std::mismatch(3)

NAME
std::mismatch - std::mismatch

Synopsis
   Defined in header <algorithm>
   template< class InputIt1, class InputIt2 >
   std::pair<InputIt1,InputIt2> (until
   mismatch( InputIt1 first1, InputIt1 last1, C++20)
   InputIt2 first2 );
   template< class InputIt1, class InputIt2 >
   constexpr std::pair<InputIt1,InputIt2> (since
   mismatch( InputIt1 first1, InputIt1 last1, C++20)
   InputIt2 first2 );
   template< class ExecutionPolicy, class
   ForwardIt1, class ForwardIt2 >
   std::pair<ForwardIt1,ForwardIt2> (2) (since
   mismatch( ExecutionPolicy&& policy, C++17)
   ForwardIt1 first1, ForwardIt1 last1,
   ForwardIt2 first2 );
   template< class InputIt1, class InputIt2,
   class BinaryPredicate >
   std::pair<InputIt1,InputIt2> (until
   mismatch( InputIt1 first1, InputIt1 last1, C++20)
   InputIt2 first2,
   BinaryPredicate p );
   template< class InputIt1, class InputIt2,
   class BinaryPredicate >
   constexpr std::pair<InputIt1,InputIt2> (since
   mismatch( InputIt1 first1, InputIt1 last1, C++20)
   InputIt2 first2,
   BinaryPredicate p );
   template< class ExecutionPolicy, class
   ForwardIt1, class ForwardIt2,
   class BinaryPredicate >
   std::pair<ForwardIt1,ForwardIt2> (4) (since
   mismatch( ExecutionPolicy&& policy, C++17)
   ForwardIt1 first1, ForwardIt1 last1,
   ForwardIt2 first2,
   BinaryPredicate p ); (1)
   template< class InputIt1, class InputIt2 >
                        (since
   std::pair<InputIt1,InputIt2> C++14)
   mismatch( InputIt1 first1, InputIt1 last1, (until
                        C++20)
   InputIt2 first2, InputIt2 last2 );
   template< class InputIt1, class InputIt2 >
   constexpr std::pair<InputIt1,InputIt2> (since
   mismatch( InputIt1 first1, InputIt1 last1, (3) C++20)
   InputIt2 first2, InputIt2 last2 );
   template< class ExecutionPolicy, class
   ForwardIt1, class ForwardIt2 >
   std::pair<ForwardIt1,ForwardIt2> (6) (since
   mismatch( ExecutionPolicy&& policy, C++17)
   ForwardIt1 first1, ForwardIt1 last1,
   ForwardIt2 first2, ForwardIt2 last2 );
   template< class InputIt1, class InputIt2,
   class BinaryPredicate >
                        (5) (since
   std::pair<InputIt1,InputIt2> C++14)
   mismatch( InputIt1 first1, InputIt1 last1, (until
   InputIt2 first2, InputIt2 last2, C++20)
   BinaryPredicate p );
   template< class InputIt1, class InputIt2,
   class BinaryPredicate >
   constexpr std::pair<InputIt1,InputIt2> (since
   mismatch( InputIt1 first1, InputIt1 last1, (7) C++20)
   InputIt2 first2, InputIt2 last2,
   BinaryPredicate p );
   template< class ExecutionPolicy, class
   ForwardIt1, class ForwardIt2,
   class BinaryPredicate >
   std::pair<ForwardIt1,ForwardIt2> (8) (since
   mismatch( ExecutionPolicy&& policy, C++17)
   ForwardIt1 first1, ForwardIt1 last1,
   ForwardIt2 first2, ForwardIt2 last2,
   BinaryPredicate p );
   Returns the first mismatching pair of elements from two ranges: one defined by
   [first1, last1) and another defined by [first2,last2). If last2 is not provided
   (overloads (1-4)), it denotes first2 + (last1 - first1).
   1,5) Elements are compared using operator==.
   3,7) Elements are compared using the given binary predicate p.
   2,4,6,8) Same as (1,3,5,7), but executed according to 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.

Parameters
   first1, last1 - the first range of the elements
   first2, last2 - the second range of the elements
   policy - the execution policy to use. See execution policy for details.
                   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);
   p - 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 objects of types
                   InputIt1 and InputIt2 can be dereferenced and then implicitly
                   converted to Type1 and Type2 respectively.

Type requirements
   -
   InputIt1 must meet the requirements of LegacyInputIterator.
   -
   InputIt2 must meet the requirements of LegacyInputIterator.
   -
   ForwardIt1 must meet the requirements of LegacyForwardIterator.
   -
   ForwardIt2 must meet the requirements of LegacyForwardIterator.
   -
   BinaryPredicate must meet the requirements of BinaryPredicate.

Return value
   std::pair with iterators to the first two non-equal elements.
   If no mismatches are found when the comparison reaches last1, the pair
   holds last1 and the corresponding iterator from the second range. The (until C++14)
   behavior is undefined if the second range is shorter than the first
   range.
   If no mismatches are found when the comparison reaches last1 or last2,
   whichever happens first, the pair holds the end iterator and the (since C++14)
   corresponding iterator from the other range.

Complexity
   1-4) At most last1 - first1 applications of operator== or the predicate p
   5-8) At most min(last1 - first1, last2 - first2) applications of operator== or the
   predicate p.

Exceptions
   The overloads with a template parameter named ExecutionPolicy report errors as
   follows:
     * If execution of a function invoked as part of the algorithm throws an exception
       and ExecutionPolicy is one of the standard policies, std::terminate is called.
       For any other ExecutionPolicy, the behavior is implementation-defined.
     * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

First version
template<class InputIt1, class InputIt2> std::pair<InputIt1, InputIt2>
    mismatch(InputIt1 first1, InputIt1 last1, InputIt2 first2) {
    while (first1 != last1 && *first1 == *first2) {
        ++first1, ++first2;
    }
    return std::make_pair(first1, first2); }

Second version
template<class InputIt1, class InputIt2, class BinaryPredicate> std::pair<InputIt1, InputIt2>
    mismatch(InputIt1 first1, InputIt1 last1, InputIt2 first2, BinaryPredicate p) {
    while (first1 != last1 && p(*first1, *first2)) {
        ++first1, ++first2;
    }
    return std::make_pair(first1, first2); }
                        Third version template<class InputIt1, class InputIt2> std::pair<InputIt1, InputIt2>
    mismatch(InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2) {
    while (first1 != last1 && first2 != last2 && *first1 == *first2) {
        ++first1, ++first2;
    }
    return std::make_pair(first1, first2); }
                        Fourth version template<class InputIt1, class InputIt2, class BinaryPredicate> std::pair<InputIt1, InputIt2>
    mismatch(InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, BinaryPredicate p) {
    while (first1 != last1 && first2 != last2 && p(*first1, *first2)) {
        ++first1, ++first2;
    }
    return std::make_pair(first1, first2); }

Example
   This program determines the longest substring that is simultaneously found at the
   very beginning of the given string and at the very end of it, in reverse order
   (possibly overlapping) // Run this code
 #include <iostream>
 #include <string>
 #include <algorithm>
 std::string mirror_ends(const std::string& in)
 {
     return std::string(in.begin(),
                        std::mismatch(in.begin(), in.end(), in.rbegin()).first);
 }
 int main()
 {
     std::cout << mirror_ends("abXYZba") << '\n'
               << mirror_ends("abca") << '\n'
               << mirror_ends("aba") << '\n';
 }

Output:
 ab
 a
 aba

See also
   equal determines if two sets of elements are the same
                        (function template)
   find
   find_if finds the first element satisfying specific criteria
   find_if_not (function template)
   (C++11)
                        returns true if one range is lexicographically less than
   lexicographical_compare another
                        (function template)
   search searches for a range of elements
                        (function template)
   ranges::mismatch finds the first position where two ranges differ
   (C++20) (niebloid)