From 7db5a066e9d011c19e1df8b3e07aa37e3b7e9c99 Mon Sep 17 00:00:00 2001
From: Pranjal Raihan <praihan@meta.com>
Date: Thu, 6 Jun 2024 16:12:48 -0700
Subject: [PATCH] TypeErasedRef

Summary: The doc block explains it well. It's essentially a `void*` with an accompanying `std::type_info` which provides some basic runtime type safety.

Reviewed By: sethdelliott

Differential Revision: D58217404

fbshipit-source-id: 609414a0e21fe59c514d8d1340a6bc5c95a34a89
---
 thrift/lib/cpp2/util/TypeErasedRef.h          | 94 +++++++++++++++++++
 .../lib/cpp2/util/test/TypeErasedRefTest.cpp  | 42 +++++++++
 2 files changed, 136 insertions(+)
 create mode 100644 thrift/lib/cpp2/util/TypeErasedRef.h
 create mode 100644 thrift/lib/cpp2/util/test/TypeErasedRefTest.cpp

diff --git a/thrift/lib/cpp2/util/TypeErasedRef.h b/thrift/lib/cpp2/util/TypeErasedRef.h
new file mode 100644
index 00000000000..27acfa11be5
--- /dev/null
+++ b/thrift/lib/cpp2/util/TypeErasedRef.h
@@ -0,0 +1,94 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * 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 <string>
+#include <type_traits>
+#include <typeinfo>
+#include <utility>
+
+#include <folly/Traits.h>
+#include <folly/lang/SafeAssert.h>
+
+namespace apache::thrift::util {
+
+/**
+ * A type-safe type-erased const reference to an object.
+ *
+ * Unlike TypeErasedValue, the referred-to object is not owned by this class.
+ * The user must ensure that the aforementioned object outlives any access
+ * through an object of this class.
+ *
+ * Access is provided through the templated value<T>() method which checks that
+ * the referred-to object is indeed of type T. This is the main difference
+ * between TypeErasedRef and const void* — the latter has no safety checks.
+ *
+ * TypeErasedRef is copyable. The copy points to the same object as the
+ * original.
+ */
+class TypeErasedRef {
+ public:
+  const std::type_info& type() const noexcept { return *typeInfo_; }
+  const void* ptr() const noexcept { return ptr_; }
+
+  template <class T>
+  bool holds_alternative() const noexcept {
+    return type() == typeid(T);
+  }
+
+  template <class T>
+  const folly::remove_cvref_t<T>& value() const {
+    if (!holds_alternative<T>()) {
+      throw std::bad_cast();
+    }
+    return value_unchecked<T>();
+  }
+
+  template <class T>
+  const folly::remove_cvref_t<T>& value_unchecked() const noexcept {
+    FOLLY_SAFE_DCHECK(
+        holds_alternative<T>(),
+        "Tried to call value_unchecked() on TypeErasedRef with incompatible type");
+    return *reinterpret_cast<const folly::remove_cvref_t<T>*>(ptr_);
+  }
+
+  TypeErasedRef(const TypeErasedRef& other) noexcept = default;
+  TypeErasedRef& operator=(const TypeErasedRef& other) noexcept = default;
+
+  template <class T>
+  static TypeErasedRef of(folly::remove_cvref_t<T>&&) = delete;
+
+  template <class T>
+  static TypeErasedRef of(const folly::remove_cvref_t<T>& object) noexcept {
+    return fromTypeInfoUnchecked(
+        static_cast<const void*>(std::addressof(object)), typeid(T));
+  }
+
+  static TypeErasedRef fromTypeInfoUnchecked(
+      const void* ptr, const std::type_info& typeInfo) noexcept {
+    return TypeErasedRef(ptr, typeInfo);
+  }
+
+ private:
+  TypeErasedRef(const void* ptr, const std::type_info& typeInfo) noexcept
+      : ptr_(ptr), typeInfo_(&typeInfo) {}
+
+  const void* ptr_;
+  const std::type_info* typeInfo_;
+};
+
+} // namespace apache::thrift::util
diff --git a/thrift/lib/cpp2/util/test/TypeErasedRefTest.cpp b/thrift/lib/cpp2/util/test/TypeErasedRefTest.cpp
new file mode 100644
index 00000000000..a33339f6487
--- /dev/null
+++ b/thrift/lib/cpp2/util/test/TypeErasedRefTest.cpp
@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * 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.
+ */
+
+#include <folly/portability/GTest.h>
+
+#include <thrift/lib/cpp2/util/TypeErasedRef.h>
+
+using apache::thrift::util::TypeErasedRef;
+
+TEST(TypeErasedRefTest, Basic) {
+  std::string str = "hello";
+  auto ref1 = TypeErasedRef::of<std::string>(str);
+  auto ref1Copy = ref1;
+
+  std::string str2 = "world";
+  auto ref2 = TypeErasedRef::of<std::string>(str2);
+  EXPECT_EQ(ref2.value<std::string>(), str2);
+  ref2 = ref1;
+
+  for (auto& ref : {ref1, ref1Copy, ref2}) {
+    EXPECT_EQ(std::uintptr_t(ref.ptr()), std::uintptr_t(&str));
+    EXPECT_EQ(ref.type(), typeid(std::string));
+    EXPECT_TRUE(ref.holds_alternative<std::string>());
+    EXPECT_FALSE(ref.holds_alternative<int>());
+    EXPECT_EQ(ref.value<std::string>(), str);
+    EXPECT_EQ(ref.value<const std::string&>(), str);
+    EXPECT_THROW(ref.value<int>(), std::bad_cast);
+  }
+}