# std::includes

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class InputIt1, class InputIt2 > bool includes( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2 ); |  | (until C++20)
template< class InputIt1, class InputIt2 > constexpr bool includes( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2 ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > bool includes( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, ForwardIt2 last2 ); | (2) | (since C++17)
| (3) |
template< class InputIt1, class InputIt2, class Compare > bool includes( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, Compare comp ); |  | (until C++20)
template< class InputIt1, class InputIt2, class Compare > constexpr bool includes( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, Compare comp ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class Compare > bool includes( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, ForwardIt2 last2, Compare comp ); | (4) | (since C++17)

Returns true if the sorted range `[first2, last2)` is a subsequence of the sorted range `[first1, last1)`. (A subsequence need not be contiguous.)

1) Both ranges must be sorted with operator<.
3) Both ranges must be sorted with the given comparison function `comp`.
2,4) Same as (1,3), 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 sorted range of elements to examine
first2, last2 | - | the sorted range of elements to search for
policy | - | the execution policy to use. See execution policy for details.

comp

-

comparison function object (i.e. an object that satisfies the requirements of Compare) which returns ​true if the first argument is less than (i.e. is ordered before) the second.

The signature of the comparison function should be equivalent to the following:

bool cmp(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 an object of type InputIt can be dereferenced and then implicitly converted to both of them. ​

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

### Return value

true if `[first2, last2)` is a subsequence of `[first1, last1)`; otherwise false.

### Complexity

At most 2·(N1+N2-1) comparisons, where N1 = std::distance(first1, last1) and N2 = std::distance(first2, last2).

### 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>
    bool includes(InputIt1 first1, InputIt1 last1,
                  InputIt2 first2, InputIt2 last2)
    {
        for (; first2 != last2; ++first1)
        {
            if (first1 == last1 || *first2 < *first1)
                return false;
            if ( !(*first1 < *first2) )
                ++first2;
        }
        return true;
    }

Second version

    template<class InputIt1, class InputIt2, class Compare>
    bool includes(InputIt1 first1, InputIt1 last1,
                  InputIt2 first2, InputIt2 last2, Compare comp)
    {
        for (; first2 != last2; ++first1)
        {
            if (first1 == last1 || comp(*first2, *first1))
                return false;
            if (!comp(*first1, *first2))
                ++first2;
        }
        return true;
    }

### Example

Run this code

    #include <iostream>
    #include <algorithm>
    #include <cctype>
    #include <vector>

    int main()
    {
      std::vector<char> v1 {'a', 'b', 'c', 'f', 'h', 'x'};
      std::vector<char> v2 {'a', 'b', 'c'};
      std::vector<char> v3 {'a', 'c'};
      std::vector<char> v4 {'g'};
      std::vector<char> v5 {'a', 'c', 'g'};

      for (auto i : v1) std::cout << i << ' ';
      std::cout << "\nincludes:\n" << std::boolalpha;

      for (auto i : v2) std::cout << i << ' ';
      std::cout << ": " << std::includes(v1.begin(), v1.end(), v2.begin(), v2.end()) << '\n';
      for (auto i : v3) std::cout << i << ' ';
      std::cout << ": " << std::includes(v1.begin(), v1.end(), v3.begin(), v3.end()) << '\n';
      for (auto i : v4) std::cout << i << ' ';
      std::cout << ": " << std::includes(v1.begin(), v1.end(), v4.begin(), v4.end()) << '\n';
      for (auto i : v5) std::cout << i << ' ';
      std::cout << ": " << std::includes(v1.begin(), v1.end(), v5.begin(), v5.end()) << '\n';

      auto cmp_nocase = [](char a, char b) {
        return std::tolower(a) < std::tolower(b);
      };

      std::vector<char> v6 {'A', 'B', 'C'};
      for (auto i : v6) std::cout << i << ' ';
      std::cout << ": (case-insensitive) "
                << std::includes(v1.begin(), v1.end(), v6.begin(), v6.end(), cmp_nocase)
                << '\n';
    }

Output:

    a b c f h x
    includes:
    a b c : true
    a c : true
    g : false
    a c g : false
    A B C : (case-insensitive) true

### See also

set_difference | computes the difference between two sets (function template)
search | searches for a range of elements (function template)

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