# std::equal

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt1, class InputIt2 > bool equal( InputIt1 first1, InputIt1 last1, InputIt2 first2 ); | (1) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > bool equal( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2 ); | (2) | (since C++17)
template< class InputIt1, class InputIt2, class BinaryPred > bool equal( InputIt1 first1, InputIt1 last1, InputIt2 first2, BinaryPred p ); | (3) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPred > bool equal( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, BinaryPred p ); | (4) | (since C++17)
template< class InputIt1, class InputIt2 > bool equal( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2 ); | (5) | (since C++14) (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > bool equal( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, ForwardIt2 last2 ); | (6) | (since C++17)
template< class InputIt1, class InputIt2, class BinaryPred > bool equal( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, BinaryPred p ); | (7) | (since C++14) (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPred > bool equal( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, ForwardIt2 last2, BinaryPred p ); | (8) | (since C++17)

Checks whether `[`first1`,` last1`)` and a range starting from first2 are equal:

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

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:

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)

### Parameters

first1, last1 | - | the first range of the elements to compare
first2, last2 | - | the second range of the elements to compare
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 objects of types InputIt1 and InputIt2 can be dereferenced and then implicitly converted to Type1 and Type2 respectively. ​

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

### Return value

1-4) If each corresponding elements in the two ranges are equal, returns true. Otherwise returns false.
5-8) If std::distance (first1, last1) and std::distance (first2, last2) are equal, and each corresponding elements in the two ranges are equal, returns true. Otherwise returns false.

### Complexity

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

1) At most \(\scriptsize N_1\)N1 comparisons using operator==.
2) \(\scriptsize O(N_1)\)O(N1) comparisons using operator==.
3) At most \(\scriptsize N_1\)N1 applications of the predicate p.
4) \(\scriptsize O(N_1)\)O(N1) applications of the predicate p.
5-8) If `InputIt1` and `InputIt2` are both LegacyRandomAccessIterator, and last1 - first1 != last2 - first2 is true, no comparison will be made.
Otherwise, given \(\scriptsize N\)N as \(\scriptsize \min(N_1,N_2)\)min(N1,N2):
5) At most \(\scriptsize N\)N comparisons using operator==.
6) \(\scriptsize O(N)\)O(N) comparisons using operator==.
7) At most \(\scriptsize N\)N applications of the predicate p.
8) \(\scriptsize O(N)\)O(N) applications of 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

equal (1)

    template<class InputIt1, class InputIt2>
    constexpr //< since C++20
    bool equal(InputIt1 first1, InputIt1 last1, InputIt2 first2)
    {
        for (; first1 != last1; ++first1, ++first2)
            if (!(*first1 == *first2))
                return false;

        return true;
    }

equal (3)

    template<class InputIt1, class InputIt2, class BinaryPred>
    constexpr //< since C++20
    bool equal(InputIt1 first1, InputIt1 last1,
               InputIt2 first2, BinaryPred p)
    {
        for (; first1 != last1; ++first1, ++first2)
            if (!p(*first1, *first2))
                return false;

        return true;
    }

equal (5)

    namespace detail
    {
        // random-access iterator implementation (allows quick range size detection)
        template<class RandomIt1, class RandomIt2>
        constexpr //< since C++20
        bool equal(RandomIt1 first1, RandomIt1 last1, RandomIt2 first2, RandomIt2 last2,
                   std::random_access_iterator_tag, std::random_access_iterator_tag)
        {
            if (last1 - first1 != last2 - first2)
                return false;

            for (; first1 != last1; ++first1, ++first2)
                if (!(*first1 == *first2))
                    return false;

            return true;
        }

        // input iterator implementation (needs to manually compare with “last2”)
        template<class InputIt1, class InputIt2>
        constexpr //< since C++20
        bool equal(InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2,
                   std::input_iterator_tag, std::input_iterator_tag)
        {
            for (; first1 != last1 && first2 != last2; ++first1, ++first2)
                if (!(*first1 == *first2))
                    return false;

            return first1 == last1 && first2 == last2;
        }
    }

    template<class InputIt1, class InputIt2>
    constexpr //< since C++20
    bool equal(InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2)
    {
        details::equal(first1, last1, first2, last2,
                       typename std::iterator_traits<InputIt1>::iterator_category(),
                       typename std::iterator_traits<InputIt2>::iterator_category());
    }

equal (7)

    namespace detail
    {
        // random-access iterator implementation (allows quick range size detection)
        template<class RandomIt1, class RandomIt2, class BinaryPred>
        constexpr //< since C++20
        bool equal(RandomIt1 first1, RandomIt1 last1,
                   RandomIt2 first2, RandomIt2 last2, BinaryPred p,
                   std::random_access_iterator_tag, std::random_access_iterator_tag)
        {
            if (last1 - first1 != last2 - first2)
                return false;

            for (; first1 != last1; ++first1, ++first2)
                if (!p(*first1, *first2))
                    return false;

            return true;
        }

        // input iterator implementation (needs to manually compare with “last2”)
        template<class InputIt1, class InputIt2, class BinaryPred>
        constexpr //< since C++20
        bool equal(InputIt1 first1, InputIt1 last1,
                   InputIt2 first2, InputIt2 last2, BinaryPred p,
                   std::input_iterator_tag, std::input_iterator_tag)
        {
            for (; first1 != last1 && first2 != last2; ++first1, ++first2)
                if (!p(*first1, *first2))
                    return false;

            return first1 == last1 && first2 == last2;
        }
    }

    template<class InputIt1, class InputIt2, class BinaryPred>
    constexpr //< since C++20
    bool equal(InputIt1 first1, InputIt1 last1,
               InputIt2 first2, InputIt2 last2, BinaryPred p)
    {
        details::equal(first1, last1, first2, last2, p,
                       typename std::iterator_traits<InputIt1>::iterator_category(),
                       typename std::iterator_traits<InputIt2>::iterator_category());
    }

### Notes

`std::equal` should not be used to compare the ranges formed by the iterators from std::unordered_set, std::unordered_multiset, std::unordered_map, or std::unordered_multimap because the order in which the elements are stored in those containers may be different even if the two containers store the same elements.

When comparing entire containers or string views(since C++17) for equality, operator== for the corresponding type are usually preferred.

Sequential `std::equal` is not guaranteed to be short-circuit. E.g. if the first pair elements of both ranges do not compare equal, the rest of elements may also be compared. Non-short-circuit comparison may happen when the ranges are compared with std::memcmp or implementation-specific vectorized algorithms.

### Example

The following code uses `std::equal` to test if a string is a palindrome.

Run this code

    #include <algorithm>
    #include <iomanip>
    #include <iostream>
    #include <string_view>

    constexpr bool is_palindrome(const std::string_view& s)
    {
        return std::equal(s.cbegin(), s.cbegin() + s.size() / 2, s.crbegin());
    }

    void test(const std::string_view& s)
    {
        std::cout << std::quoted(s)
                  << (is_palindrome(s) ? " is" : " is not")
                  << " a palindrome\n";
    }

    int main()
    {
        test("radar");
        test("hello");
    }

Output:

    "radar" is a palindrome
    "hello" is not a palindrome

### See also

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)
mismatch | finds the first position where two ranges differ (function template)
search | searches for the first occurrence of a range of elements (function template)
ranges::equal (C++20) | determines if two sets of elements are the same (algorithm function object)
equal_to | function object implementing x == y (class template)
equal_range | returns range of elements matching a specific key (function template)

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