std::includes() algorithm

since C++20

// (1)
template< class InputIt1, class InputIt2 >
constexpr bool includes( InputIt1 first1, InputIt1 last1,
                         InputIt2 first2, InputIt2 last2 );

// (2)
template< class InputIt1, class InputIt2, class Compare >
constexpr bool includes( InputIt1 first1, InputIt1 last1,
                         InputIt2 first2, InputIt2 last2, Compare comp );


// (3)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
bool includes( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1,
               ForwardIt2 first2, ForwardIt2 last2 );

// (4)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2,
          class Compare >
bool includes( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1,
               ForwardIt2 first2, ForwardIt2 last2, Compare comp );

since C++17

// (1)
template< class InputIt1, class InputIt2 >
bool includes( InputIt1 first1, InputIt1 last1,
               InputIt2 first2, InputIt2 last2 );

// (2)
template< class InputIt1, class InputIt2, class Compare >
bool includes( InputIt1 first1, InputIt1 last1,
               InputIt2 first2, InputIt2 last2, Compare comp );


// (3)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
bool includes( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1,
               ForwardIt2 first2, ForwardIt2 last2 );

// (4)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2,
          class Compare >
bool includes( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1,
               ForwardIt2 first2, ForwardIt2 last2, Compare comp );

until C++17

// (1)
template< class InputIt1, class InputIt2 >
bool includes( InputIt1 first1, InputIt1 last1,
               InputIt2 first2, InputIt2 last2 );

// (2)
template< class InputIt1, class InputIt2, class Compare >
bool includes( InputIt1 first1, InputIt1 last1,
               InputIt2 first2, InputIt2 last2, Compare comp );

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<.

• (2) Both ranges must be sorted with comp.

• (3 - 4) Same as (1) and (2), but executed according to policy.

  Overload Resolution

  These overloads participate in overload resolution only if 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)

This includes function is stable, which means that for equivalent elements in the original two ranges, the elements from the first range precede the elements from the second range, preserving their original order.

Undefined Behaviour

The behavior is undefined

if the destination range overlaps either of the input ranges (the input ranges may overlap each other).

Parameters

first1 last2	The sorted range of elements to examine.
first2 last3	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). The signature of the comparison function should be equivalent to the following: bool cmp(const Type1 &a, const Type2 &b); The signature does not need to have const&, but must not modify arguments. Must accept all values of type (possibly const) Type and Type2, regardless of value category (so 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 RandomIt can be implicitly converted to both of them.

Type requirements

InputIt1 InputIt2	LegacyInputIterator
ForwardIt1 ForwardIt2 ForwardIt3	LegacyForwardIterator

Return value

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

Complexity

Given N1 as std::distance(first1, last1) and N2 as std::distance(first2, last2):

(1, 3) At most 2 * (N1 + N2 − 1) comparisons using operator<.

(2, 4) At most 2 * (N1 + N2 − 1) applications of the comparison function comp.

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

includes(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;
}

includes(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;
}

Examples

Main.cpp

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

template<class Os, class Co>
Os& operator<<(Os& os, const Co& v)
{
    for (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";
}

Possible 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)