# BlockSpentOutputs The spent-output (undo) data for every transaction in a block — the collection of UTXOs that this block’s transactions consumed. ``` final class BlockSpentOutputs ``` ## Overview Spent outputs are what the kernel writes to `rev*.dat` files alongside `blk*.dat` so it can undo a block during a chain reorganization. Obtain via [`readBlockSpentOutputs(at:)`](/documentation/BitcoinKernel/ChainstateManager/readBlockSpentOutputs(at:)); genesis blocks have no undo data and the read returns `nil` there. The index aligns with the block’s transaction list, except that the coinbase (always index 0 in the block) contributes no spent outputs. Wraps the opaque `btck_BlockSpentOutputs` type; `deinit` calls `btck_block_spent_outputs_destroy` when the last Swift reference drops.