From 0f9e12f86181c05d6e33b3c22bbebab24e358233 Mon Sep 17 00:00:00 2001
From: Craig Tiller <ctiller@google.com>
Date: Thu, 11 Aug 2022 17:08:15 -0700
Subject: [PATCH] [no_destruct] Improve ergonomics (#30564)

* [no_destruct] Improve ergonomics

* comment

* fix

* Automated change: Fix sanity tests

Co-authored-by: ctiller <ctiller@users.noreply.github.com>
---
 src/core/lib/gprpp/no_destruct.h    | 22 ++++++++++++++--------
 test/core/gprpp/no_destruct_test.cc | 23 +++++++++++++++++++++++
 2 files changed, 37 insertions(+), 8 deletions(-)

diff --git a/src/core/lib/gprpp/no_destruct.h b/src/core/lib/gprpp/no_destruct.h
index a7d2188ef4b..0e839e3ca73 100644
--- a/src/core/lib/gprpp/no_destruct.h
+++ b/src/core/lib/gprpp/no_destruct.h
@@ -58,25 +58,31 @@ class NoDestruct {
   NoDestruct& operator=(const NoDestruct&) = delete;
   ~NoDestruct() = default;
 
-  T* operator->() { return reinterpret_cast<T*>(&space_); }
-  const T* operator->() const { return *reinterpret_cast<const T*>(&space_); }
-  T& operator*() { return *reinterpret_cast<T*>(&space_); }
-  const T& operator*() const { return *reinterpret_cast<const T*>(&space_); }
+  T* operator->() { return get(); }
+  const T* operator->() const { return get(); }
+  T& operator*() { return *get(); }
+  const T& operator*() const { return *get(); }
+
+  T* get() { return reinterpret_cast<T*>(&space_); }
+  const T* get() const { return reinterpret_cast<const T*>(&space_); }
 
  private:
   typename std::aligned_storage<sizeof(T), alignof(T)>::type space_;
 };
 
-// Helper for when a program desires a single instance of a default constructed
-// T to be always available.
-// T is constructed eagerly at program startup, so it's essentially free to load
-// the pointer to the instance.
+// Helper for when a program desires a single *process wide* instance of a
+// default constructed T to be always available.
+// The instance is constructed eagerly at program startup, so it's essentially
+// free to load the pointer to the instance.
 template <typename T>
 class NoDestructSingleton {
  public:
   static T* Get() { return &*value_; }
 
  private:
+  NoDestructSingleton() = delete;
+  ~NoDestructSingleton() = delete;
+
   static NoDestruct<T> value_;
 };
 
diff --git a/test/core/gprpp/no_destruct_test.cc b/test/core/gprpp/no_destruct_test.cc
index 954e5a84c32..1cda14a44f3 100644
--- a/test/core/gprpp/no_destruct_test.cc
+++ b/test/core/gprpp/no_destruct_test.cc
@@ -41,6 +41,29 @@ TEST(NoDestruct, CrashOnDestructionIsAccessible) {
   g_test_crash_on_destruction->Exists();
 }
 
+bool g_thing_constructed = false;
+
+class Thing {
+ public:
+  Thing() {
+    EXPECT_FALSE(g_thing_constructed);
+    g_thing_constructed = true;
+  }
+
+  int Add(int i, int j) { return i + j; }
+
+ private:
+  ~Thing() = delete;
+};
+
+TEST(GlobalSingleton, Works) {
+  // Thing should be eagerly constructed, so we should not observe it being not
+  // constructed.
+  EXPECT_TRUE(g_thing_constructed);
+  // We should be able to fetch the global Thing and use it.
+  EXPECT_EQ(NoDestructSingleton<Thing>::Get()->Add(1, 2), 3);
+}
+
 }  // namespace
 }  // namespace testing
 }  // namespace grpc_core