[CXX-2625] Bring our own make_unique (#1028)

* Bring our own make_unique

Implements std::make_unique and std::make_unique_for_overwrite

Author: vector-of-bool

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

@@ -14,52 +14,124 @@
 
 #pragma once
 
-#include <bsoncxx/config/prelude.hpp>
-
-#if defined(BSONCXX_POLY_USE_MNMLSTC)
-
-#include <core/memory.hpp>
-
-namespace bsoncxx {
-BSONCXX_INLINE_NAMESPACE_BEGIN
-namespace stdx {
-
-using ::core::make_unique;
-
-}  // namespace stdx
-BSONCXX_INLINE_NAMESPACE_END
-}  // namespace bsoncxx
-
-#elif defined(BSONCXX_POLY_USE_BOOST)
-
-#include <boost/smart_ptr/make_unique.hpp>
-
-namespace bsoncxx {
-BSONCXX_INLINE_NAMESPACE_BEGIN
-namespace stdx {
-
-using ::boost::make_unique;
-
-}  // namespace stdx
-BSONCXX_INLINE_NAMESPACE_END
-}  // namespace bsoncxx
-
-#elif __cplusplus >= 201402L || (defined(_MSVC_LANG) && _MSVC_LANG >= 201402L)
-
+#include <cstddef>
 #include <memory>
+#include <type_traits>
+#include <utility>
+
+#include <bsoncxx/config/prelude.hpp>
 
 namespace bsoncxx {
 BSONCXX_INLINE_NAMESPACE_BEGIN
 namespace stdx {
 
-using ::std::make_unique;
+namespace detail {
+
+// Switch backend of make_unique by the type we are creating.
+// It would be easier to 'if constexpr' on whether we are an array and whether to direct-init or
+// value-init, but we don't have if-constexpr and we need it to guard against an uterance of a
+// possibly-illegal 'new' expression.
+template <typename T>
+struct make_unique_impl {
+    // For make_unique:
+    template <typename... Args,
+              // Guard on constructible-from:
+              typename = decltype(new T(std::declval<Args>()...))>
+    static std::unique_ptr<T> make(std::true_type /* direct-init */, Args&&... args) {
+        return std::unique_ptr<T>(new T(std::forward<Args>(args)...));
+    }
+
+    // For make_unique_for_overwrite:
+    template <typename U = T,
+              // Guard on whether T is value-initializable:
+              // (Hide behind a deduced 'U' to defer the evaluation of
+              // this default template argument until overload resolution)
+              typename = decltype(new U)>
+    static std::unique_ptr<T> make(std::false_type /* value-init */) {
+        return std::unique_ptr<T>(new T);
+    }
+};
+
+// For unbounded arrays:
+template <typename Elem>
+struct make_unique_impl<Elem[]> {
+    template <typename ShouldDirectInit,
+              // Guard on whether the new-expression will be legal:
+              typename = decltype(new Elem[std::declval<std::size_t>()])>
+    static std::unique_ptr<Elem[]> make(ShouldDirectInit, std::size_t count) {
+        // These can share a function via a plain if, because both new expressions
+        // must be semantically valid
+        if (ShouldDirectInit()) {
+            return std::unique_ptr<Elem[]>(new Elem[count]());
+        } else {
+            return std::unique_ptr<Elem[]>(new Elem[count]);
+        }
+    }
+};
+
+// Bounded arrays are disallowed:
+template <typename Elem, std::size_t N>
+struct make_unique_impl<Elem[N]> {};
+
+// References are nonsense:
+template <typename T>
+struct make_unique_impl<T&> {};
+
+// References are nonsense:
+template <typename T>
+struct make_unique_impl<T&&> {};
+
+}  // namespace detail
+
+/**
+ * @brief Create a new std::unique_ptr that points-to the given type, direct-initialized based on
+ * the given arguments.
+ *
+ * @tparam T Any non-array object type or any array of unknown bound.
+ * @param args If T is a non-array object type, these are the constructor arguments used to
+ * direct-initialize the object. If T is an array of unknown bound, then the sole argument must be a
+ * single std::size_t that specifies the number of objects to allocate in the array.
+ *
+ * Requires:
+ * - If T is an array of unknown bounds, then args... must be a single size_t and the element type
+ * of T must be value-initializable.
+ * - Otherwise, if T is a non-array object type, then T must be direct-initializable with arguments
+ * `args...`
+ * - Otherwise, this function is excluded from overload resolution.
+ */
+template <typename T,
+          typename... Args,
+          typename Impl = detail::make_unique_impl<T>,
+          typename = decltype(Impl::make(std::true_type{}, std::declval<Args>()...))>
+std::unique_ptr<T> make_unique(Args&&... args) {
+    return Impl::make(std::true_type{}, std::forward<Args>(args)...);
+}
+
+/**
+ * @brief Create a new std::unique_ptr that points-to a default-initialized instance of the given
+ * type.
+ *
+ * @tparam T A non-array object type or an array of unknown bound
+ * @param args If T is an object type, then args... must no arguments are allowed.
+ * If T is an array of unknown bound, then args... must be a single size_t specifying
+ * the length of the array to allocate.
+ *
+ * Requires:
+ * - T must be default-initializable
+ * - If T is an array of unknown bounds, then args... must be a single size_t
+ * - Otherwise, if T is a non-array object type, args... must be empty
+ * - Otherwise, this function is excluded from overload resolution
+ */
+template <typename T,
+          typename... Args,
+          typename Impl = detail::make_unique_impl<T>,
+          typename = decltype(Impl::make(std::false_type{}, std::declval<Args>()...))>
+std::unique_ptr<T> make_unique_for_overwrite(Args&&... args) {
+    return Impl::make(std::false_type{}, std::forward<Args>(args)...);
+}
 
 }  // namespace stdx
 BSONCXX_INLINE_NAMESPACE_END
 }  // namespace bsoncxx
 
-#else
-#error "Cannot find a valid polyfill for make_unique"
-#endif
-
 #include <bsoncxx/config/postlude.hpp>

src/bsoncxx/test/CMakeLists.txt

@@ -37,6 +37,7 @@ add_executable(test_bson
     json.cpp
     oid.cpp
     view_or_value.cpp
+    make_unique.test.cpp
 )
 
 # Common target properties for test executables.
@@ -72,4 +73,5 @@ set_dist_list(src_bsoncxx_test_DIST
     oid.cpp
     to_string.hh
     view_or_value.cpp
+    make_unique.test.cpp
 )

src/bsoncxx/test/make_unique.test.cpp

@@ -0,0 +1,40 @@
+#include <algorithm>
+#include <cstdint>
+#include <new>
+
+#include <bsoncxx/stdx/make_unique.hpp>
+#include <bsoncxx/test/catch.hh>
+
+namespace {
+
+struct something {
+    something(int val) : value(val) {}
+    int value;
+};
+
+TEST_CASE("Create a unique_ptr") {
+    auto ptr = bsoncxx::stdx::make_unique<int>(12);
+    REQUIRE(ptr);
+    CHECK(*ptr == 12);
+
+    auto thing = bsoncxx::stdx::make_unique<something>(5);
+    REQUIRE(thing);
+    CHECK(thing->value == 5);
+}
+
+TEST_CASE("Create a unique_ptr<T[]>") {
+    const unsigned length = 12;
+    auto ptr = bsoncxx::stdx::make_unique<int[]>(length);
+    REQUIRE(ptr);
+    // All elements are direct-initialized, which produces '0' for `int`
+    CHECK(ptr[0] == 0);
+    auto res = std::equal_range(ptr.get(), ptr.get() + length, 0);
+    CHECK(res.first == ptr.get());
+    CHECK(res.second == (ptr.get() + length));
+
+    ptr = bsoncxx::stdx::make_unique_for_overwrite<int[]>(length);
+    std::fill_n(ptr.get(), length, 42);
+    CHECK(std::all_of(ptr.get(), ptr.get() + length, [](int n) { return n == 42; }));
+}
+
+}  // namespace