CXX-2745 add immortal.hh and type_traits.hh (#1402)

Author: eramongodb

src/bsoncxx/lib/CMakeLists.txt

@@ -104,11 +104,13 @@ set_dist_list(src_bsoncxx_lib_DIST
     bsoncxx/private/convert.hh
     bsoncxx/private/export.hh
     bsoncxx/private/helpers.hh
+    bsoncxx/private/immortal.hh
     bsoncxx/private/itoa.hh
     bsoncxx/private/bson.hh
     bsoncxx/private/make_unique.hh
     bsoncxx/private/stack.hh
     bsoncxx/private/suppress_deprecation_warnings.hh
+    bsoncxx/private/type_traits.hh
     bsoncxx/private/version.hh
     bsoncxx/v_noabi/bsoncxx/types/bson_value/value.hh
     bsoncxx/v1/config/config.hpp.in

src/bsoncxx/lib/bsoncxx/private/immortal.hh

@@ -0,0 +1,62 @@
+// Copyright 2009-present MongoDB, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#pragma once
+
+#include <bsoncxx/v1/detail/macros.hpp>
+
+#include <new>
+
+namespace bsoncxx {
+
+//
+// Used to construct an object whose destructor is never invoked (hence, "immortal").
+//
+// This is primarily to avoid generating exit-time destructors for error category objects:
+//
+// ```cpp
+// std::error_category const& name() noexcept {
+//     class type final : public std::error_category { ... };
+//     static bsoncxx::immortal<type> const instance;
+//     return instance.value();
+// }
+// ```
+//
+template <typename T>
+class immortal {
+   private:
+    alignas(T) unsigned char _storage[sizeof(T)];
+
+   public:
+    ~immortal() = default;
+    immortal(immortal&&) = delete;
+    immortal& operator=(immortal&&) = delete;
+    immortal(immortal const&) = delete;
+    immortal& operator=(immortal const&) = delete;
+
+    template <typename... Args>
+    explicit immortal(Args&&... args) {
+        new (_storage) T(BSONCXX_PRIVATE_FWD(args)...);
+    }
+
+    T const& value() const {
+        return *reinterpret_cast<T const*>(_storage);
+    }
+
+    T& value() {
+        return *reinterpret_cast<T*>(_storage);
+    }
+};
+
+} // namespace bsoncxx

src/bsoncxx/lib/bsoncxx/private/type_traits.hh

@@ -0,0 +1,78 @@
+// Copyright 2009-present MongoDB, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#pragma once
+
+#include <bsoncxx/v1/detail/type_traits.hpp>
+
+namespace bsoncxx {
+
+template <typename LHS, typename RHS>
+using is_assignable_from_expr = decltype(std::declval<LHS>() = std::declval<RHS>());
+
+// Approximates the `assignable_from` concept in C++20.
+template <typename LHS, typename RHS>
+struct is_assignable_from : detail::conjunction<
+                                std::is_lvalue_reference<LHS>,
+                                std::is_same<detail::detected_t<is_assignable_from_expr, LHS, RHS>, LHS>> {};
+
+// Equivalent to `is_assignable_from` but also requires noexceptness.
+template <typename LHS, typename RHS>
+struct is_nothrow_assignable_from
+    : detail::bool_constant<
+          is_assignable_from<LHS, RHS>::value && noexcept(std::declval<LHS>() = std::declval<RHS>())> {};
+
+// Approximates the `movable` concept in C++20.
+template <typename T>
+struct is_movable : detail::conjunction<
+                        std::is_object<T>,
+                        std::is_move_constructible<T>,
+                        is_assignable_from<T&, T>,
+                        detail::is_swappable<T>> {};
+
+// Equivalent to `is_moveable` but additionally requires noexceptness.
+template <typename T>
+struct is_nothrow_moveable : detail::conjunction<
+                                 std::is_object<T>,
+                                 std::is_nothrow_move_constructible<T>,
+                                 is_nothrow_assignable_from<T&, T>,
+                                 detail::is_nothrow_swappable<T>> {};
+
+// Approximates the `copyable` concept in C++20.
+template <typename T>
+struct is_copyable : detail::conjunction<
+                         std::is_copy_constructible<T>,
+                         is_movable<T>,
+                         is_assignable_from<T&, T&>,
+                         is_assignable_from<T&, T const&>,
+                         is_assignable_from<T&, T const>> {};
+
+// Approximates the `semiregular` concept in C++20.
+template <typename T>
+struct is_semiregular : detail::conjunction<is_copyable<T>, std::is_default_constructible<T>> {};
+
+// Approximates the `regular` concept in C++20.
+template <typename T>
+struct is_regular : detail::conjunction<is_semiregular<T>, detail::is_equality_comparable<T>> {};
+
+// Equivalent to `is_trivial` without requiring trivial default constructibility.
+template <typename T>
+struct is_semitrivial : detail::conjunction<
+                            std::is_trivially_destructible<T>,
+                            std::is_trivially_move_constructible<T>,
+                            std::is_trivially_move_assignable<T>,
+                            std::is_trivially_copy_constructible<T>,
+                            std::is_trivially_copy_assignable<T>> {};
+
+} // namespace bsoncxx