 Movable memory object

A movable memory object is a live allocation that its memory-management system may relocate while preserving its identity and valid references. Moving the stored bytes is only part of the operation: references must be updated, or an indirection mechanism must continue to locate the object. This capability permits memory compaction. Microsoft's primary runtime documentation describes separate marking, reference-relocation and object-compaction phases of garbage collection.^[ref]

For an illustrative16-byte object initially starting at offset24, compaction may place its bytes at offset0. A reference to its beginning then changes from24 to0; a reference five bytes inside it changes from29 to5. In general, moving the base from $a$ to $a^{\prime }$ maps a tracked interior reference $a+k$ to $a^{\prime }+k$. An unrelated integer with value29 must not be rewritten merely because it resembles the old address.

The memory manager therefore needs reliable knowledge of roots and references, including those held in other live objects. A garbage collection root can keep an object reachable while its address changes. If an external consumer requires the address to stay fixed, a pinned memory object imposes a relocation constraint instead. Movability is a property of the reference-management arrangement, not simply of whether the object contains bytes that can be copied.

A raw C programming language pointer returned by an ordinary allocator generally prevents arbitrary hidden movement, because the allocator does not control all copies of that address.^[ref] Physical-page movement under virtual memory is another layer: an operating system may preserve virtual addresses while changing the underlying physical mapping. It need not make the application-level object movable in its own address space.