NAME¶

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

Synopsis¶

Defined in header <algorithm>
Call signature
template< std::input_iterator I, std::sentinel_for<I> S,

class T,
class Proj = std::identity > (since
requires std::indirect_binary_predicate<ranges::equal_to, C++23)
std::projected<I, Proj>, (until
const T*> C++26)

constexpr bool contains( I first, S last, const T& value,
Proj proj = {} );
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, (since
std::projected<I, Proj>, C++26)
const T*>

constexpr bool contains( I first, S last, const T& value,
Proj proj = {} );
template< ranges::input_range R,

class T,
class Proj = std::identity > (since
requires std::indirect_binary_predicate<ranges::equal_to, C++23)
(until
std::projected<ranges::iterator_t<R>, Proj>, C++26)
const T*>

constexpr bool contains( R&& r, const T& value, Proj proj =
{} );
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, (1) (since
C++26)
std::projected<ranges::iterator_t<R>, Proj>,
const T*>

constexpr bool contains( R&& r, const T& value, Proj proj =
{} );
template< std::forward_iterator I1, std::sentinel_for<I1>
S1,

std::forward_iterator I2, std::sentinel_for<I2> (2)
S2,
class Pred = ranges::equal_to,
class Proj1 = std::identity, class Proj2 =
std::identity > (3) (since
requires std::indirectly_comparable<I1, I2, Pred, Proj1, C++23)
Proj2>
constexpr bool contains_subrange( I1 first1, S1 last1, I2
first2, S2 last2,
Pred pred = {},

Proj1 proj1 = {}, Proj2
proj2 = {} );
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>, (4) (since
ranges::iterator_t<R2>, C++23)
Pred, Proj1, Proj2>
constexpr bool contains_subrange( R1&& r1, R2&& r2, Pred
pred = {},

Proj1 proj1 = {}, Proj2
proj2 = {} );

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 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.

In practice, they may be implemented as function objects, or with special compiler
extensions.

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 Searchers (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
}

See also¶

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