From 107d5798b72d5822b1cdb69e51a3a217fc1a0a27 Mon Sep 17 00:00:00 2001
From: K1ngst0m <kingstom.chen@gmail.com>
Date: Sun, 29 Oct 2023 00:14:45 +0800
Subject: [PATCH] add singleton base class

---
 engine/common/singleton.h | 33 ++++++++++++++++++
 tests/singleton.cpp       | 70 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 103 insertions(+)
 create mode 100644 engine/common/singleton.h
 create mode 100644 tests/singleton.cpp

diff --git a/engine/common/singleton.h b/engine/common/singleton.h
new file mode 100644
index 00000000..a98ea671
--- /dev/null
+++ b/engine/common/singleton.h
@@ -0,0 +1,33 @@
+#ifndef APH_SINGLETON_H
+#define APH_SINGLETON_H
+
+#include <mutex>
+#include <memory>
+#include <stdexcept>
+
+namespace aph
+{
+
+template <typename Derived>
+class Singleton
+{
+public:
+    static Derived& GetInstance()
+    {
+        // Meyers' Singleton with guaranteed thread-safety since C++11
+        static Derived instance;
+        return instance;
+    }
+
+    Singleton(const Singleton&)                = delete;
+    Singleton& operator=(const Singleton&)     = delete;
+    Singleton(Singleton&&) noexcept            = delete;
+    Singleton& operator=(Singleton&&) noexcept = delete;
+
+protected:
+    Singleton()          = default;
+    virtual ~Singleton() = default;
+};
+
+}  // namespace aph
+#endif
diff --git a/tests/singleton.cpp b/tests/singleton.cpp
new file mode 100644
index 00000000..1a74de6d
--- /dev/null
+++ b/tests/singleton.cpp
@@ -0,0 +1,70 @@
+#include "common/singleton.h"
+#include <catch2/catch_test_macros.hpp>
+#include <thread>
+#include <set>
+
+using namespace aph;
+
+class MySingleton : public Singleton<MySingleton>
+{
+    friend class Singleton<MySingleton>;
+
+public:
+    void setValue(int v) { value = v; }
+    int  getValue() const { return value; }
+
+private:
+    MySingleton() = default;
+    int value{};
+};
+
+TEST_CASE("Basic Usage of Singleton", "[Singleton]")
+{
+    SECTION("Ensure we can create an instance")
+    {
+        MySingleton& instance1 = MySingleton::GetInstance();
+        instance1.setValue(10);
+        REQUIRE(instance1.getValue() == 10);
+    }
+
+    SECTION("Ensure repeated calls return same instance")
+    {
+        MySingleton& instance1 = MySingleton::GetInstance();
+        MySingleton& instance2 = MySingleton::GetInstance();
+        REQUIRE(&instance1 == &instance2); // Compare addresses
+    }
+}
+
+TEST_CASE("Thread Safety of Singleton", "[Singleton]")
+{
+    std::vector<std::thread> threads;
+    std::vector<MySingleton*> addresses;
+
+    // Mutex to ensure synchronized access to the addresses vector
+    std::mutex mtx;
+
+    const int numThreads = 100;
+
+    for (int i = 0; i < numThreads; ++i)
+    {
+        threads.push_back(std::thread([&]()
+        {
+            MySingleton& instance = MySingleton::GetInstance();
+            std::lock_guard<std::mutex> lock(mtx);
+            addresses.push_back(&instance);
+        }));
+    }
+
+    for (auto& thread : threads)
+    {
+        thread.join();
+    }
+
+    SECTION("All threads get the same instance")
+    {
+        for (std::size_t i = 1; i < addresses.size(); ++i)
+        {
+            REQUIRE(addresses[i] == addresses[0]);
+        }
+    }
+}