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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../algorithm/mismatch.html below:

std::mismatch - cppreference.com

Returns a pair of iterators to the first mismatching of elements from [first1, last1) and a range starting from first2:

• For overloads (1-4), the second range has std::distance(first1, last1) elements.
• For overloads (5-8), the second range is [first2, last2).

• If std::distance(first1, last1) and std::distance(first2, last2) are different, the comparison stops when last1 or last2 is reached.

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 participate in overload resolution only if all following conditions are satisfied:

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 objects of types InputIt1 and InputIt2 can be dereferenced and then implicitly converted to Type1 and Type2 respectively. ​

std::pair with iterators to the first two non-equal elements.

If last1 is reached, the second iterator in the pair is the std::distance(first1, last1)th iterator after first2.

For overloads (5-8), if last2 is reached, the first iterator in the pair is the std::distance(first2, last2)th iterator after first1.

Given \(\scriptsize N_1\)N1 as std::distance(first1, last1) and \(\scriptsize N_2\)N2 as std::distance(first2, last2):

1,2) At most \(\scriptsize N_1\)N1 comparisons using operator==.

3,4) At most \(\scriptsize N_1\)N1 applications of the predicate p.

5,6) At most \(\scriptsize \min(N_1,N_2)\)min(N1,N2) comparisons using operator==.

7,8) At most \(\scriptsize \min(N_1,N_2)\)min(N1,N2) applications of the predicate p.

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.

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);
}

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

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);
}

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

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).

#include <algorithm>
#include <iostream>
#include <string>
 
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:

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