; return tmp;
Similarly, operator - is used for some combinations of iterators and sentinel types for which it does not have to be defined.
If [a, b) denotes a range, the semantics of b - a in these cases are the same as those of iter_difference_t<decltype(a)> n = 0; for (auto tmp = a; tmp != b; ++tmp) ++n; return n; and if [b, a) denotes a range, the same as those of iter_difference_t<decltype(b)> n = 0; for (auto tmp = b; tmp != a; ++tmp) --n; return n;
In the description of the algorithms, given an iterator a whose difference type is D, and an expression n of integer-like type other than cv D, the semantics of a + n and a - n are, respectively, those of a + D(n) and a - D(n).
In the description of algorithm return values, a sentinel value s denoting the end of a range [i, s) is sometimes returned where an iterator is expected.
In these cases, the semantics are as if the sentinel is converted into an iterator using ranges​::​next(i, s).
Overloads of algorithms that take range arguments ([range.range]) behave as if they are implemented by calling ranges​::​begin and ranges​::​end on the range(s) and dispatching to the overload in namespace ranges that takes separate iterator and sentinel arguments.
The well-formedness and behavior of a call to an algorithm with an explicitly-specified template argument list is unspecified, except where explicitly stated otherwise.
[Note 3:
Consequently, an implementation can declare an algorithm with different template parameters than those presented.
— end note]
214)214)
The decision whether to include a copying version was usually based on complexity considerations.
When the cost of doing the operation dominates the cost of copy, the copying version is not included.
For example, sort_copy is not included because the cost of sorting is much more significant, and users can invoke copy followed by sort.