When each sentence or similar grammatical structure ends with each line, it is referred to as end-stopping. Love is not love Which alters when it alteration finds, Or bends with the remover to remove: The last two lines are known as end-stopped lines, which end with punctuation that facilitates a pause in the reading. The degree of enjambment can vary depending on where the line break occurs within the structure of the sentence.
Terminology[ edit ] The terminology of "finalizer" and "finalization" versus "destructor" and "destruction" varies between authors and is sometimes unclear.
For languages that implement garbage collection via reference countingterminology varies, with some languages such as Objective-C and Perl using "destructor", and other languages such as Python using "finalizer" per spec, Python is garbage collected, but the reference CPython implementation uses reference counting.
This reflects the fact that reference counting results in semi-deterministic object lifetime: In certain narrow technical usage, "constructor" and "destructor" are language-level terms, meaning "methods defined in a class", while "initializer" and "finalizer" are implementation-level terms, meaning "methods called during object creation or destruction".
Thus for example the original specification for the C language referred to "destructors", even though C is garbage-collected, but the specification for the Common Language Infrastructure CLIand the implementation of its runtime environment as the Common Language Runtime CLRreferred to "finalizers".
This is reflected in the C language committee's notes, which read in part: Although D classes are garbage collected, their cleanup functions are called destructors. It is also possible for there to be little or no explicit user-specified finalization, but significant implicit finalization, performed by the compiler, interpreter, or runtime; this is common in case of automatic reference counting, as in the CPython reference implementation of Python, or in Automatic Reference Counting in Apple's implementation of Objective-Cwhich both automatically break references during finalization.
A finalizer can include arbitrary code; a particularly complex use is to automatically return the object to an object pool. This latter can cause problems due to the garbage collector not being able to track these external resources, so they will not be collected aggressively enough, and can cause out-of-memory errors due to exhausting unmanaged memory — this can be avoided by treating native memory as a resource The destructors an analysis using the dispose patternas discussed below.
Finalizers are generally both much less necessary and much less used than destructors. They are much less necessary because garbage collection automates memory managementand much less used because they are not generally executed deterministically — they may not be called in a timely manner, or even at all, and the execution environment cannot be predicted — and thus any cleanup that must be done in a deterministic way must instead be done by some other method, most frequently manually via the dispose pattern.
Notably, both Java and Python do not guarantee that finalizers will ever be called, and thus they cannot be relied on for cleanup. Due to the lack of programmer control over their execution, it is usually recommended to avoid finalizers for any but the most trivial operations. In particular, operations often performed in destructors are not usually appropriate for finalizers.
A common anti-pattern is to write finalizers as if they were destructors, which is both unnecessary and ineffectual, due to differences between finalizers and destructors. Syntax varies significantly by language. In Java a finalizer is a method called finalize, which overrides the Object.
In Go finalizers are applied to a single pointer by calling the runtime.
SetFinalizer function in the standard library. 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 methodsdue to needing to do object-specific operations. The garbage collector must also account for the possibility of object resurrection.
Most commonly this is done by first executing finalizers, then checking whether any objects have been resurrected, and if so, aborting their destruction. This additional check is potentially expensive — a simple implementation re-checks all garbage if even a single object has a finalizer — and thus both slows down and complicates garbage collection.
For this reason, objects with finalizers may be collected less frequently than objects without finalizers only on certain cyclesexacerbating problems caused by relying on prompt finalization, such as resource leaks.
If an object is resurrected, there is the further question of whether its finalizer is called again, when it is next destroyed — unlike destructors, finalizers are potentially called multiple times. If finalizers are called for resurrected objects, objects may repeatedly resurrect themselves and be indestructible; this occurs in the CPython implementation of Python prior to Python 3.
To avoid this, in many languages, including Java, Objective-C at least in recent Apple implementationsand Python from Python 3. In other cases, notably CLR languages like Cfinalization is tracked separately from the objects themselves, and objects can be repeatedly registered or deregistered for finalization.
Problems[ edit ] Finalizers can cause a significant number of problems, and are thus strongly discouraged by a number of authorities. Finalizers may result in object resurrectionwhich is often a programming error and whose very possibility significantly slows down and complicates garbage collection.
Finalizers are run based on garbage collection, which is generally based on managed memory pressure — they are not run in case of other resource scarcity, and thus are not suited for managing other scarce resources. Finalizers do not run in a specified order, and cannot rely on class invariants as they may refer to other objects that have already been finalized.
Slow finalizers may delay other finalizers. Exceptions within finalizers generally cannot be handled, because the finalizer is run in an unspecified environment, and may be either ignored or cause uncontrolled program termination.
Finalizers may reference and accidentally finalize live objects, violating program invariants. Finalizers may cause synchronization issue, even in otherwise sequential single-threaded programs, as finalization may be done concurrently concretely, in one or more separate threads.
Finalizers that are run during program termination cannot rely on the usual runtime environment, and thus may fail due to incorrect assumptions — for this reason finalizers are often not run during termination.In The Destructors by Graham Greene we have the theme of control, pride, power, acceptance, change, jealousy and insecurity.
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