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    std::includes

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    C++
     
    Algorithm library
    Constrained algorithms and algorithms on ranges (C++20)
    Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
    Non-modifying sequence operations    
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    (C++17)
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    (C++11)
    (C++11)
    (C++11)
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    (until C++17)(C++11)
    (C++20)(C++20)
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    (C++17)

    Sorting and related operations
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    (C++11)    
    (C++11)

    Sorting operations
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    (on partitioned ranges)
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    (C++11)
    (C++17)
    Lexicographical comparison operations
    (C++20)
    Permutation operations


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

    Checks if the sorted target range [first2, last2) is a subsequence of the sorted source range [first1, last1).

    A sequence S is a subsequence of another sequence T if S can be obtained from T by removing any number of T’s elements and keeping the remaining elements in the same order.

    1) If any of these two ranges is not sorted with respect to operator<(until C++20)std::less{}(since C++20), the behavior is undefined.
    2) If any of these two ranges is not sorted with respect to comp, the behavior is undefined.
    3,4) Same as (1,2), but executed according to policy.
    These overloads participate in overload resolution only if the value of the following expression is true:

    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)

    Parameters

    first1, last1 - the pair of iterators defining the source range
    first2, last2 - the pair of iterators defining the target range
    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. ​

    policy - the execution policy to use
    Type requirements
    -
    InputIt1, InputIt2 must meet the requirements of LegacyInputIterator.
    -
    ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.
    -
    Compare must meet the requirements of Compare.

    Return value

    true if the target range is a subsequence of the source range; otherwise false.

    Complexity

    Given

    • N1 as std::distance(first1, last1),
    • N2 as std::distance(first2, last2):
    1) At most 2⋅(N1+N2)-1 comparisons using operator<(until C++20)std::less{}(since C++20).
    2) At most 2⋅(N1+N2)-1 applications of the comparison function comp.
    3) 𝓞(N1+N2) comparisons using operator<(until C++20)std::less{}(since C++20).
    4) 𝓞(N1+N2) applications of the comparison function comp.

    Exceptions

    3,4) During the execution process:
    • If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
    • If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

    Possible implementation

    include (1) 
    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;
}
    include (2) 
    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 <algorithm>
#include <cctype>
#include <iostream>

template<class Os, class Co>
Os& operator<<(Os& os, const Co& v)
{
    for (const auto& i : v)
        os << i << ' ';
    return os << '\t';
}

int main()
{
    const auto
        v1 = {'a', 'b', 'c', 'f', 'h', 'x'},
        v2 = {'a', 'b', 'c'},
        v3 = {'a', 'c'},
        v4 = {'a', 'a', 'b'},
        v5 = {'g'},
        v6 = {'a', 'c', 'g'},
        v7 = {'A', 'B', 'C'};
    
    auto no_case = [](char a, char b) { return std::tolower(a) < std::tolower(b); };
    
    std::cout
    << v1 << "\nincludes:\n" << std::boolalpha
    << v2 << ": " << std::includes(v1.begin(), v1.end(), v2.begin(), v2.end()) << '\n'
    << v3 << ": " << std::includes(v1.begin(), v1.end(), v3.begin(), v3.end()) << '\n'
    << v4 << ": " << std::includes(v1.begin(), v1.end(), v4.begin(), v4.end()) << '\n'
    << v5 << ": " << std::includes(v1.begin(), v1.end(), v5.begin(), v5.end()) << '\n'
    << v6 << ": " << std::includes(v1.begin(), v1.end(), v6.begin(), v6.end()) << '\n'
    << v7 << ": " << std::includes(v1.begin(), v1.end(), v7.begin(), v7.end(), no_case)
          << " (case-insensitive)\n";
}

    Output: 

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

    Defect reports

    The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

    DR Applied to Behavior as published Correct behavior
    LWG 1205 C++98 the return value was unclear if the target range is empty returns true in this case

    See also