Sketching rc.

sketching/september_2025/dll_rc/dll_rc.dm

@@ -0,0 +1,81 @@
+pub class Rc<T> : Drop, Copy
+
+    class Inner<T>(t: T)
+        mut count = 1
+    end
+
+    inner: &weak Inner = Null
+
+    pub ctor(t: T)
+        inner = Inner(t)
+    end
+
+    ctor(parent: &Self)
+        parent.inner++
+        inner = &weak parent.inner
+    end
+
+    pub fn copy() -> Self
+        Self(self)
+    end
+
+    impl fn drop(self)
+        if inner.upgrade() is Some(inner) then
+            inner.count--
+            if inner.count == 0 then
+                drop inner
+            end
+        else
+            throw Exception('rc.inner could not be converted from a weak reference')
+        end
+    end
+
+end
+
+class Game(player: Player) end
+
+class Player
+    pub mut game: &weak Game = Null
+end
+
+fn circular()
+    let player = Player()
+    let game = Game(player)
+
+    expectThrows(EmptyWeakReferenceException) {
+        player.game.upgrade()
+    }
+
+    player.game = &weak game
+
+    expectDoesNotThrow {
+        player.game.upgrade()
+    }
+end
+
+class Board
+    pub fn flip()
+        println 'Flipped!'
+    end
+end
+
+class ChessGame(pub board: rc Board) end
+
+class ChessPlayer(pub board: rc Board) end
+
+fn getGameAndPlayer() -> (ChessGame, ChessPlayer)
+    let board = rc Board() // Rc<Board> count is 1
+    let game = ChessGame(board) // Rc<Board> copied, count is 2
+    let player = ChessPlayer(board) // Rc<Board> copied, count is 3
+    (game, player) // initial Rc<Board> dropped before fn return, count now 2
+end
+
+fn playChess()
+    let (game, player) = getGameAndPlayer()
+    // Rc<Board> count is 2
+    let board = game.board // Copy, so count is now 3
+    board.flip()
+    drop board // count now 2
+    drop game // count now 1
+    drop player // count now 0, board memory finally dropped
+end