std::ranges:: contains, std::ranges:: contains_subrange

From cppreference.com

Defined in header `<algorithm>`
Call signature
	(1)
template < std:: input_iterator I, std:: sentinel_for < I > S, class T, class Proj = std:: identity > requires std:: indirect_binary_predicate < ranges:: equal_to , std :: projected < I, Proj > , const T * > constexpr bool contains ( I first, S last, const T & value, Proj proj = { } ) ;			(since C++23) (until C++26)
template < std:: input_iterator I, std:: sentinel_for < I > S, class Proj = std:: identity , class T = std :: projected_value_t < I, Proj > > requires std:: indirect_binary_predicate < ranges:: equal_to , std :: projected < I, Proj > , const T * > constexpr bool contains ( I first, S last, const T & value, Proj proj = { } ) ;			(since C++26)
		(2)
template < ranges:: input_range R, class T, class Proj = std:: identity > requires std:: indirect_binary_predicate < ranges:: equal_to , std :: projected < ranges:: iterator_t < R > , Proj > , const T * > constexpr bool contains ( R && r, const T & value, Proj proj = { } ) ;				(since C++23) (until C++26)
template < ranges:: input_range R, class Proj = std:: identity , class T = std :: projected_value_t < ranges:: iterator_t < R > , Proj > > requires std:: indirect_binary_predicate < ranges:: equal_to , std :: projected < ranges:: iterator_t < R > , Proj > , const T * > constexpr bool contains ( R && r, const T & value, Proj proj = { } ) ;				(since C++26)
template < std:: forward_iterator I1, std:: sentinel_for < I1 > S1, std:: forward_iterator I2, std:: sentinel_for < I2 > S2, class Pred = ranges:: equal_to , class Proj1 = std:: identity , class Proj2 = std:: identity > requires std:: indirectly_comparable < I1, I2, Pred, Proj1, Proj2 > constexpr bool contains_subrange ( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = { } , Proj1 proj1 = { } , Proj2 proj2 = { } ) ;			(3)	(since C++23)
template < ranges:: forward_range R1, ranges:: forward_range R2, class Pred = ranges:: equal_to , class Proj1 = std:: identity , class Proj2 = std:: identity > requires std:: indirectly_comparable < ranges:: iterator_t < R1 > , ranges:: iterator_t < R2 > , Pred, Proj1, Proj2 > constexpr bool contains_subrange ( R1 && r1, R2 && r2, Pred pred = { } , Proj1 proj1 = { } , Proj2 proj2 = { } ) ;			(4)	(since C++23)

1) Search-based algorithm that checks whether or not a given range contains a value with iterator-sentinel pairs.

2) Same as (1) but uses r as the source range, as if using ranges:: begin ( r ) as first and ranges:: end ( r ) as last .

3) Search-based algorithm that checks whether or not a given range is a subrange of another range with iterator-sentinel pairs.

4) Same as (3) but uses r1 as the first source range and r2 as the second source range, as if using ranges:: begin ( r1 ) as first1 , ranges:: end ( r1 ) as last1 , ranges:: begin ( r2 ) as first2 , and ranges:: end ( r2 ) as last2 .

The function-like entities described on this page are algorithm function objects (informally known as niebloids ), that is:

Explicit template argument lists cannot be specified when calling any of them.
None of them are visible to argument-dependent lookup .
When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Parameters

first, last	-	the range of elements to examine
r	-	the range of the elements to examine
value	-	value to compare the elements to
pred	-	predicate to apply to the projected elements
proj	-	projection to apply to the elements

Return value

1,2) : ranges:: find ( std :: move ( first ) , last, value, proj ) ! = last

3,4) : first2 == last2 || ! ranges:: search ( first1, last1, first2, last2, pred, proj1, proj2 ) . empty ( )

Complexity

At most last - first applications of the predicate and projection.

Notes

Up until C++20, we've had to write std :: ranges:: find ( r, value ) ! = std :: ranges:: end ( r ) to determine if a single value is inside a range. And to check if a range contains a subrange of interest, we use not std :: ranges:: search ( haystack, needle ) . empty ( ) . While this is accurate, it isn't necessarily convenient, and it hardly expresses intent (especially in the latter case). Being able to say std :: ranges :: contains ( r, value ) addresses both of these points.

ranges::contains_subrange , same as ranges::search , but as opposed to std::search , provides no access to Searcher s (such as Boyer-Moore ).

Feature-test macro	Value	Std	Feature
`__cpp_lib_ranges_contains`	202207L	(C++23)	`std::ranges::contains` and `ranges::contains_subrange`
`__cpp_lib_algorithm_default_value_type`	202403	(C++26)	List-initialization for algorithms ( 1,2 )

Possible implementation

contains (1,2)

struct __contains_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S,
             class Proj = std::identity,
             class T = std::projected_value_t<I, Proj>>
    requires std::indirect_binary_predicate<ranges::equal_to, std::projected<I, Proj>,
                                            const T*>
    constexpr bool operator()(I first, S last, const T& value, Proj proj = {}) const
    {
        return ranges::find(std::move(first), last, value, proj) != last;
    }
 
    template<ranges::input_range R,
             class Proj = std::identity,
             class T = std::projected_value_t<ranges::iterator_t<R>, Proj>>
    requires std::indirect_binary_predicate<ranges::equal_to,
                                            std::projected<ranges::iterator_t<R>, Proj>,
                                            const T*>
    constexpr bool operator()(R&& r, const T& value, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::move(value), proj);
    }
};
 
inline constexpr __contains_fn contains {};

contains_subrange (3,4)

struct __contains_subrange_fn
{
    template<std::forward_iterator I1, std::sentinel_for<I1> S1,
             std::forward_iterator I2, std::sentinel_for<I2> S2,
             class Pred = ranges::equal_to,
             class Proj1 = std::identity, class Proj2 = std::identity>
    requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
    constexpr bool operator()(I1 first1, S1 last1,
                              I2 first2, S2 last2,
                              Pred pred = {},
                              Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        return (first2 == last2) ||
               !ranges::search(first1, last1, first2, last2, pred, proj1, proj2).empty();
    }
 
    template<ranges::forward_range R1, ranges::forward_range R2,
             class Pred = ranges::equal_to,
             class Proj1 = std::identity, class Proj2 = std::identity>
    requires std::indirectly_comparable<ranges::iterator_t<R1>,
                                        ranges::iterator_t<R2>, Pred, Proj1, Proj2>
    constexpr bool operator()(R1&& r1, R2&& r2,
                              Pred pred = {},
                              Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        return (*this)(ranges::begin(r1), ranges::end(r1),
                       ranges::begin(r2), ranges::end(r2), std::move(pred),
                       std::move(proj1), std::move(proj2));
    }
};
 
inline constexpr __contains_subrange_fn contains_subrange {};

Example

Run this code

#include <algorithm>
#include <array>
#include <complex>
 
namespace ranges = std::ranges;
 
int main()
{
    constexpr auto haystack = std::array{3, 1, 4, 1, 5};
    constexpr auto needle = std::array{1, 4, 1};
    constexpr auto bodkin = std::array{2, 5, 2};
 
    static_assert(
        ranges::contains(haystack, 4) &&
       !ranges::contains(haystack, 6) &&
        ranges::contains_subrange(haystack, needle) &&
       !ranges::contains_subrange(haystack, bodkin)
    );
 
    constexpr std::array<std::complex<double>, 3> nums{{{1, 2}, {3, 4}, {5, 6}}};
    #ifdef __cpp_lib_algorithm_default_value_type
        static_assert(ranges::contains(nums, {3, 4}));
    #else
        static_assert(ranges::contains(nums, std::complex<double>{3, 4}));
    #endif
}

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

ranges::find ranges::find_if ranges::find_if_not (C++20) (C++20) (C++20)	finds the first element satisfying specific criteria (algorithm function object)
ranges::search (C++20)	searches for the first occurrence of a range of elements (algorithm function object)
ranges::binary_search (C++20)	determines if an element exists in a partially-ordered range (algorithm function object)
ranges::includes (C++20)	returns true if one sequence is a subsequence of another (algorithm function object)
ranges::all_of ranges::any_of ranges::none_of (C++20) (C++20) (C++20)	checks if a predicate is true for all, any or none of the elements in a range (algorithm function object)