std::ranges::range_adaptor_closure

std::ranges::range_adaptor_closure - cppreference.com

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`template< class D > requires std::is_object_v<D> && std::same_as<D, std::remove_cv_t<D>> class range_adaptor_closure {};`		(since C++23)

std::ranges::range_adaptor_closure is a helper class template for defining a RangeAdaptorClosureObject.

Let t be the object of type T, the implementation ensures that t is a range adaptor closure object if all the requirements are met:

t is a unary function object that takes one range argument.
T has exactly one public base class ranges::range_adaptor_closure<T>, and T has no base classes of type ranges::range_adaptor_closure<U> for any other type U.
T does not satisfy range.

Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_ranges`	`202202L`	(C++23)	`std::ranges::range_adaptor_closure`

Example

#include <ranges>
#include <string_view>

// Define Slice as a range adaptor closure
struct Slice : std::ranges::range_adaptor_closure<Slice>
{
    std::size_t start = 0;
    std::size_t end = std::string_view::npos;

    constexpr std::string_view operator()(std::string_view sv) const
    {
        return sv.substr(start, end - start);
    }
};

int main()
{
    constexpr std::string_view str = "01234567";

    constexpr Slice slicer{.start = 1, .end = 6};

    // use slicer as a normal function object
    constexpr auto sv1 = slicer(str);
    static_assert(sv1 == "12345");

    // use slicer as a range adaptor closure object
    constexpr auto sv2 = str | slicer;
    static_assert(sv2 == "12345");

    // range adaptor closures can be composed
    constexpr auto slice_and_drop = slicer | std::views::drop(2);
    static_assert(std::string_view(str | slice_and_drop) == "345");
}