Editing Tracing garbage collection (section)

=== Precise vs. conservative and internal pointers ===
Some collectors can correctly identify all pointers (references) in an object; these are called ''precise'' (also ''exact'' or ''accurate'') collectors, the opposite being a ''conservative'' or ''partly conservative'' collector. Conservative collectors assume that any bit pattern in memory could be a pointer if, interpreted as a pointer, it would point into an allocated object. Conservative collectors may produce false positives, where unused memory is not released because of improper pointer identification. This is not always a problem in practice unless the program handles a lot of data that could easily be misidentified as a pointer. False positives are generally less problematic on [[64-bit computing|64-bit]] systems than on [[32-bit]] systems because the range of valid memory addresses tends to be a tiny fraction of the range of 64-bit values. Thus, an arbitrary 64-bit pattern is unlikely to mimic a valid pointer. A false negative can also happen if pointers are "hidden", for example using an [[XOR linked list]]. Whether a precise collector is practical usually depends on the type safety properties of the programming language in question. An example for which a conservative garbage collector would be needed is the [[C (programming language)|C language]], which allows typed (non-void) pointers to be type cast into untyped (void) pointers, and vice versa.

A related issue concerns ''internal pointers'', or pointers to fields within an object. If the semantics of a language allow internal pointers, then there may be many different addresses that can refer to parts of the same object, which complicates determining whether an object is garbage or not. An example for this is the [[C++]] language, in which multiple inheritance can cause pointers to base objects to have different addresses. In a tightly optimized program, the corresponding pointer to the object itself may have been overwritten in its register, so such internal pointers need to be scanned.