# std::ranges::viewable_range

From cppreference.com
< cpp‎ | ranges

Defined in header `<ranges>` |  |
template< class T > concept viewable_range = ranges::range <T> && (( ranges::view < std::remove_cvref_t <T>> && std::constructible_from < std::remove_cvref_t <T>, T>) || (! ranges::view < std::remove_cvref_t <T>> && ( std::is_lvalue_reference_v <T> || ( std::movable < std::remove_reference_t <T>> && !/*is-initializer-list*/<T>)))); |  | (since C++20)

The `viewable_range` concept is a refinement of `range` that describes a range that can be converted into a `view` through `views::all`.

The constant /*is-initializer-list*/<T> is true if and only if std::remove_cvref_t <T> is a specialization of std::initializer_list.

### Example

Run this code

    #include <ranges>
    #include <string>
    #include <vector>

    struct valid_result {};
    struct invalid_result {};

    template <typename T>
    concept valid_viewable_range = std::same_as<T, valid_result>;

    template <typename T>
    concept invalid_viewable_range = std::same_as<T, invalid_result>;

    auto test_viewable_range(std::ranges::viewable_range auto &&) -> valid_result;
    auto test_viewable_range(auto&&) -> invalid_result;

    int main()
    {
        auto il = {1, 2, 3};
        int arr []{1, 2, 3};
        std::vector vec{1, 2, 3};
        std::ranges::ref_view r{arr};
        std::ranges::owning_view o{std::string("Hello")};

        static_assert(requires {
            { test_viewable_range(il) } -> valid_viewable_range;
            { test_viewable_range(std::move(il)) } -> invalid_viewable_range;
            { test_viewable_range(arr) } -> valid_viewable_range;
            { test_viewable_range(std::move(arr)) } -> invalid_viewable_range;
            { test_viewable_range(vec) } -> valid_viewable_range;
            { test_viewable_range(std::move(vec)) } -> valid_viewable_range;
            { test_viewable_range(r) } -> valid_viewable_range;
            { test_viewable_range(std::move(r)) } -> valid_viewable_range;
            { test_viewable_range(o) } -> invalid_viewable_range;
            { test_viewable_range(std::move(o)) } -> valid_viewable_range;
            { test_viewable_range(std::ranges::ref_view(o)) } -> valid_viewable_range;
        });
    }

### 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 3481 | C++20 | `viewable_range` accepted an lvalue of a move-only view | rejects
P2415R2 | C++20 | `viewable_range` only accepted non-`view` rvalues that are `borrowed_range` | accepts more types

### See also

views::all_tviews::all (C++20) | a `view` that includes all elements of a `range` (alias template) (range adaptor object)

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