It's up to the programmer to make sure that memory is deallocated when the program quits.
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Objective-C uses ARC (Automatic Reference Counting) for deallocating unused objects. In Objective-J, objects are automatically deallocated by JavaScript's Garbage Collector.
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This requires that each object will have to manually be deallocated, in order for the garbage collector to release the memory safely.
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The garbage collector is able to determine if an object is reachable; any object that is determined to no longer be reachable can be deallocated. Many programming languages (for example, Java, C#, D, Dylan) use automatic garbage collection. In contrast, when memory becomes unreachable in dynamic memory allocation implementations that require explicit deallocation, the memory can no longer be explicitly deallocated. Unreachable memory in systems that use manual memory management results in a memory leak.
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A Stack-dynamic variable is known as local variable, which is bound when the declaration statement is executed, and it is deallocated when the procedure returns. The main examples are local variables in C subprograms and Java methods. Explicit Heap-Dynamic variables are nameless (abstract) memory cells that are allocated and deallocated by explicit run-time instructions specified by the programmer. The main examples are dynamic objects in C++ (via new and delete) and all objects in Java.
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Like traditional heap allocation, these schemes do not provide memory safety; it is possible for a programmer to access a region after it is deallocated through a dangling pointer, or to forget to deallocate a region, causing a memory leak.
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When the data is deallocated, the tombstone is set to a null (or, more generally, to a value that is illegal for a pointer in the given runtime environment), indicating that the variable no longer exists. This prevents the use of invalid pointers, which would otherwise access the memory area that once belonged to the now deallocated variable, although it may already contain other data, in turn leading to corruption of in-memory data. Depending on the operating system, the CPU can automatically detect such an invalid access (e. g. for the null value: a null pointer dereference error).
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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 is up to the developer to break cycles by using weak references. Apple Inc.
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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 with the reference that created them.
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TI-RTOS Kernel provides tooling to set up an embedded system's memory map and also to allow memory buffers to be allocated and deallocated while the system runs. The type of memory manager used during runtime is actually configurable so that memory fragmentation can be minimized if necessary.
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In computer programming, a static variable is a variable that has been allocated "statically", meaning that its lifetime (or "extent") is the entire run of the program. This is in contrast to shorter-lived automatic variables, whose storage is stack allocated and deallocated on the call stack; and in contrast to objects, whose storage is dynamically allocated and deallocated in heap memory. Variable lifetime is contrasted with scope (where a variable can be used): "global" and "local" refer to scope, not lifetime, but scope often implies lifetime. In many languages, global variables are always static, but in some languages they are dynamic, while local variables are generally automatic, but may be static.
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Object resurrection occurs via the following process. First, an object becomes garbage when it is no longer reachable from the program, and may be collected (destroyed and deallocated). Then, during object destruction, before the garbage collector deallocates the object, a finalizer method may be run, which may in turn make that object or another garbage object (reachable from the object with a finalizer) reachable again by creating references to it, as a finalizer may contain arbitrary code. If this happens, the referenced object – which is not necessarily the finalized object – is no longer garbage, and cannot be deallocated, as otherwise the references to it would become dangling references and cause errors when used, generally program crash or unpredictable behavior.
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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 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 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.
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Memory allocated with the must be deallocated with the operator, rather than . Using the inappropriate form results in undefined behavior. C++ compilers are not required to generate a diagnostic message for using the wrong form. The C++11 standard specifies an additional syntax, p = new T[N] {initializer1, ..., initializerN}; that initializes each to .
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S`. Whiley uses a structural rather than nominal type system. Modula-3, Go and Ceylon are examples of other languages which support structural typing in some form. Whiley supports reference lifetimes similar to those found in Rust. Lifetimes can be given when allocating new objects to indicate when they can be safely deallocated.
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Many real-time operating systems use memory pools, such as the Transaction Processing Facility. Some systems, like the web server Nginx, use the term memory pool to refer to a group of variable-size allocations which can be later deallocated all at once. This is also known as a region; see region-based memory management.
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References to such objects must then include lifetime identifier to prevent dangling references. Every method has an implicit lifetime referred to by `this`. A variable of type `&this;:T` represents a reference to an object of type `T` which is deallocated with the enclosing method. Subtyping between lifetimes is determined by the outlives relation.
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Undefined (means, unpredictable) data in C and similar languages may appear in poorly designed programs or as a result of an unexpected fault, and represent a severe danger, particularly pointers to deallocated memory and null pointers to arrays or structures. Even an attempt to read a value, which a garbage pointer refers to, can crash a program.
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For example, Microsoft SQL Server implements cursors by creating a temporary table and populating it with the query's result set. If a cursor is not properly closed (deallocated), the resources will not be freed until the SQL session (connection) itself is closed. This wasting of resources on the server can lead to performance degradations and failures.
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Incremental, concurrent, and real-time garbage collectors have been developed, such as Baker's algorithm or Lieberman's algorithm."GC FAQ". In Baker's algorithm, the allocation is done in either half of a single region of memory. When it becomes half full, a garbage collection is performed which moves the live objects into the other half and the remaining objects are implicitly deallocated.
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C programs are compiled as separate object files, which are then linked into an executable or library via a linker. Thus name resolution is split across the compiler, which resolves names within a translation unit (more loosely, "compilation unit", but this is properly a different concept), and the linker, which resolves names across translation units; see linkage for further discussion. In C, variables with block scope enter context when they are declared (not at the top of the block), go out of context if any (non-nested) function is called within the block, come back into context when the function returns, and go out of context at the end of the block. In the case of automatic local variables, they are also allocated on declaration and deallocated at the end of the block, while for static local variables, they are allocated at program initialization and deallocated at program termination.
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In 1994 this work was generalized in a seminal work by Tofte and Talpin to support type polymorphism and higher- order functions in Standard ML, a functional programming language, using a different algorithm based on type inference and the theoretical concepts of polymorphic region types and the region calculus. On Citeseer Their work introduced an extension of the lambda calculus including regions, adding two constructs: :e1 at ρ: Compute the result of the expression e1 and store it in region ρ; :letregion ρ in e2 end: Create a region and bind it to ρ; evaluate e2; then deallocate the region. Due to this syntactic structure, regions are nested, meaning that if r2 is created after r1, it must also be deallocated before r1; the result is a stack of regions. Moreover, regions must be deallocated in the same function in which they are created.
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If the program does not deallocate an object, a memory leak occurs. If the program attempts to access or deallocate memory that has already been deallocated, the result is undefined and difficult to predict, and the program is likely to become unstable or crash. This can be partially remedied by the use of smart pointers, but these add overhead and complexity. Note that garbage collection does not prevent logical memory leaks, i.e.
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Loading a library causes memory to be allocated; the library must be deallocated in order to avoid a memory leak. Additionally, failure to unload a library can prevent filesystem operations on the file which contains the library. Unloading the library is accomplished with `FreeLibrary` on Windows and with `dlclose` on UNIX-like operating systems. However, unloading a DLL can lead to program crashes if objects in the main application refer to memory allocated within the DLL.
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However, most modern automatic memory management is non-deterministic, making no guarantees that objects will be destroyed promptly or even at all! This is because it is cheaper to leave some garbage allocated than to precisely collect each object immediately on its becoming garbage. Secondly, releasing resources during object destruction means that an object must have a finalizer (in deterministic memory management known as a destructor) – the object cannot simply be deallocated – which significantly complicates and slows garbage collection.
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C99 standardised variable-length arrays (VLAs) within block scope. Such array variables are allocated based on the value of an integer value at runtime upon entry to a block, and are deallocated at the end of the block. As of C11 this feature is no longer required to be implemented by the compiler. int n = ...; int a[n]; a[3] = 10; This syntax produces an array whose size is fixed until the end of the block.
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Every access to the pointer compares these two values, and access is allowed only if the values match. When a variable is deallocated, the key of its pointer is modified to hold a value different from the variable's cell. From then on, any attempt to dereference the pointer can be flagged as an error. Since copying a pointer also copies its cell value, changing the key of the ordered pair safely disables all copies of the pointer.
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If methods for a non-existent object are called, a null pointer exception is thrown. One of the ideas behind Java's automatic memory management model is that programmers can be spared the burden of having to perform manual memory management. In some languages, memory for the creation of objects is implicitly allocated on the stack or explicitly allocated and deallocated from the heap. In the latter case, the responsibility of managing memory resides with the programmer.
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A finalizer is called when an object is garbage collected – after an object has become garbage (unreachable), but before its memory is deallocated. Finalization occurs non-deterministically, at the discretion of the garbage collector, and might never occur. This contrasts with destructors, which are called deterministically as soon as an object is no longer in use, and are always called, except in case of uncontrolled program termination. Finalizers are most frequently instance methods, due to needing to do object-specific operations.
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Alternatively, when a region is deallocated, its list of blocks can be appended to a global freelist from which other regions may later allocate new blocks. With this simple scheme, it is not possible to deallocate individual objects in regions. The overall cost per allocated byte of this scheme is very low; almost all allocations involve only a comparison and an update to the next-free-position pointer. Deallocating a region is a constant-time operation, and is done rarely.
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When a process ends via `exit`, all of the memory and resources associated with it are deallocated so they can be used by other processes. However, the process's entry in the process table remains. The parent can read the child's exit status by executing the `wait` system call, whereupon the zombie is removed. The `wait` call may be executed in sequential code, but it is commonly executed in a handler for the SIGCHLD signal, which the parent receives whenever a child has died.
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A language implementation cannot easily support full closures if its run-time memory model allocates all automatic variables on a linear stack. In such languages, a function's automatic local variables are deallocated when the function returns. However, a closure requires that the free variables it references survive the enclosing function's execution. Therefore, those variables must be allocated so that they persist until no longer needed, typically via heap allocation, rather than on the stack, and their lifetime must be managed so they survive until all closures referencing them are no longer in use.
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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 garbage collection is the most common type of garbage collection – so much so that "garbage collection" often refers to tracing garbage collection, rather than other methods such as reference counting – and there are a large number of algorithms used in implementation.
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Such blocks were created when a lesson came into use and deallocated when the lesson became inactive. In contrast, named common blocks were associated with a block of a lesson (a disk file). Shared memory was addressed as `nc1` through `nc1500` (for integers) or `vc1` through `vc1500` (for floating point numbers).see pages X-1 to X-3 and X-6 of The TUTOR Language, Sherwood, 1974 Where 150 student variables was insufficient, a lesson could use the `storage` command to create an additional private memory segment of up to 1000 words.
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This may be automatic, or a special destruction method may be called on the object. In class-based languages with deterministic object lifetime, notably C++, a destructor is a method called when an instance of a class is deleted, before the memory is deallocated. In C++, destructors differs from constructors in various ways: they cannot be overloaded, must have no arguments, need not maintain class invariants, and can cause program termination if they throw exceptions. In garbage collecting languages, objects may be destroyed when they can no longer be reached by the running code.
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However, in 2014, the coal block land was deallocated by the Supreme Court resulting in sharp drops in the company's stocks. Its original allocation in 2006 is part of the Indian coal allocation scam. During investigations, the Ministry of Power stated that it had opposed a role for Hindalco in the mine, quote: Ministry of Power [told the CBI] that it did not recommend the 2006 allocation of Mahan coal block to Hindalco. Instead, it claimed to have asked the Ministry of Coal to allocate Mahan to Essar Power.
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Instead, the compiler assumes that the object to which an unowned reference points is not deallocated as long the reference itself remains allocated. This is typically used in situations where the target object itself holds a reference to the object that holds the unowned reference. var strongReference: MyClass // Strong reference, cannot be nil weak var weakReference: MyClass? // Weak reference, can be nil unowned var unownedReference: MyClass // Weak reference, cannot be nil Swift also differs from Objective-C in its usage and encouragement of value types instead of reference types.
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In computer science, a finalizer or finalize method is a special method that performs finalization, generally some form of cleanup. A finalizer is executed during object destruction, prior to the object being deallocated, and is complementary to an initializer, which is executed during object creation, following allocation. Finalizers are strongly discouraged by some, due to difficulty in proper use and the complexity they add, and alternatives are suggested instead, primarily the dispose pattern – see problems with finalizers. The term "finalizer" is primarily used in object-oriented and functional languages that use garbage collection, of which the archetype is Smalltalk.
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Simple explicit regions are straightforward to implement; the following description is based on Hanson. Each region is implemented as a linked list of large blocks of memory; each block should be large enough to serve many allocations. The current block maintains a pointer to the next free position in the block, and if the block is filled, a new one is allocated and added to the list. When the region is deallocated, the next-free-position pointer is reset to the beginning of the first block, and the list of blocks can be reused for the next region to be created.
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As a result of widespread destruction and human invasion into the protected areas, Togolese authorities reformed the park boundaries since 1999. Peripheral areas deemed too much destroyed to be re-naturalized have been excluded from the national park and officially deallocated for human development. This shrunk the size of the national park, now named the Oti-Kéran National Park, from 179550 to 69000 hectares. What remained is planned to form part of a future biosphere reserve, linked by the Oti-Mandouri National Park to the WAP (W, Arli, Pendjari) protected area system in Burkina Faso, Benin, and Niger.
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The returned pointer can be used (recast) into a (typed) dynamic array, by means of a slice (however resizing array, including appending must be avoided; and for obvious reasons they must not be returned from the function). A `scope` keyword can be used both to annotate parts of code, but also variables and classes/structs, to indicate they should be destroyed (destructor called) immediately on scope exit. Whatever the memory is deallocated also depends on implementation and class- vs-struct differences. `std.experimental.allocator` contains a modular and composable allocator templates, to create custom high performance allocators for special use cases.
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Finalization is formally complementary to initialization – initialization occurs at the start of lifetime, finalization at the end – but differs significantly in practice. Both variables and objects are initialized, primarily to assign values, but in general only objects are finalized, and in general there is no need to clear values – the memory can simply be deallocated and reclaimed by the operating system. Beyond assigning initial values, initialization is primarily used to acquire resources or to register an object with some service (like an event handler). These actions have symmetric release or unregister actions, and these can symmetrically be handled in a finalizer, which is done in RAII.
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When one function calls another during a typical program's execution, the local state of the caller (including parameters and local variables) must be preserved in order for execution to proceed after the callee returns. In most compiled programs, this local state is stored on the call stack in a data structure called a stack frame or activation record. This stack frame is pushed, or allocated, as prelude to calling another function, and is popped, or deallocated, when the other function returns to the function that did the call. The upwards funarg problem arises when the calling function refers to the called/exited function's state after that function has returned.
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In computer science, region-based memory management is a type of memory management in which each allocated object is assigned to a region. A region, also called a zone, arena, area, or memory context, is a collection of allocated objects that can be efficiently deallocated all at once. Like stack allocation, regions facilitate allocation and deallocation of memory with low overhead; but they are more flexible, allowing objects to live longer than the stack frame in which they were allocated. In typical implementations, all objects in a region are allocated in a single contiguous range of memory addresses, similarly to how stack frames are typically allocated.
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Systems using regions may experience issues where regions become very large before they are deallocated and contain a large proportion of dead data; these are commonly called "leaks" (even though they are eventually freed). Eliminating leaks may involve restructuring the program, typically by introducing new, shorter-lifetime regions. Debugging this type of problem is especially difficult in systems using region inference, where the programmer must understand the underlying inference algorithm, or examine the verbose intermediate representation, to diagnose the issue. Tracing garbage collectors are more effective at deallocating this type of data in a timely manner without program changes; this was one justification for hybrid region/GC systems.
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It is generally the case that after an object is used, it is removed from memory to make room for other programs or objects to take that object's place. However, if there is sufficient memory or a program has a short run time, object destruction may not occur, memory simply being deallocated at process termination. In some cases object destruction simply consists of deallocating the memory, particularly in garbage-collected languages, or if the "object" is actually a plain old data structure. In other cases some work is performed prior to deallocation, particularly destroying member objects (in manual memory management), or deleting references from the object to other objects to decrement reference counts (in reference counting).
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Then one can know, and quite literally prove, that the object will not be deallocated prematurely, and that memory leaks will not occur. The benefit of the ATS system is that since all theorem proving occurs strictly within the compiler, it has no effect on the speed of the executable program. ATS code is often harder to compile than standard C code, but once it compiles the programmer can be certain that it is running correctly to the degree specified by their proofs (assuming the compiler and runtime system are correct). In ATS proofs are separate from implementation, so it is possible to implement a function without proving it if the programmer so desires.
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In 1988, researchers began investigating how to use regions for safe memory allocation by introducing the concept of region inference, where the creation and deallocation of regions, as well as the assignment of individual static allocation expressions to particular regions, is inserted by the compiler at compile-time. The compiler is able to do this in such a way that it can guarantee dangling pointers and leaks do not occur. In an early work by Ruggieri and Murtagh, a region is created at the beginning of each function and deallocated at the end. They then use data flow analysis to determine a lifetime for each static allocation expression, and assign it to the youngest region that contains its entire lifetime.
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Because Silver Spring (whose postal area includes Wheaton) has its own prefix, 209, there was no need to apply the reshuffling to Silver Spring; instead, all mail going to 209xx ZIP Codes was simply rerouted to the new sectional center facility. On the other hand, depopulation may cause a post office to close and its associated ZIP Code to be deallocated. For example, Centralia, Pennsylvania's ZIP Code, 17927, was retired in 2002, and ZIP Codes for Onoville (14764), Quaker Bridge (14771) and Red House (14773) in New York were prevented from going into use in 1964 in preparation for the Kinzua Dam's completion. Elkins Park, Pennsylvania, was originally issued the 19117 ZIP Code, although it lies in Montgomery County, Pennsylvania.
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The following program demonstrates a variable with block scope coming into context partway through the block, then exiting context (and in fact being deallocated) when the block ends: #include int main(void) { char x = 'm'; printf("%c ", x); { printf("%c ", x); char x = 'b'; printf("%c ", x); } printf("%c ", x); } The program outputs: m m b m There are other levels of scope in C."Scope", XL C/C++ V8.0 for Linux, IBM Variable names used in a function prototype have function prototype visibility, and exit context at the end of the function prototype. Since the name is not used, this is not useful for compilation, but may be useful for documentation. Label names for GOTO statement have function scope, while case label names for switch statements have block scope (the block of the switch).
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He devised a savvy strategy to stem the tide of the epidemic, and saved thousands of lives. In 1954 the disease was arrested.It’s utterly incomprehensible and outright iniquitous that following Dr. Oriedo’s precipitously inexplicable death at age 34 the postcolonial Kenyan authorities and those who succeeded him were derelict of the valuable progress, successes, and a robustly dynamic public health infrastructure he’d attained and implemented via his ardent and industrious efforts to stem not just the black fever disease but also a plethora of other tropical diseases including malaria, typhoid, and malnutrition. Not only did the postcolonial Kenyan authorities and those who succeeded him fail to sustain his achievements, but also critical resources were deallocated leading to dilapidation of civil infrastructure and a lack of a meaningful public health strategy.
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These do not have analogous language- level concepts for variables: variable lifetime ends implicitly (for automatic variables, on stack unwind; for static variables, on program termination), and at this time (or later, depending on implementation) memory is deallocated, but no finalization is done in general. However, when an object's lifetime is tied to a variable's lifetime, the end of the variable's lifetime causes finalization of the object; this is a standard paradigm in C++. Together these yield four implementation-level steps: :allocation, initialization, finalization, deallocation These steps may be done automatically by the language runtime, interpreter, or virtual machine, or may be manually specified by the programmer in a subroutine, concretely via methods – the frequency of this varies significantly between steps and languages. Initialization is very commonly programmer-specified in class-based languages, while in strict prototype-based languages initialization is automatically done by copying.
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