std::mismatch

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Concepts and utilities: std::Sortable, std::projected, ...
Constrained algorithms: std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
is_execution_policy	execution::seqexecution::parexecution::par_unseqexecution::unseq (C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++20)
Non-modifying sequence operations
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each for_each_n (C++17)	countcount_if mismatch equal adjacent_find	findfind_iffind_if_not (C++11) find_end find_first_of	search search_n lexicographical_compare lexicographical_compare_3way (C++20)
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) shift_leftshift_right (C++20)(C++20) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	removeremove_if replacereplace_if reverse rotate unique random_shuffle (until C++17)	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
Operations on uninitialized storage
uninitialized_copy uninitialized_move (C++17) uninitialized_fill	uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) destroy (C++17)	uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17) destroy_n (C++17)
Partitioning operations
is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
Sorting operations
is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
Binary search operations
lower_bound	upper_bound	binary_search	equal_range
Set operations (on sorted ranges)
merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
Heap operations
is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
Minimum/maximum operations
max max_element	min min_element	minmax (C++11) minmax_element (C++11)	clamp (C++17) compare_3way (C++20)
Permutations
is_permutation (C++11)	next_permutation	prev_permutation
Numeric operations
iota (C++11) inner_product adjacent_difference	accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17) exclusive_scan (C++17)	transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
C library
qsort	bsearch

 

Defined in header <algorithm>
	(1)
template< class InputIt1, class InputIt2 > std::pair<InputIt1,InputIt2>     mismatch( InputIt1 first1, InputIt1 last1,               InputIt2 first2 );			(until C++20)
template< class InputIt1, class InputIt2 > constexpr std::pair<InputIt1,InputIt2>               mismatch( InputIt1 first1, InputIt1 last1,                         InputIt2 first2 );			(since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > std::pair<ForwardIt1,ForwardIt2>     mismatch( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1,               ForwardIt2 first2 );		(2)	(since C++17)
		(3)
template< class InputIt1, class InputIt2, class BinaryPredicate > std::pair<InputIt1,InputIt2>     mismatch( InputIt1 first1, InputIt1 last1,               InputIt2 first2,               BinaryPredicate p );				(until C++20)
template< class InputIt1, class InputIt2, class BinaryPredicate > constexpr std::pair<InputIt1,InputIt2>               mismatch( InputIt1 first1, InputIt1 last1,                         InputIt2 first2,                         BinaryPredicate p );				(since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPredicate > std::pair<ForwardIt1,ForwardIt2>     mismatch( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1,               ForwardIt2 first2,               BinaryPredicate p );			(4)	(since C++17)
			(5)
template< class InputIt1, class InputIt2 > std::pair<InputIt1,InputIt2>     mismatch( InputIt1 first1, InputIt1 last1,               InputIt2 first2, InputIt2 last2 );					(since C++14) (until C++20)
template< class InputIt1, class InputIt2 > constexpr std::pair<InputIt1,InputIt2>               mismatch( InputIt1 first1, InputIt1 last1,                         InputIt2 first2, InputIt2 last2 );					(since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > std::pair<ForwardIt1,ForwardIt2>     mismatch( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1,               ForwardIt2 first2, ForwardIt2 last2 );				(6)	(since C++17)
				(7)
template< class InputIt1, class InputIt2, class BinaryPredicate > std::pair<InputIt1,InputIt2>     mismatch( InputIt1 first1, InputIt1 last1,               InputIt2 first2, InputIt2 last2,               BinaryPredicate p );						(since C++14) (until C++20)
template< class InputIt1, class InputIt2, class BinaryPredicate > constexpr std::pair<InputIt1,InputIt2>               mismatch( InputIt1 first1, InputIt1 last1,                         InputIt2 first2, InputIt2 last2,                         BinaryPredicate p );						(since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPredicate > std::pair<ForwardIt1,ForwardIt2>     mismatch( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1,               ForwardIt2 first2, ForwardIt2 last2,               BinaryPredicate p );					(8)	(since C++17)

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. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> 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.
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 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 behavior is undefined if the second range is shorter than the first range.	(until C++14)
If no mismatches are found when the comparison reaches last1 or last2, whichever happens first, the pair holds the end iterator and the corresponding iterator from the other range.	(since C++14)

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)
findfind_iffind_if_not (C++11)	finds the first element satisfying specific criteria (function template)
lexicographical_compare	returns true if one range is lexicographically less than another (function template)
search	searches for a range of elements (function template)