The SRC Modula-3 garbage collector is partially copying which means that some objects change their memory location over time as they may be moved during collection. The only objects which are guaranteed to remain in their position during collection are those pointed to by ambiguous roots. An ambiguous root is a value in memory which is conservatively treated by the collector as a pointer. Examples of ambiguous pointers are values on the stack or saved registers if those values can be interpreted as a pointer to some objects in the traced heap. A strong reference is an additional reference to an object created to make the object uncollectible even if no other reference exists and to prevent the object from being moved by the collector. Strong references are used where collectible (traced) objects are embedded in data structures that are not scanned by the collector (e.g., passed to C code which places that in its global variables or embedded inside of run-time generated code).
PROCEDURE Add(ref: REFANY); PROCEDURE Remove(ref: REFANY); PROCEDURE IsThere(ref: REFANY) : BOOLEAN;The StrongRef interface maintains a counter of how many strong references were created for a given reference. If the count is greater than zero then the object pointed to by the reference become uncollectible and immobilized. The object becomes collectible and mobile again after the count drops to zero. A strong reference is created by a call to StrongRef.Add and disposed of by a call to StrongRef.Remove. Strong references are kept internally by the implementation of the StrongRef interface and are not exported to its users. The original reference to an object for which a strong reference is created is passed to this interface. A call to IsThere returns TRUE if a strong reference for the argument reference currently exists.
Przemek Pardyak,