Described are computer-based methods and apparatuses, including computer program products, for garbage collection. A garbage collection data structure is provided for deleting unused data objects. One or more object identifiers are stored in a first data structure in the garbage collection data structure. Each object identifier represents a data object about to be created but not yet assigned any references from other data objects. The first data structure prevents the data object from being deleted during creation of the data object but before one or more references are created to the data object. Data indicative of one or more objects is stored in a second data structure in the garbage collection data structure. The data includes one or more object identifiers, each object identifier representing a created data object. The data also includes one or more references to created data objects.