From a0691f92471f0d34052d2adea8491774cccc360f Mon Sep 17 00:00:00 2001
From: Justin King <jcking@google.com>
Date: Wed, 14 Feb 2024 13:55:39 -0800
Subject: [PATCH] Add aliasing constructor to `Shared`

PiperOrigin-RevId: 607096421
---
 common/memory.h       | 25 +++++++++++++++++++++
 common/memory_test.cc | 52 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 77 insertions(+)

diff --git a/common/memory.h b/common/memory.h
index 994ca2477..367048448 100644
--- a/common/memory.h
+++ b/common/memory.h
@@ -119,6 +119,27 @@ class ABSL_ATTRIBUTE_TRIVIAL_ABI Shared final {
   // NOLINTNEXTLINE(google-explicit-constructor)
   explicit Shared(SharedView<U> other);
 
+  // An aliasing constructor. The resulting `Shared` shares ownership
+  // information with `alias`, but holds an unmanaged pointer to `T`.
+  //
+  // Usage:
+  //   Shared<Object> object;
+  //   Shared<Member> member = Shared<Member>(object, &object->member);
+  template <typename U>
+  Shared(const Shared<U>& alias, T* ptr)
+      : value_(ptr), refcount_(alias.refcount_) {
+    common_internal::StrongRef(refcount_);
+  }
+
+  // An aliasing constructor. The resulting `Shared` shares ownership
+  // information with `alias`, but holds an unmanaged pointer to `T`.
+  template <typename U>
+  Shared(Shared<U>&& alias, T* ptr) noexcept
+      : value_(ptr), refcount_(alias.refcount_) {
+    alias.value_ = nullptr;
+    alias.refcount_ = nullptr;
+  }
+
   ~Shared() { common_internal::StrongUnref(refcount_); }
 
   Shared& operator=(const Shared& other) {
@@ -245,6 +266,10 @@ class ABSL_ATTRIBUTE_TRIVIAL_ABI SharedView final {
   SharedView(const Shared<U>& other ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept
       : value_(other.value_), refcount_(other.refcount_) {}
 
+  template <typename U>
+  SharedView(SharedView<U> alias, T* ptr)
+      : value_(ptr), refcount_(alias.refcount_) {}
+
   template <
       typename U,
       typename = std::enable_if_t<std::conjunction_v<
diff --git a/common/memory_test.cc b/common/memory_test.cc
index cb175cde4..581a1d5f3 100644
--- a/common/memory_test.cc
+++ b/common/memory_test.cc
@@ -233,6 +233,8 @@ class Object {
     }
   }
 
+  int member = 0;
+
  private:
   bool* deleted_;
 };
@@ -260,6 +262,56 @@ TEST_P(MemoryManagerTest, Shared) {
   Finish();
 }
 
+TEST_P(MemoryManagerTest, SharedAliasCopy) {
+  bool deleted = false;
+  {
+    auto object = memory_manager().MakeShared<Object>(deleted);
+    EXPECT_TRUE(object);
+    EXPECT_FALSE(deleted);
+    {
+      auto member = Shared<int>(object, &object->member);
+      EXPECT_TRUE(object);
+      EXPECT_FALSE(deleted);
+      EXPECT_TRUE(member);
+    }
+    EXPECT_TRUE(object);
+    EXPECT_FALSE(deleted);
+  }
+  switch (memory_management()) {
+    case MemoryManagement::kPooling:
+      EXPECT_FALSE(deleted);
+      break;
+    case MemoryManagement::kReferenceCounting:
+      EXPECT_TRUE(deleted);
+      break;
+  }
+  Finish();
+}
+
+TEST_P(MemoryManagerTest, SharedAliasMove) {
+  bool deleted = false;
+  {
+    auto object = memory_manager().MakeShared<Object>(deleted);
+    EXPECT_TRUE(object);
+    EXPECT_FALSE(deleted);
+    {
+      auto member = Shared<int>(std::move(object), &object->member);
+      EXPECT_FALSE(object);
+      EXPECT_FALSE(deleted);
+      EXPECT_TRUE(member);
+    }
+    switch (memory_management()) {
+      case MemoryManagement::kPooling:
+        EXPECT_FALSE(deleted);
+        break;
+      case MemoryManagement::kReferenceCounting:
+        EXPECT_TRUE(deleted);
+        break;
+    }
+  }
+  Finish();
+}
+
 TEST_P(MemoryManagerTest, SharedCopyConstruct) {
   bool deleted = false;
   {