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78-410: ARC differs from tracing garbage collection in that there is no background process that deallocates the objects asynchronously at runtime. Unlike tracing garbage collection, ARC does not handle reference cycles automatically. This means that as long as there are "strong" references to an object, it will not be deallocated. Strong cross-references can accordingly create deadlocks and memory leaks . It

156-433: A compromise between the upsides and downsides of the mark and sweep and the stop and copy strategies. It has been empirically observed that in many programs, the most recently created objects are also those most likely to become unreachable quickly (known as infant mortality or the generational hypothesis ). A generational GC (also known as ephemeral GC) divides objects into generations and, on most cycles, will place only

234-449: A conservative garbage collector would be needed is the 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

312-550: A dangling pointer. The programmer typically had to ensure that all possible weak references to an object were set to nil manually when it was being deallocated. Zeroing weak references obviates the need to do this. Zeroing weak references are indicated by using the declared property attribute weak or by using the variable attribute __weak . Zeroing weak references are only available in Mac OS X Lion (10.7) or later and iOS 5 or later, because they require additional support from

390-477: A new object, the runtime system may either need to suspend it until the collection cycle is complete, or somehow notify the garbage collector that there exists a new, reachable object. 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

468-455: A null pointer), in Java it is only possible to have a StringBuilder , which is implicitly a reference to a mutable string (unless it's a null reference). While C++'s approach is more flexible, use of non-references can lead to problems such as object slicing , at least when inheritance is used; in languages where objects belong to reference types, these problems are automatically avoided, at

546-444: 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 systems than on 32-bit systems because

624-476: A poor fit for interactive use, or any other situation where low latency is a priority. However, incremental garbage collectors can provide hard real-time guarantees, and on systems with frequent idle time and sufficient free memory, such as personal computers, garbage collection can be scheduled for idle times and have minimal impact on interactive performance. Manual memory management (as in C++) and reference counting have

702-579: A program that allocates an object X {\displaystyle X} , runs an arbitrary input program P {\displaystyle P} , and uses X {\displaystyle X} if and only if P {\displaystyle P} finishes would require a semantic garbage collector to solve the halting problem . Although conservative heuristic methods for semantic garbage detection remain an active research area, essentially all practical garbage collectors focus on syntactic garbage. Another complication with this approach

780-627: A regular hash table, a weak hash table maintains an association between pairs of objects, where each pair is understood to be a key and value. However, the hash table does not actually maintain a strong reference on these objects. Special behavior takes place when either the key or value or both become garbage: the hash table entry is spontaneously deleted. There exist further refinements such as hash tables which have only weak keys (value references are ordinary, strong references) or only weak values (key references are strong). Weak hash tables are important for maintaining associations between objects, such that

858-456: A runtime error. Swift also differs from Objective-C in its usage and encouragement of value types instead of reference types . Most types in the Swift standard library are value types and they are copied by value, whereas classes and closures are reference types and passed by reference. Because value types are copied when passed around, they are deallocated automatically when the program leaves

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936-562: A safe value (usually null ). An unsafe reference that is not known to the garbage collector will simply remain dangling by continuing to refer to the address where the object previously resided. This is not a weak reference. In some implementations, weak references are divided into subcategories. For example, the Java Virtual Machine provides three forms of weak references, namely soft references , phantom references , and regular weak references. A softly referenced object

1014-435: A similar issue of arbitrarily long pauses in case of deallocating a large data structure and all its children, though these only occur at fixed times, not depending on garbage collection. It is difficult to compare the two cases directly, as their behavior depends on the situation. For example, in the best case for a garbage collecting system, allocation just increments a pointer, but in the best case for manual heap allocation,

1092-399: A single bit) reserved for garbage collection use only. This flag is always cleared , except during the collection cycle. The first stage is the mark stage which does a tree traversal of the entire 'root set' and marks each object that is pointed to by a root as being 'in-use'. All objects that those objects point to, and so on, are marked as well, so that every object that is reachable via

1170-583: A way to convert code to ARC. As of Xcode 4.5, it is found by choosing Edit > Refactor > Convert to Objective-C ARC... Although Xcode will automatically convert most code, some code may have to be converted manually. Xcode will inform the developer when more complex use cases arise, such as when a variable is declared inside an autorelease pool and used outside it or when two objects need to be toll-free bridged with special casts. In Swift, references to objects are strong, unless they are declared weak or unowned . Swift requires explicit handling of nil with

1248-466: Is a primitive value. One common solution is the use of tagged pointers . The garbage collector can reclaim only objects that have no references pointing to them either directly or indirectly from the root set. However, some programs require weak references , which should be usable for as long as the object exists but should not prolong its lifetime. In discussions about weak references, ordinary references are sometimes called strong references . An object

1326-412: Is always of type Optional, as the object can be deallocated and the reference automatically be set to nil. Unowned references are like weak references but are not set to nil automatically by ARC. They can be either non-Optional or Optional. An unowned reference is expected to always have a value, so accessing the value of an unowned reference after the referenced instance has been deallocated, will result in

1404-494: Is always the case in Java, and is the case by default in C#), a value of a reference type is intrinsically a reference; so if a parameter belongs to a reference type, the resulting behavior bears some resemblance to "call by reference" semantics. This behavior is sometimes called call by sharing . Call by sharing resembles call by reference in the case where a function mutates an object that it received as an argument: when that happens,

1482-408: Is common for cycles to be triggered when there is not enough free memory for the memory manager to satisfy an allocation request. But cycles can often be requested by the mutator directly or run on a time schedule. The original method involves a naïve mark-and-sweep in which the entire memory set is touched several times. In the naive mark-and-sweep method, each object in memory has a flag (typically

1560-431: Is eligible for garbage collection if there are no strong (i.e. ordinary) references to it, even though there still might be some weak references to it. A weak reference is not merely just any pointer to the object that a garbage collector does not care about. The term is usually reserved for a properly managed category of special reference objects which are safe to use even after the object disappears because they lapse to

1638-470: 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. Performance of tracing garbage collectors – both latency and throughput – depends significantly on

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1716-644: Is generally nondeterministic, it is possible to use it in hard real-time systems. A real-time garbage collector should guarantee that even in the worst case it will dedicate a certain number of computational resources to mutator threads. Constraints imposed on a real-time garbage collector are usually either work based or time based. A time based constraint would look like: within each time window of duration T {\displaystyle T} , mutator threads should be allowed to run at least for T m {\displaystyle T_{m}} time. For work based analysis, MMU (minimal mutator utilization)

1794-458: Is incredibly space efficient since it only requires one bit per allocated pointer (which most allocation algorithms require anyway). However, this upside is somewhat mitigated, since most of the time large portions of memory are wrongfully marked black (used), making it hard to give resources back to the system (for use by other allocators, threads, or processes) in times of low memory usage. The mark and don't sweep strategy can therefore be seen as

1872-476: Is kept as a separate list or using another bit. As the reference tree is traversed during a collection cycle (the "mark" phase), these bits are manipulated by the collector. A final "sweep" of the memory areas then frees white objects. The mark and sweep strategy has the advantage that, once the condemned set is determined, either a moving or non-moving collection strategy can be pursued. This choice of strategy can be made at runtime, as available memory permits. It has

1950-443: Is not optimal, insofar as the last time a program uses an object could be long before that object falls out of the environment scope. A distinction is sometimes drawn between syntactic garbage , those objects the program cannot possibly reach, and semantic garbage , those objects the program will in fact never again use. For example: The problem of precisely identifying semantic garbage can easily be shown to be partially decidable :

2028-400: Is only eligible for reclamation if the garbage collector decides that the program is low on memory. Unlike a soft reference or a regular weak reference, a phantom reference does not provide access to the object that it references. Instead, a phantom reference is a mechanism that allows the garbage collector to notify the program when the referenced object has become phantom reachable . An object

2106-415: Is phantom reachable, if it still resides in memory and it is referenced by a phantom reference, but its finalizer has already executed. Similarly, Microsoft.NET provides two subcategories of weak references, namely long weak references (tracks resurrection) and short weak references. Data structures can also be devised which have weak tracking features. For instance, weak hash tables are useful. Like

2184-574: Is possible to avoid both garbage collection and heap management overhead by preallocating pools of memory and using a custom, lightweight scheme for allocation/deallocation. The overhead of write barriers is more likely to be noticeable in an imperative -style program which frequently writes pointers into existing data structures than in a functional -style program which constructs data only once and never changes them. Some advances in garbage collection can be understood as reactions to performance issues. Early collectors were stop-the-world collectors, but

2262-518: Is that it is both simpler to implement and faster than incremental garbage collection. Incremental and concurrent garbage collectors are designed to reduce this disruption by interleaving their work with activity from the main program. Incremental garbage collectors perform the garbage collection cycle in discrete phases, with program execution permitted between each phase (and sometimes during some phases). Concurrent garbage collectors do not stop program execution at all, except perhaps briefly when

2340-405: Is that, in languages with both reference types and unboxed value types , the garbage collector needs to somehow be able to distinguish which variables on the stack or fields in an object are regular values and which are references: in memory, an integer and a reference might look alike. The garbage collector then needs to know whether to treat the element as a reference and follow it, or whether it

2418-507: Is the most common type of garbage collection – so much so that "garbage collection" often refers to the tracing method, rather than others such as reference counting – and there are a large number of algorithms used in implementation. Informally, an object is reachable if it is referenced by at least one variable in the program, either directly or through references from other reachable objects. More precisely, objects can be reachable in only two ways: The reachability definition of "garbage"

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2496-565: Is up to the developer to break cycles by using weak references . Apple Inc. deploys ARC in their operating systems, such as macOS ( OS X ) and iOS . Limited support (ARCLite) has been available since Mac OS X Snow Leopard and iOS 4 , with complete support following in Mac OS X Lion and iOS 5 . Garbage collection was declared deprecated in OS X Mountain Lion , in favor of ARC, and removed from

2574-774: Is usually used as a real-time constraint for the garbage collection algorithm. One of the first implementations of hard real-time garbage collection for the JVM was based on the Metronome algorithm , whose commercial implementation is available as part of the IBM WebSphere Real Time . Another hard real-time garbage collection algorithm is Staccato, available in the IBM 's J9 JVM , which also provides scalability to large multiprocessor architectures, while bringing various advantages over Metronome and other algorithms which, on

2652-499: The .NET Framework ) usually use some hybrid of the various strategies that have been described thus far; for example, most collection cycles might look only at a few generations, while occasionally a mark-and-sweep is performed, and even more rarely a full copying is performed to combat fragmentation. The terms "minor cycle" and "major cycle" are sometimes used to describe these different levels of collector aggression. Simple stop-the-world garbage collectors completely halt execution of

2730-406: The tri-color marking abstraction , but simple collectors (such as the mark-and-sweep collector) often do not make this abstraction explicit. Tri-color marking works as described below. Three sets are created – white , black and grey : In many algorithms, initially the black set starts as empty, the grey set is the set of objects which are directly referenced from roots and

2808-474: The unreachable objects and leave everything else as it is, or it may copy some or all of the reachable objects into a new area of memory, updating all references to those objects as needed. These are called "non-moving" and "moving" (or, alternatively, "non-compacting" and "compacting") garbage collectors, respectively. At first, a moving algorithm may seem inefficient compared to a non-moving one, since much more work would appear to be required on each cycle. But

2886-410: The "from space", and vice versa. The objects reachable from the root set are copied from the "from space" to the "to space". These objects are scanned in turn, and all objects that they point to are copied into "to space", until all reachable objects have been copied into "to space". Once the program continues execution, new objects are once again allocated in the "to space" until it is once again full and

2964-549: The Objective-C runtime library in macOS Sierra . The following rules are enforced by the compiler when ARC is turned on: ARC introduces some new property declaration attributes, some of which replace the old attributes. Zeroing weak references is a feature in Objective-C ARC that automatically clears (sets to nil ) weak-reference local variables, instance variables, and declared properties immediately before

3042-501: The Objective-C runtime. However, some OS X classes do not currently support weak references. Code that uses ARC but needs to support versions of the OS older than those above cannot use zeroing weak references, and therefore must use unsafe_unretained weak references. There exists a third-party library called PLWeakCompatibility [1] that allows one to use zeroing weak references even on these older OS versions. Xcode 4.2 or later provides

3120-523: The Optional type: a value type that can either have a value or be nil. An Optional type must be handled by "unwrapping" it with a conditional statement , allowing safe usage of the value, if present. Conversely, any non-Optional type will always have a value and cannot be nil. Accordingly, a strong reference to an object can be of both Optional and non-Optional type (optionality and reference strength are different, albeit related, concepts). A weak reference

3198-457: The above example is run, a and b are the same Foo object with prop being 3 , while c is a copy of the original a with prop being 1 . In C#, apart from the distinction between value types and reference types, there is also a separate concept called reference variables. A reference variable, once declared and bound, behaves as an alias of the original variable, but it can also be rebounded to another variable by using

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3276-476: The algorithm preserves an important invariant – no black objects reference white objects. This ensures that the white objects can be freed once the grey set is empty. This is called the tri-color invariant . Some variations on the algorithm do not preserve this invariant but use a modified form for which all the important properties hold. The tri-color method has an important advantage – it can be performed "on-the-fly", without halting

3354-532: The allocator maintains freelists of specific sizes and allocation only requires following a pointer. However, this size segregation usually cause a large degree of external fragmentation, which can have an adverse impact on cache behaviour. Memory allocation in a garbage collected language may be implemented using heap allocation behind the scenes (rather than simply incrementing a pointer), so the performance advantages listed above don't necessarily apply in this case. In some situations, most notably embedded systems , it

3432-630: The amortized cost can be extremely low, in some cases even lower than one instruction per allocation or collection, outperforming stack allocation. Manual memory management requires overhead due to explicit freeing of memory, and reference counting has overhead from incrementing and decrementing reference counts, and checking if the count has overflowed or dropped to zero. In terms of latency, simple stop-the-world garbage collectors pause program execution for garbage collection, which can happen at arbitrary times and take arbitrarily long, making them unusable for real-time computing , notably embedded systems, and

3510-421: The black/white bit is then inverted (for example, 0=black, 1=white). Everything becomes white. This momentarily breaks the invariant that reachable objects are black, but a full marking phase follows immediately, to mark them black again. Once this is done, all unreachable memory is white. No "sweep" phase is necessary. The mark and don't sweep strategy requires cooperation between the allocator and collector, but

3588-575: The contrary, require specialized hardware. Value type In certain computer programming languages , data types are classified as either value types or reference types , where reference types are always implicitly accessed via references , whereas value type variables directly contain the values themselves. Even among languages that have this distinction, the exact properties of value and reference types vary from language to language, but typical properties include: Even when function arguments are passed using "call by value" semantics (which

3666-423: The cost of removing some options from the programmer. In most programming languages, it is possible to change the variable of a reference type to refer to another object, i.e. to rebind the variable to another object. For example, in the following Java code: Foo is a reference type, where a is initially assigned a reference of a new object, and b is assigned to the same object reference, i.e. bound to

3744-414: The disadvantage of "bloating" objects by a small amount, as in, every object has a small hidden memory cost because of the list/extra bit. This can be somewhat mitigated if the collector also handles allocation, since then it could potentially use unused bits in the allocation data structures. Or, this "hidden memory" can be eliminated by using a Tagged pointer , trading the memory cost for CPU time. However,

3822-403: The distinction between value type and reference types is no longer clear, because the object itself cannot be modified, but only replaced as a whole (for value type) / with the reference pointed to another object (for reference type). Passing such immutable objects between variables have no observable differences if the object is copied or passed by reference, unless the object identity is taken. In

3900-487: The entire region can then be overwritten with fresh objects. This technique permits very fast incremental garbage collection, since the garbage collection of only one region at a time is all that is typically required. Ungar 's classic generation scavenger has two generations. It divides the youngest generation, called "new space", into a large "eden" in which new objects are created and two smaller "survivor spaces", past survivor space and future survivor space. The objects in

3978-483: The entire system must be suspended during collection; no mutation of the working set can be allowed. This can cause programs to 'freeze' periodically (and generally unpredictably), making some real-time and time-critical applications impossible. In addition, the entire working memory must be examined, much of it twice, potentially causing problems in paged memory systems. Because of these performance problems, most modern tracing garbage collectors implement some variant of

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4056-455: The garbage collector must copy the object contents to a location outside of the garbage collected region of memory. An alternative approach is to pin the object in memory, preventing the garbage collector from moving it and allowing the memory to be directly shared with native pointers (and possibly allowing pointer arithmetic). Not only do collectors differ in whether they are moving or non-moving, they can also be categorized by how they treat

4134-408: The generation being collected (by the hypothesis), leaving it to be used to allocate new objects. When a collection doesn't collect many objects (the hypothesis doesn't hold, for example because the program has computed a large collection of new objects it does want to retain), some or all of the surviving objects that are referenced from older memory regions are promoted to the next highest region, and

4212-422: The generational hypothesis holds, this results in much faster collection cycles while still reclaiming most unreachable objects. In order to implement this concept, many generational garbage collectors use separate memory regions for different ages of objects. When a region becomes full, the objects in it are traced, using the references from the older generation(s) as roots. This usually results in most objects in

4290-436: The implementation, workload, and environment. Naive implementations or use in very memory-constrained environments, notably embedded systems, can result in very poor performance compared with other methods, while sophisticated implementations and use in environments with ample memory can result in excellent performance. In terms of throughput, tracing by its nature requires some implicit runtime overhead , though in some cases

4368-544: The mark and sweep collector. In a "mark and don't sweep" collector, all reachable objects are always black. An object is marked black at the time it is allocated, and it will stay black even if it becomes unreachable. A white object is unused memory and may be allocated. Second, the interpretation of the black/white bit can change. Initially, the black/white bit may have the sense of (0=white, 1=black). If an allocation operation ever fails to find any available (white) memory, that means all objects are marked used (black). The sense of

4446-402: The mark and sweep is the only strategy that readily cooperates with external allocators in the first place. A mark and don't sweep garbage collector, like the mark-and-sweep, keeps a bit with each object to record if it is white or black; the grey set is kept as a separate list or using another bit. There are two key differences here. First, black and white mean different things than they do in

4524-514: The moving algorithm leads to several performance advantages, both during the garbage collection cycle itself and during program execution: One disadvantage of a moving garbage collector is that it only allows access through references that are managed by the garbage collected environment, and does not allow pointer arithmetic . This is because any pointers to objects will be invalidated if the garbage collector moves those objects (they become dangling pointers ). For interoperability with native code,

4602-417: The mutation will be visible to the caller as well, because the caller and the function have references to the same object. It differs from call by reference in the case where a function assigns its parameter to a different reference; when that happens, this assignment will not be visible to the caller, because the caller and the function have separate references, even though both references initially point to

4680-457: The object being pointed to starts deallocating. This ensures that the pointer goes to either a valid object or nil , and avoids dangling pointers . Prior to the introduction of this feature, "weak references" referred to references that were not retaining, but were not set to nil when the object they pointed to was deallocated (equivalent to unsafe_unretained in ARC), thus possibly leading to

4758-480: The objects engaged in the association can still become garbage if nothing in the program refers to them any longer (other than the associating hash table). The use of a regular hash table for such a purpose could lead to a "logical memory leak": the accumulation of reachable data which the program does not need and will not use. Tracing collectors are so called because they trace through the working set of memory. These garbage collectors perform collection in cycles. It

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4836-419: The objects of a subset of generations into the initial white (condemned) set. Furthermore, the runtime system maintains knowledge of when references cross generations by observing the creation and overwriting of references. When the garbage collector runs, it may be able to use this knowledge to prove that some objects in the initial white set are unreachable without having to traverse the entire reference tree. If

4914-526: The older generation that may reference objects in new space are kept in a "remembered set". On each scavenge, the objects in new space are traced from the roots in the remembered set and copied to future survivor space. If future survivor space fills up, the objects that do not fit are promoted to old space, a process called "tenuring". At the end of the scavenge, some objects reside in future survivor space, and eden and past survivor space are empty. Then future survivor space and past survivor space are exchanged and

4992-426: The performance of this approach was distracting in interactive applications. Incremental collection avoided this disruption, but at the cost of decreased efficiency due to the need for barriers. Generational collection techniques are used with both stop-the-world and incremental collectors to increase performance; the trade-off is that some garbage is not detected as such for longer than normal. While garbage collection

5070-420: The process is repeated. This approach is very simple, but since only one semi space is used for allocating objects, the memory usage is twice as high compared to other algorithms. The technique is also known as stop-and-copy . Cheney's algorithm is an improvement on the semi-space collector. A mark and sweep garbage collector keeps a bit or two with each object to record if it is white or black. The grey set

5148-469: The program continues, allocating objects in eden. In Ungar's original system, eden is 5 times larger than each survivor space. Generational garbage collection is a heuristic approach, and some unreachable objects may not be reclaimed on each cycle. It may therefore occasionally be necessary to perform a full mark and sweep or copying garbage collection to reclaim all available space. In fact, runtime systems for modern programming languages (such as Java and

5226-431: The program to run a collection cycle, thus guaranteeing that new objects are not allocated and objects do not suddenly become unreachable while the collector is running. This has the disadvantage that the program can perform no useful work while a collection cycle is running (sometimes called the "embarrassing pause" ). Stop-the-world garbage collection is therefore mainly suitable for non-interactive programs. Its advantage

5304-405: The program's execution stack is scanned. However, the sum of the incremental phases takes longer to complete than one batch garbage collection pass, so these garbage collectors may yield lower total throughput. Careful design is necessary with these techniques to ensure that the main program does not interfere with the garbage collector and vice versa; for example, when the program needs to allocate

5382-414: 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

5460-430: The reference assignment operator = ref . The variable itself can be of any type, including value types and reference types, i.e. by passing a variable of a reference type by reference (alias) to a function, the object where the reference-type variable points to can also be changed, in addition to the object itself (if it is mutable). If an object is immutable and object equality is tested on content rather than identity,

5538-403: The root set is marked. In the second stage, the sweep stage , all memory is scanned from start to finish, examining all free or used blocks; those not marked as being 'in-use' are not reachable by any roots, and their memory is freed. For objects which were marked in-use, the in-use flag is cleared, preparing for the next cycle. This method has several disadvantages, the most notable being that

5616-424: The same object as a , therefore, changes through a is also visible to b as well. Afterwards, a is assigned a reference (rebound) to another new object, and now a and b refer to different objects. At the end, a refers to the second object with its prop field having the value 1 , while b refers to the first object with its prop field having the value 3 . However, such as C++,

5694-446: The same object. Many languages have explicit pointers or references. Reference types differ from these in that the entities they refer to are always accessed via references; for example, whereas in C++ it's possible to have either a std :: string and a std :: string * , where the former is a mutable string and the latter is an explicit pointer to a mutable string (unless it's

5772-416: The scope that contains them. Tracing garbage collection In computer programming , tracing garbage collection is a form of automatic memory management that consists of determining which objects should be deallocated ("garbage collected") by tracing which objects are reachable by a chain of references from certain "root" objects, and considering the rest as "garbage" and collecting them. Tracing

5850-433: The system for significant periods of time. This is accomplished by marking objects as they are allocated and during mutation, maintaining the various sets. By monitoring the size of the sets, the system can perform garbage collection periodically, rather than as needed. Also, the need to touch the entire working set on each cycle is avoided. Once the unreachable set has been determined, the garbage collector may simply release

5928-438: The term "reference type" is used to mean an alias, and it is not possible to rebind a variable of a reference type once it is created, as it is an alias to the original object. In C++, all non-reference class types have value semantics. In the above example, b is declared to be a reference (alias) of a , and for all purposes, a and b are the same thing. It is impossible to rebind b to become something else. After

6006-471: The white set includes all other objects. Every object in memory is at all times in exactly one of the three sets. The algorithm proceeds as following: When the grey set is empty, the scan is complete; the black objects are reachable from the roots, while the white objects are not and can be garbage-collected. Since all objects not immediately reachable from the roots are added to the white set, and objects can only move from white to grey and from grey to black,

6084-407: The white, grey and black object sets during a collection cycle. The most straightforward approach is the semi-space collector , which dates to 1969. In this moving collector, memory is partitioned into an equally sized "from space" and "to space". Initially, objects are allocated in "to space" until it becomes full and a collection cycle is triggered. At the start of the cycle, the "to space" becomes

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