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Showing content from https://en.cppreference.com/w/cpp/language/../named_req/../ranges/viewable_range.html below:

std::ranges::viewable_range - cppreference.com

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.

#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;
    });
}

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