A small amount of std::ranges from the future

Author: vector-of-bool

src/bsoncxx/include/CMakeLists.txt

@@ -61,6 +61,7 @@ set_dist_list(src_bsoncxx_include_DIST
     bsoncxx/v_noabi/bsoncxx/stdx/make_unique.hpp
     bsoncxx/v_noabi/bsoncxx/stdx/optional.hpp
     bsoncxx/v_noabi/bsoncxx/stdx/operators.hpp
+    bsoncxx/v_noabi/bsoncxx/stdx/ranges.hpp
     bsoncxx/v_noabi/bsoncxx/stdx/string_view.hpp
     bsoncxx/v_noabi/bsoncxx/stdx/type_traits.hpp
     bsoncxx/v_noabi/bsoncxx/string/to_string.hpp

src/bsoncxx/include/bsoncxx/v_noabi/bsoncxx/stdx/ranges.hpp

@@ -0,0 +1,247 @@
+/**
+ * @file ranges.hpp
+ * @brief A backport of a small amount of std::ranges from C++20
+ * @date 2023-11-14
+ *
+ * @copyright Copyright (c) 2023
+ */
+#pragma once
+
+#include <cstddef>
+#include <initializer_list>
+#include <iterator>
+#include <memory>
+#include <type_traits>
+#include <utility>
+
+#include <bsoncxx/stdx/iterator.hpp>
+#include <bsoncxx/stdx/type_traits.hpp>
+
+#include <bsoncxx/config/prelude.hpp>
+
+namespace bsoncxx {
+inline namespace v_noabi {
+namespace detail {
+
+/**
+ * XXX: These _decay_xyz functions are required for MSVC 14.0 (VS2015) compat.
+ *
+ * The requirements on each of the below invocable objects is that the decay-copied
+ * return type meet certain requirements. It would be easier to include these
+ * requirements inline directly via enable_if/requires_t, but it is difficult to
+ * position them in a way that doesn't confuse MSVC 14.0. The most reliable
+ * configuration is to use a SFINAE-disappearing return type, so these functions
+ * serve that purpose while simultaneously performing the required decay-copy.
+ *
+ * It's possible that these are actually easier on the eyes than the inline
+ * requirements…
+ */
+template <typename I>
+constexpr requires_t<I, is_iterator<I>> _decay_iterator(I i) noexcept {
+    return i;
+}
+
+template <typename I, typename S>
+constexpr requires_t<S, is_sentinel_for<I, S>> _decay_sentinel(S s) noexcept {
+    return s;
+}
+
+template <typename Sz>
+constexpr requires_t<Sz, std::is_integral<Sz>> _decay_integral(Sz s) noexcept {
+    return s;
+}
+
+template <typename P>
+constexpr requires_t<P, std::is_pointer<P>> _decay_copy_pointer(P p) noexcept {
+    return p;
+}
+
+/**
+ * @brief Access the beginning iterator of a range-like object.
+ *
+ * Requires that the result is a valid iterator type
+ *
+ * Based on std::ranges::begin()
+ */
+static constexpr struct _begin_fn {
+    // 1: Object is an array
+    template <typename El, std::size_t N>
+    static constexpr auto impl(El (&arr)[N], rank<5>) bsoncxx_returns(arr + 0);
+
+    // 2: Object has member .begin() returning an iterator
+    template <typename R>
+    static constexpr auto impl(R& rng, rank<4>) bsoncxx_returns(_decay_iterator(rng.begin()));
+
+    // 3: Object has an ADL-visible begin(x) returning an iterator
+    template <typename R>
+    static constexpr auto impl(R& rng, rank<3>) bsoncxx_returns(_decay_iterator(begin(rng)));
+
+    template <typename R>
+    constexpr auto operator()(R&& rng) const bsoncxx_returns((impl)(rng, rank<10>{}));
+} begin;
+
+/**
+ * @brief Yields the iterator result from calling begin(R&), if that expression
+ * is valid
+ */
+template <typename R>
+using iterator_t = decltype(begin(std::declval<R&>()));
+
+/**
+ * @brief Access the sentinel value for a range-like object.
+ *
+ * Requires that a valid begin() is also available, and that the end() returns
+ * a type which is_sentinel_for the type iterator_t<R>
+ *
+ * Based on std::ranges::end().
+ */
+static constexpr struct _end_fn {
+    // 1: Range is an array
+    template <typename Iter, typename El, std::size_t N>
+    static constexpr auto impl(El (&arr)[N], rank<5>) bsoncxx_returns(arr + N);
+
+    // 2: Range has member .end() returning a valid sentinel
+    template <typename Iter, typename R>
+    static constexpr auto impl(R& rng, rank<4>) bsoncxx_returns(_decay_sentinel<Iter>(rng.end()));
+
+    // 3: Range has ADL-found end(x) returning a valid sentinel
+    template <typename Iter, typename R>
+    static constexpr auto impl(R& r, rank<3>) bsoncxx_returns(_decay_sentinel<Iter>(end(r)));
+
+    template <typename R>
+    constexpr auto operator()(R&& rng) const bsoncxx_returns((impl<iterator_t<R>>)(rng, rank<5>{}));
+} end;
+
+/**
+ * @brief Yield the type resulting from end(R&), if that expression is valid
+ */
+template <typename R>
+using sentinel_t = decltype(end(std::declval<R&>()));
+
+/**
+ * @brief Obtain the size of the given range `rng`.
+ *
+ * Returns the first valid of:
+ * - The bounds of the array, if `rng` is an array
+ * - The result of rng.size() on R, if that returns an integral type
+ * - The result of size(rng), if such a name is visible via ADL and returns
+ *   an integral type.
+ * - The value of (end(rng) - begin(rng)) as an unsigned integer if `rng` is
+ *   forward-iterable and the sentinel is a sized sentinel type.
+ */
+static constexpr struct _size_fn {
+    // 1: Array of known bound
+    template <typename Element, std::size_t N>
+    static constexpr auto impl(Element (&)[N], rank<5>) bsoncxx_returns(N);
+
+    // 2: Range with member .size()
+    template <typename R>
+    static constexpr auto impl(R& rng, rank<4>) bsoncxx_returns(_decay_integral(rng.size()));
+
+    // 3: Range with ADL-found size(x)
+    template <typename R>
+    static constexpr auto impl(R& rng, rank<3>) bsoncxx_returns(_decay_integral(size(rng)));
+
+    // 4: Range is a forward-range and has a sized sentinel type
+    template <
+        typename R,
+        typename Iter = iterator_t<R>,
+        typename Sentinel = sentinel_t<R>,
+        // Require a forward iterator:
+        requires_t<int, std::is_base_of<std::forward_iterator_tag, iterator_concept_t<Iter>>> = 0,
+        // Require a sized sentinel:
+        requires_t<int, is_sized_sentinel_for<Sentinel, Iter>> = 0,
+        // We cast to an unsigned type from the difference type:
+        typename Sz = make_unsigned_t<difference_t<Sentinel, Iter>>>
+    static constexpr auto impl(R& rng, rank<3>)
+        bsoncxx_returns(static_cast<Sz>(end(rng) - begin(rng)));
+
+    template <typename R>
+    constexpr auto operator()(R&& rng) const bsoncxx_returns((impl)(rng, rank<10>{}));
+} size;
+
+/**
+ * @brief Obtain the size type of the given range
+ */
+template <typename R>
+using range_size_t = decltype(size(std::declval<R&>()));
+
+/**
+ * @brief Obtain the size of the given range as a signed integer type
+ */
+static constexpr struct _ssize_fn {
+    template <typename R,
+              typename Unsigned = range_size_t<R>,
+              typename Signed = make_signed_t<Unsigned>,
+              typename RetDiff =
+                  conditional_t<(sizeof(Signed) > sizeof(std::ptrdiff_t)), Signed, std::ptrdiff_t>>
+    constexpr auto operator()(R&& rng) const bsoncxx_returns(static_cast<RetDiff>(size(rng)));
+} ssize;
+
+/**
+ * @brief Obtain the difference type of the given range
+ */
+template <typename R>
+using range_difference_t = iter_difference_t<iterator_t<R>>;
+
+/**
+ * @brief Obtain a pointer-to-data for the given `rng`.
+ *
+ * Returns the first valid of:
+ * - `rng.data()` if such expression yields a pointer type
+ * - `data(rng)` if such expression is visible via ADL and returns a pointer type
+ * - `to_address(begin(rng))` if such expression is valid and iterator_t<R> is
+ *   a contiguous_iterator.
+ */
+static constexpr struct _data_fn {
+    template <typename R>
+    static constexpr auto impl(R&& rng, rank<2>) bsoncxx_returns(_decay_copy_pointer(rng.data()));
+
+    template <typename R, requires_t<int, is_contiguous_iterator<iterator_t<R>>> = 0>
+    static constexpr auto impl(R&& rng, rank<1>) bsoncxx_returns(to_address(begin(rng)));
+
+    template <typename R>
+    constexpr auto operator()(R&& rng) const bsoncxx_returns((impl)(rng, rank<10>{}));
+} data;
+
+/**
+ * @brief Get the type returned by data(R&), if valid
+ */
+template <typename R>
+using range_data_t = decltype(data(std::declval<R&>()));
+
+/**
+ * @brief Get the value type of the range
+ *
+ * Equivalent: iter_value_t<iterator_t<R>>
+ */
+template <typename R>
+using range_value_t = iter_value_t<iterator_t<R>>;
+
+template <typename R>
+using range_concept_t = iterator_concept_t<iterator_t<R>>;
+
+/**
+ * @brief Trait detects if the given type is a range
+ */
+template <typename R>
+struct is_range : conjunction<is_detected<iterator_t, R>, is_detected<sentinel_t, R>> {};
+
+/**
+ * @brief Detect if the given range is contiguous-ish.
+ *
+ * This is not quite like ranges::contiguous_range, as it does not require that
+ * the iterator be configuous. It only looks for a valid data(R) and size(R).
+ * Without stdlib support, we cannot fully detect contiguous_iterators, but we
+ * want to be able to support, basic_string<C>, vector<T>, array<T>, etc, which
+ * have .data() and .size()
+ */
+template <typename R>
+struct is_contiguous_range
+    : conjunction<is_range<R>, is_detected<range_data_t, R>, is_detected<range_size_t, R>> {};
+
+}  // namespace detail
+}  // namespace v_noabi
+}  // namespace bsoncxx
+
+#include <bsoncxx/config/postlude.hpp>

src/bsoncxx/include/bsoncxx/v_noabi/bsoncxx/stdx/type_traits.hpp

@@ -459,8 +459,7 @@ struct rank<0> {};
 struct _decay_copy_fn {
     template <typename T>
     constexpr auto operator()(T&& arg) const
-        noexcept(std::is_nothrow_constructible<decay_t<T>, T&&>{})
-            -> requires_t<decay_t<T>, std::is_constructible<decay_t<T>, T&&>> {
+        noexcept(noexcept(static_cast<decay_t<T>>(static_cast<T&&>(arg)))) -> decay_t<T> {
         return static_cast<decay_t<T>>(static_cast<T&&>(arg));
     }
 };

src/bsoncxx/test/CMakeLists.txt

@@ -38,6 +38,7 @@ add_executable(test_bson
     oid.cpp
     view_or_value.cpp
     make_unique.test.cpp
+    ranges.test.cpp
     type_traits.test.cpp
     iterator.test.cpp
 )
@@ -144,4 +145,5 @@ set_dist_list(src_bsoncxx_test_DIST
     type_traits.test.cpp
     make_unique.test.cpp
     iterator.test.cpp
+    ranges.test.cpp
 )

src/bsoncxx/test/ranges.test.cpp

@@ -0,0 +1,50 @@
+#include <cstddef>
+#include <deque>
+#include <vector>
+
+#include <bsoncxx/stdx/iterator.hpp>
+#include <bsoncxx/stdx/ranges.hpp>
+#include <third_party/catch/include/catch.hpp>
+
+namespace ranges = bsoncxx::detail;
+
+static_assert(ranges::is_range<std::vector<int>>{}, "fail");
+static_assert(ranges::is_contiguous_range<std::vector<int>>{}, "fail");
+static_assert(ranges::is_contiguous_range<std::vector<int>&>{}, "fail");
+static_assert(ranges::is_contiguous_range<std::vector<int> const&>{}, "fail");
+
+static_assert(
+    std::is_same<ranges::range_difference_t<std::vector<int>>, std::vector<int>::difference_type>{},
+    "fail");
+
+static_assert(ranges::is_range<int (&)[2]>{}, "fail");
+static_assert(ranges::is_contiguous_range<int (&)[2]>{}, "fail");
+static_assert(std::is_same<ranges::range_difference_t<int[21]>, std::ptrdiff_t>{}, "fail");
+
+static_assert(ranges::is_range<std::deque<int>>{}, "fail");
+static_assert(!ranges::is_contiguous_range<std::deque<int>>{}, "fail");
+
+TEST_CASE("Range from vector") {
+    std::vector<int> nums = {1, 2, 3, 4};
+    auto it = ranges::begin(nums);
+    REQUIRE(it != ranges::end(nums));
+    static_assert(std::is_same<decltype(it), std::vector<int>::iterator>::value, "fail");
+    CHECK(*it == 1);
+    ++it;
+    CHECK(it == std::next(ranges::begin(nums)));
+    CHECK(*it == 2);
+    CHECK(ranges::size(nums) == 4);
+    CHECK(ranges::ssize(nums) == 4);
+    CHECK(ranges::data(nums) == nums.data());
+    CHECK(ranges::to_address(ranges::begin(nums)) == nums.data());
+}
+
+TEST_CASE("Range from array") {
+    int array[] = {1, 2, 3, 4};
+    auto it = ranges::begin(array);
+    CHECK(it == array + 0);
+    CHECK(ranges::end(array) == array + 4);
+    CHECK(ranges::size(array) == 4);
+    CHECK(ranges::ssize(array) == 4);
+    CHECK(ranges::data(array) == ranges::begin(array));
+}