Defined in header <algorithm>  

(1)  
template< class ForwardIt > ForwardIt adjacent_find( ForwardIt first, ForwardIt last );  (until C++20)  
template< class ForwardIt > constexpr ForwardIt adjacent_find( ForwardIt first, ForwardIt last );  (since C++20)  
template< class ExecutionPolicy, class ForwardIt > ForwardIt adjacent_find( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last );  (2)  (since C++17) 
(3)  
template< class ForwardIt, class BinaryPredicate> ForwardIt adjacent_find( ForwardIt first, ForwardIt last, BinaryPredicate p );  (until C++20)  
template< class ForwardIt, class BinaryPredicate> constexpr ForwardIt adjacent_find( ForwardIt first, ForwardIt last, BinaryPredicate p );  (since C++20)  
template< class ExecutionPolicy, class ForwardIt, class BinaryPredicate> ForwardIt adjacent_find( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, BinaryPredicate p );  (4)  (since C++17) 
Searches the range [first, last)
for two consecutive equal elements.
operator==
.p
.policy
. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
(until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
(since C++20) is true.first, last    the range of elements to examine 
policy    the execution policy to use. See execution policy for details. 
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:
While the signature does not need to have 
Type requirements  
ForwardIt must meet the requirements of LegacyForwardIterator. 
an iterator to the first of the first pair of identical elements, that is, the first iterator it
such that *it == *(it+1)
for the first version or p(*it, *(it + 1)) != false
for the second version.
If no such elements are found, last
is returned.
min((resultfirst)+1, (lastfirst)1)
applications of the predicate where result
is the return value.O(lastfirst)
applications of the corresponding predicate.The overloads with a template parameter named ExecutionPolicy
report errors as follows:
ExecutionPolicy
is one of the standard policies, std::terminate
is called. For any other ExecutionPolicy
, the behavior is implementationdefined. std::bad_alloc
is thrown. First version 

template<class ForwardIt> ForwardIt adjacent_find(ForwardIt first, ForwardIt last) { if (first == last) { return last; } ForwardIt next = first; ++next; for (; next != last; ++next, ++first) { if (*first == *next) { return first; } } return last; } 
Second version 
template<class ForwardIt, class BinaryPredicate> ForwardIt adjacent_find(ForwardIt first, ForwardIt last, BinaryPredicate p) { if (first == last) { return last; } ForwardIt next = first; ++next; for (; next != last; ++next, ++first) { if (p(*first, *next)) { return first; } } return last; } 
#include <algorithm> #include <iostream> #include <vector> #include <functional> int main() { std::vector<int> v1{0, 1, 2, 3, 40, 40, 41, 41, 5}; auto i1 = std::adjacent_find(v1.begin(), v1.end()); if (i1 == v1.end()) { std::cout << "No matching adjacent elements\n"; } else { std::cout << "The first adjacent pair of equal elements is at " << std::distance(v1.begin(), i1) << ", *i1 = " << *i1 << '\n'; } auto i2 = std::adjacent_find(v1.begin(), v1.end(), std::greater<int>()); if (i2 == v1.end()) { std::cout << "The entire vector is sorted in ascending order\n"; } else { std::cout << "The last element in the nondecreasing subsequence is at " << std::distance(v1.begin(), i2) << ", *i2 = " << *i2 << '\n'; } }
Output:
The first adjacent pair of equal elements is at 4, *i1 = 40 The last element in the nondecreasing subsequence is at 7, *i2 = 41
removes consecutive duplicate elements in a range (function template) 

(C++20)  finds the first two adjacent items that are equal (or satisfy a given predicate) (niebloid) 
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