# std::ranges::views::common, std::ranges::common_view

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< cpp‎ | ranges

Defined in header `<ranges>` |  |
template< ranges::view V > requires (not ranges::common_range <V> and std::copyable < ranges::iterator_t <V>>) class common_view : public ranges::view_interface <common_view<V>> | (1) | (since C++20)
namespace views { inline constexpr /* unspecified */ common = /* unspecified */; } | (2) | (since C++20)
Call signature |  |
template< ranges::viewable_range R > requires /* see below */ constexpr ranges::view auto common( R&& r ); |  | (since C++20)

1) Adapts a given `view` with different types for iterator/sentinel pair into a `view` that is also a `common_range`. A `common_view` always has the same iterator/sentinel type.
2) RangeAdaptorObject. Let e be a subexpression. Then the expression views::common(e) is expression-equivalent to:

- views::all (e), if it is a well-formed expression and decltype((e)) models `common_range`;
- common_view{e} otherwise.

### Data members

Member name | Definition
`base_` (private) | The underlying view of type `V`. (exposition-only member object*)

### Member functions

(constructor) | constructs a `common_view` (public member function)
base | returns a copy of the underlying (adapted) view (public member function)
begin | returns an iterator to the beginning (public member function)
end | returns an iterator to the end (public member function)
size | returns the number of elements. Provided only if the underlying (adapted) range satisfies `sized_range`. (public member function)

##### Inherited from std::ranges::view_interface

empty | returns whether the derived view is empty. Provided if it satisfies `sized_range` or `forward_range`. (public member function of `std::ranges::view_interface<D>`)
cbegin (C++23) | returns a constant iterator to the beginning of the range. (public member function of `std::ranges::view_interface<D>`)
cend (C++23) | returns a sentinel for the constant iterator of the range. (public member function of `std::ranges::view_interface<D>`)
operator bool | returns whether the derived view is not empty. Provided if ranges::empty is applicable to it. (public member function of `std::ranges::view_interface<D>`)
data | gets the address of derived view's data. Provided if its iterator type satisfies `contiguous_iterator`. (public member function of `std::ranges::view_interface<D>`)
front | returns the first element in the derived view. Provided if it satisfies `forward_range`. (public member function of `std::ranges::view_interface<D>`)
back | returns the last element in the derived view. Provided if it satisfies `bidirectional_range` and `common_range`. (public member function of `std::ranges::view_interface<D>`)
operator[] | returns the `n`th element in the derived view. Provided if it satisfies `random_access_range`. (public member function of `std::ranges::view_interface<D>`)

### Deduction guides

### Helper templates

template< class T > constexpr bool enable_borrowed_range<std::ranges::common_view<T>> = ranges::enable_borrowed_range <T>; |  | (since C++20)

This specialization of std:: ranges::enable_borrowed_range makes `common_view` satisfy `borrowed_range` when the underlying view satisfies it.

### Notes

`common_view` can be useful for working with legacy algorithms that expect the iterator and sentinel are of the same type.

### Example

Run this code

    #include <iostream>
    #include <iterator>
    #include <list>
    #include <numeric>
    #include <ranges>

    int main()
    {
        auto v1 = {1, 2, 3, 4, 5};
        auto i1 = std::counted_iterator{v1.begin(), std::ssize(v1)};
        auto r1 = std::ranges::subrange{i1, std::default_sentinel};
    //  auto e1 = std::accumulate(r1.begin(), r1.end(), 0); // error: "common range" required
        auto c1 = std::ranges::common_view{r1};
        std::cout << "accumulate: " << std::accumulate(c1.begin(), c1.end(), 0) << '\n';

        // inherited from ranges::view_interface:
        std::cout << "c1.front(): " << c1.front() << '\n';
        std::cout << "c1.back(): " << c1.back() << '\n';
        std::cout << "c1.data(): " << c1.data() << '\n';
        std::cout << "c1[0]: " << c1[0] << '\n';

        auto v2 = std::list{1, 2, 3, 4, 5};
        auto i2 = std::counted_iterator{v2.begin(), std::ssize(v2)};
        auto r2 = std::ranges::subrange{i2, std::default_sentinel};
    //  auto e2 = std::accumulate(r2.begin(), r2.end(), 0); // error: "common range" required
        auto c2 = std::ranges::common_view{ r2 };
        std::cout << "accumulate: " << std::accumulate(c2.begin(), c2.end(), 0) << '\n';

        // inherited from ranges::view_interface:
        std::cout << "c2.front(): " << c2.front() << '\n';
    //  auto e3 = c2.back(); // error: "bidirectional range" required
    //  auto e4 = c2.data(); // error: "contiguous range" required
    //  auto e5 = c2[0];     // error: "random access range" required
    }

Possible output:

    accumulate: 15
    c1.front(): 1
    c1.back(): 5
    c1.data(): 0x7f19937f00d0
    c1[0]: 1
    accumulate: 15
    c2.front(): 1

### 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 3494 | C++20 | `common_view` was never a `borrowed_range` | it is a `borrowed_range` if its underlying view is

### See also

ranges::common_range (C++20) | specifies that a range has identical iterator and sentinel types (concept)
common_iterator (C++20) | adapts an iterator type and its sentinel into a common iterator type (class template)

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