Dispose pattern: Difference between revisions

Revision as of 03:00, 22 November 2016

Revision as of 03:01, 22 November 2016

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=== Wrapping resources in objects ===

Wrapping resources in objects is the object-oriented form of [[Encapsulation (computer programming)|encapsulation]], and underlies the dispose pattern.

''Hi Luke!''

Resources are typically represented by [[Handle (computing)|handles]] (abstract references), concretely usually integers, which are used to communicate with an external system that provides the resource. For example, files are provided by the [[operating system]] (specifically the [[file system]]), which in many systems represents files with a [[file descriptor]] (an integer representing the file).

sThese handles can be used directly, by storing the value insin a variable and passing it as an argument to functions that use the resource. However, it is frequently useful to abstract from the handle itself (for example, if different operating systesssystems represent files differently), and to store additional auxiliary data with the handle, so handles can be stored as a field in a [[Record (computer science)|record]], along with other data; if this in an [[opaque data type]], then this provides [[information hiding]] and the user is abstracted from the actual representation.

''Hi John''

For example, in [[C file input/output]], files are represented by objectssobjects of the <code>FILE</code> type (confusingly called "[[file handle]]s": these are a language-level abstraction), which stores an (operating system) handle to thsethe file ("[[file descriptor]]"), together with auxiliary information like I/O mode (reading, writing) and seamposition in the stream. These objects are created by calling <code>[[open (system call)|open]]</code> (in object-oriented terms, a [[factory method|factory]]), which acquires the resource, and the resource is released by calling <code>[[close (system call)|close]]</code> on the <code>FILE</code> object. In code:

<source lang="c">

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An alternative to requiring explicit disposal is to tie resource management to [[object lifetime]]: resources are acquired during [[object creation]], and released during [[object destruction]]. This approach is known as the [[Resource Acquisition Is Initialization]] (RAII) idiom, and is used in languages with deterministic memory management (e.g. [[C++]]). In this case, in the example above, the resource is acquired when the file object is created, and when the scope of the variable <code>f</code> is exited, the file object that <code>f</code> refers to is destroyed, and as part of this, the resource is released.

RAII relies on object lifetime being deterministic; however, with automatic memory management, [[object lifetime]] is not a concern of the programmer: objects are destroyed at some point after they are no longer used, but ''when'' is abstracted. Indeed, lifetime is often not deterministic, though it may be, notably if sde[[reference counting]] is used. Indeed, in some cases there is no guarantee that objects will ''ever'' be finalized: when the program terminates, it may not finalize the objects, and instead just let the operating system reclaim memory; if finalization is required (e.g., to flush buffers), data loss can occur.

Thus by not coupling resource management to object lifetime, the dispose pattern allows ''resources'' to be released promptly, while giving implementation flexibility for memory management. The cost of this is that resources must be managed manually, which can be tedious and error-prone.