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std::adjacent_find

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Defined in header <algorithm>
template< class ForwardIt >
ForwardIt adjacent_find( ForwardIt first, ForwardIt last );
 (1) (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt >

ForwardIt adjacent_find( ExecutionPolicy&& policy,

                         ForwardIt first, ForwardIt last );
 (2) (since C++17)
template< class ForwardIt, class BinaryPred >

ForwardIt adjacent_find( ForwardIt first, ForwardIt last,

                         BinaryPred p );
 (3) (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt, class BinaryPred >

ForwardIt adjacent_find( ExecutionPolicy&& policy,
                         ForwardIt first, ForwardIt last,

                         BinaryPred p );
 (4) (since C++17)

Searches the range [first, last) for two consecutive equal elements.

1) Elements are compared using operator==.
3) Elements are compared using the given binary predicate p.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

(until C++20)

std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

(since C++20)

Contents

[edit] Parameters

first, last - the pair of iterators defining the range of elements to examine
policy - the execution policy to use
p - binary predicate which returns ​true if the elements should be treated as equal.

The signature of the predicate function should be equivalent to the following:

 bool pred(const Type1 &a, const Type2 &b);

While the signature does not need to have const &, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1 & is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since C++11)).
The types Type1 and Type2 must be such that an object of type ForwardIt can be dereferenced and then implicitly converted to both of them. ​

Type requirements
-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
BinaryPred must meet the requirements of BinaryPredicate.

[edit] Return value

An iterator to the first of the first pair of identical elements, that is, the first iterator it such that *it == *(it + 1) for (1,2) or p(*it, *(it + 1)) != false for (3,4).

If no such elements are found, last is returned.

[edit] Complexity

Given result as the return value of adjacent_find, M as std::distance(first, result) and N as std::distance(first, last):

1) Exactly min(M+1,N-1) comparisons using operator==.
2) O(N) comparisons using operator==.
3) Exactly min(M+1,N-1) applications of the predicate p.
4) O(N) applications of the predicate p.

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