Change the implementation of SharedArray so that it can destruct its contents

api/sixtyfps-cpp/include/sixtyfps_sharedarray.h

@@ -9,6 +9,7 @@
 LICENSE END */
 #pragma once
 #include "sixtyfps_sharedarray_internal.h"
+#include <atomic>
 
 namespace sixtyfps {
 
@@ -16,26 +17,29 @@ template<typename T>
 struct SharedArray
 {
     SharedArray()
-    {
-        cbindgen_private::sixtyfps_shared_array_new_null(reinterpret_cast<SharedArray<uint8_t> *>(this));
-    }
+        : inner(const_cast<SharedArrayHeader*>(reinterpret_cast<const SharedArrayHeader*>(
+            cbindgen_private::sixtyfps_shared_array_empty())))
+    { }
 
     SharedArray(const SharedArray &other)
+        : inner(other.inner)
     {
-        cbindgen_private::sixtyfps_shared_array_clone(
-                reinterpret_cast<SharedArray<uint8_t> *>(this),
-                reinterpret_cast<const SharedArray<uint8_t> *>(&other));
+        if (inner->refcount > 0) {
+            ++inner->refcount;
+        }
     }
     ~SharedArray()
     {
-        cbindgen_private::sixtyfps_shared_array_drop(reinterpret_cast<SharedArray<uint8_t> *>(this));
+        drop();
     }
     SharedArray &operator=(const SharedArray &other)
     {
-        cbindgen_private::sixtyfps_shared_array_drop(reinterpret_cast<SharedArray<uint8_t> *>(this));
-        cbindgen_private::sixtyfps_shared_array_clone(
-                reinterpret_cast<SharedArray<uint8_t> *>(this),
-                reinterpret_cast<const SharedArray<uint8_t> *>(&other));
+        if (other.inner == inner) { return *this; }
+        drop();
+        inner = other.inner;
+        if (inner->refcount > 0) {
+            ++inner->refcount;
+        }
         return *this;
     }
     SharedArray &operator=(SharedArray &&other)
@@ -44,7 +48,36 @@ struct SharedArray
         return *this;
     }
 
+    const T *begin() const {
+        return reinterpret_cast<const T *>(inner + 1);
+    }
+
+    const T *end() const {
+        return begin() + inner->size;
+    }
+
 private:
-    void *inner; // opaque
+
+    void drop() {
+        if (inner->refcount > 0 && (--inner->refcount) == 0) {
+            auto b = begin(), e = end();
+            for (auto it = b; it < e; ++it) {
+                it->~T();
+            }
+        }
+        cbindgen_private::sixtyfps_shared_array_free(
+            reinterpret_cast<uint8_t *>(inner),
+            sizeof(SharedArrayHeader) + inner->capacity * sizeof(T),
+            alignof(SharedArrayHeader));
+    }
+
+    // Unfortunately, this cannot be generated by cbindgen because std::atomic is not understood
+    struct SharedArrayHeader {
+        std::atomic<std::intptr_t> refcount;
+        std::size_t size;
+        std::size_t capacity;
+    };
+    static_assert(alignof(T) <= alignof(SharedArrayHeader), "Not yet supported because we would need to add padding");
+    SharedArrayHeader *inner;
 };
 }