Add ArenaAllocator::Ptr

src/arena_allocator.hpp

@@ -302,38 +302,87 @@ public:
                                         new_size * sizeof(T), alignof(T)));
   }
 
+  /**
+   * @brief Smart pointer for arena-allocated objects with non-trivial
+   * destructors.
+   *
+   * ArenaAllocator::Ptr calls the destructor but does not free memory (assumes
+   * arena allocation). This provides RAII semantics for objects that need
+   * cleanup without the overhead of individual memory deallocation.
+   *
+   * @tparam T The type of object being managed
+   */
+  template <typename T> struct Ptr {
+    Ptr() noexcept : ptr_(nullptr) {}
+
+    explicit Ptr(T *ptr) noexcept : ptr_(ptr) {}
+
+    Ptr(const Ptr &) = delete;
+    Ptr &operator=(const Ptr &) = delete;
+
+    Ptr(Ptr &&other) noexcept : ptr_(other.ptr_) { other.ptr_ = nullptr; }
+
+    Ptr &operator=(Ptr &&other) noexcept {
+      if (this != &other) {
+        reset();
+        ptr_ = other.ptr_;
+        other.ptr_ = nullptr;
+      }
+      return *this;
+    }
+
+    ~Ptr() { reset(); }
+
+    T *operator->() const noexcept { return ptr_; }
+    T &operator*() const noexcept { return *ptr_; }
+
+    T *get() const noexcept { return ptr_; }
+
+    explicit operator bool() const noexcept { return ptr_ != nullptr; }
+
+    T *release() noexcept {
+      T *result = ptr_;
+      ptr_ = nullptr;
+      return result;
+    }
+
+    void reset(T *new_ptr = nullptr) noexcept {
+      if (ptr_) {
+        ptr_->~T();
+      }
+      ptr_ = new_ptr;
+    }
+
+  private:
+    T *ptr_;
+  };
+
   /**
    * @brief Construct an object of type T in the arena using placement new.
    *
-   * This is a convenience method that combines allocation with in-place
-   * construction. It properly handles alignment requirements for type T.
+   * This method returns different types based on whether T is trivially
+   * destructible:
+   * - For trivially destructible types: returns T* (raw pointer)
+   * - For non-trivially destructible types: returns ArenaAllocator::Ptr<T>
+   * (smart pointer that calls destructor)
    *
-   * @tparam T The type of object to construct (must be trivially destructible)
+   * @tparam T The type of object to construct
    * @tparam Args Types of constructor arguments
    * @param args Arguments to forward to T's constructor
-   * @return Pointer to the constructed object
+   * @return T* for trivially destructible types, ArenaAllocator::Ptr<T>
+   * otherwise
    * @note Prints error to stderr and calls std::abort() if memory allocation
    * fails
-   *
-   * ## Type Requirements:
-   * T must be trivially destructible (std::is_trivially_destructible_v<T>).
-   * This prevents subtle bugs since destructors are never called for objects
-   * constructed in the arena.
-   *
-   *
-   * ## Note:
-   * Objects constructed this way cannot be individually destroyed.
-   * Their destructors will NOT be called automatically - hence the requirement
-   * for trivially destructible types.
    */
-  template <typename T, typename... Args> T *construct(Args &&...args) {
-    static_assert(
-        std::is_trivially_destructible_v<T>,
-        "ArenaAllocator::construct requires trivially destructible types. "
-        "Objects constructed in the arena will not have their destructors "
-        "called.");
+  template <typename T, typename... Args> auto construct(Args &&...args) {
     void *ptr = allocate_raw(sizeof(T), alignof(T));
-    return new (ptr) T(std::forward<Args>(args)...);
+    T *obj = new (ptr) T(std::forward<Args>(args)...);
+
+    if constexpr (std::is_trivially_destructible_v<T>) {
+      return obj;
+    } else {
+      return Ptr<T>(obj);
+    }
   }
 
   /**

tests/test_arena_allocator.cpp

@@ -598,3 +598,162 @@ TEST_CASE("format function fallback codepath") {
     CHECK(result == "Valid format: 42");
   }
 }
+
+// Test object with non-trivial destructor for ArenaAllocator::Ptr testing
+class TestObject {
+public:
+  static int destructor_count;
+  static int constructor_count;
+
+  int value;
+
+  TestObject(int v) : value(v) { constructor_count++; }
+
+  ~TestObject() { destructor_count++; }
+
+  static void reset_counters() {
+    constructor_count = 0;
+    destructor_count = 0;
+  }
+};
+
+int TestObject::destructor_count = 0;
+int TestObject::constructor_count = 0;
+
+// Test struct with trivial destructor
+struct TrivialObject {
+  int value;
+  TrivialObject(int v) : value(v) {}
+};
+
+TEST_CASE("ArenaAllocator::Ptr smart pointer functionality") {
+  TestObject::reset_counters();
+
+  SUBCASE("construct returns raw pointer for trivially destructible types") {
+    ArenaAllocator arena;
+
+    auto ptr = arena.construct<TrivialObject>(42);
+    static_assert(std::is_same_v<decltype(ptr), TrivialObject *>,
+                  "construct() should return raw pointer for trivially "
+                  "destructible types");
+    CHECK(ptr != nullptr);
+    CHECK(ptr->value == 42);
+  }
+
+  SUBCASE("construct returns ArenaAllocator::Ptr for non-trivially "
+          "destructible types") {
+    ArenaAllocator arena;
+
+    auto ptr = arena.construct<TestObject>(42);
+    static_assert(
+        std::is_same_v<decltype(ptr), ArenaAllocator::Ptr<TestObject>>,
+        "construct() should return ArenaAllocator::Ptr for non-trivially "
+        "destructible types");
+    CHECK(ptr);
+    CHECK(ptr->value == 42);
+    CHECK(TestObject::constructor_count == 1);
+    CHECK(TestObject::destructor_count == 0);
+  }
+
+  SUBCASE("ArenaAllocator::Ptr calls destructor on destruction") {
+    ArenaAllocator arena;
+
+    {
+      auto ptr = arena.construct<TestObject>(42);
+      CHECK(TestObject::constructor_count == 1);
+      CHECK(TestObject::destructor_count == 0);
+    } // ptr goes out of scope
+
+    CHECK(TestObject::destructor_count == 1);
+  }
+
+  SUBCASE("ArenaAllocator::Ptr move semantics") {
+    ArenaAllocator arena;
+
+    auto ptr1 = arena.construct<TestObject>(42);
+    CHECK(TestObject::constructor_count == 1);
+
+    auto ptr2 = std::move(ptr1);
+    CHECK(!ptr1); // ptr1 should be null after move
+    CHECK(ptr2);
+    CHECK(ptr2->value == 42);
+    CHECK(TestObject::destructor_count == 0); // No destruction yet
+
+    ptr2.reset();
+    CHECK(TestObject::destructor_count == 1); // Destructor called
+  }
+
+  SUBCASE("ArenaAllocator::Ptr access operators") {
+    ArenaAllocator arena;
+
+    auto ptr = arena.construct<TestObject>(123);
+
+    // Test operator->
+    CHECK(ptr->value == 123);
+
+    // Test operator*
+    CHECK((*ptr).value == 123);
+
+    // Test get()
+    TestObject *raw_ptr = ptr.get();
+    CHECK(raw_ptr != nullptr);
+    CHECK(raw_ptr->value == 123);
+
+    // Test bool conversion
+    CHECK(ptr);
+    CHECK(static_cast<bool>(ptr) == true);
+  }
+
+  SUBCASE("ArenaAllocator::Ptr reset functionality") {
+    ArenaAllocator arena;
+
+    auto ptr = arena.construct<TestObject>(42);
+    CHECK(TestObject::constructor_count == 1);
+    CHECK(TestObject::destructor_count == 0);
+
+    ptr.reset();
+    CHECK(!ptr);
+    CHECK(TestObject::destructor_count == 1);
+
+    // Reset with new object
+    TestObject *raw_obj = arena.construct<TestObject>(84).release();
+    ptr.reset(raw_obj);
+    CHECK(ptr);
+    CHECK(ptr->value == 84);
+    CHECK(TestObject::constructor_count == 2);
+    CHECK(TestObject::destructor_count == 1);
+  }
+
+  SUBCASE("ArenaAllocator::Ptr release functionality") {
+    ArenaAllocator arena;
+
+    auto ptr = arena.construct<TestObject>(42);
+    TestObject *raw_ptr = ptr.release();
+
+    CHECK(!ptr); // ptr should be null after release
+    CHECK(raw_ptr != nullptr);
+    CHECK(raw_ptr->value == 42);
+    CHECK(TestObject::destructor_count == 0); // No destructor called
+
+    // Manually call destructor (since we released ownership)
+    raw_ptr->~TestObject();
+    CHECK(TestObject::destructor_count == 1);
+  }
+
+  SUBCASE("ArenaAllocator::Ptr move assignment") {
+    ArenaAllocator arena;
+
+    auto ptr1 = arena.construct<TestObject>(42);
+    auto ptr2 = arena.construct<TestObject>(84);
+
+    CHECK(TestObject::constructor_count == 2);
+    CHECK(TestObject::destructor_count == 0);
+
+    ptr1 = std::move(ptr2); // Should destroy first object, move second
+
+    CHECK(!ptr2); // ptr2 should be null
+    CHECK(ptr1);
+    CHECK(ptr1->value == 84);
+    CHECK(TestObject::destructor_count == 1); // First object destroyed
+  }
+}