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61-424: The dispose pattern is primarily used in languages whose runtime environment have automatic garbage collection (see motivation below). Wrapping resources in objects is the object-oriented form of encapsulation , and underlies the dispose pattern. Resources are typically represented by handles (abstract references), concretely usually integers, which are used to communicate with an external system that provides

122-445: A finalizer before memory deallocation. Destroying an object will cause any references to the object to become invalid. With manual memory management, any existing reference becomes a dangling reference . With garbage collection, objects are only destroyed when there are no references to them. Object lifetime begins when allocation completes and ends when deallocation starts. Thus, during initialization and finalization, an object

183-474: A finally block. This leads to code size bloat, and failure to do so will lead to resource leakage in the program. To make the safe use of the dispose pattern less verbose, several languages have some kind of built-in support for resources held and released in the same block of code . The C# language features the using statement that automatically calls the Dispose method on an object that implements

244-431: A try...finally construct, which ensures that the finally clause is always executed on exit: More generically: The try...finally construct is necessary for proper exception safety , since the finally block enables execution of cleanup logic regardless of if an exception is thrown or not in the try block. One disadvantage of this approach is that it requires the programmer to explicitly add cleanup code in

305-518: A constructor handles initialization. Like other methods, a constructor can be overloaded in order to support creating with different initial state. Generally, an object is removed from memory after it is no longer needed. 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 consists solely of deallocating memory, particularly with garbage-collection, or if

366-434: A destructor is called when an instance is deleted, before the memory is deallocated. In C++, destructors differ 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. With garbage collection , objects may be destroyed when they can no longer be accessed by the program. The garbage-collector calls

427-532: A P-code machine or virtual machine , that hide even the processor's instruction set . This is the approach followed by many interpreted languages such as AWK , and some languages like Java , which are meant to be compiled into some machine-independent intermediate representation code (such as bytecode ). This arrangement simplifies the task of language implementation and its adaptation to different machines, and improves efficiency of sophisticated language features such as reflective programming . It also allows

488-399: A runtime system or runtime environment is a sub-system that exists in the computer where a program is created, as well as in the computers where the program is intended to be run. The name comes from the compile time and runtime division from compiled languages , which similarly distinguishes the computer processes involved in the creation of a program (compilation) and its execution in

549-444: A constructor is a class method as there is no object (instance) available until the object is created, but destructors, initializers, and finalizers are instance methods. Further, constructors and initializers often can accept arguments, while destructors and finalizers generally do not as they are often implicitly callable. In languages with deterministic lifetime objects, lifetime may be used to piggyback resource management . This

610-629: A hierarchy of runtime systems, with the CPU itself—or actually its logic at the microcode layer or below—acting as the lowest-level runtime system. Some compiled or interpreted languages provide an interface that allows application code to interact directly with the runtime system. An example is the Thread class in the Java language . The class allows code (that is animated by one thread) to do things such as start and stop other threads. Normally, core aspects of

671-438: A language's behavior such as task scheduling and resource management are not accessible in this fashion. Higher-level behaviors implemented by a runtime system may include tasks such as drawing text on the screen or making an Internet connection. It is often the case that operating systems provide these kinds of behaviors as well, and when available, the runtime system is implemented as an abstraction layer that translates

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732-853: A lifetime that coincides with the run of a program, but the order of creation and destruction of the various static objects is generally non-deterministic. Object life cycle refers to the events that an object experiences including and between creation and destruction. Life cycle generally includes memory management and operations after allocation and before deallocation. Object creation generally consists of memory allocation and initialization where initialization includes assigning values to fields and running initialization code. Object destruction generally consists of finalization (a.k.a. cleanup) and memory deallocation (a.k.a. free). These steps generally proceed in order as: allocate, initialize, finalize, deallocate. Based on programming context, these steps may be partially or fully automated, and some of

793-559: A pointer to it; the resource is released by calling fclose on a pointer to the FILE object. In code: Note that fclose is a function with a FILE * parameter. In object-oriented programming, this is instead an instance method on a file object, as in Python: This is precisely the dispose pattern, and only differs in syntax and code structure from traditional file opening and closing. Other resources can be managed in exactly

854-556: A programming error (each object holding a resource must be disposed exactly once), it is simpler, more robust, and thus usually preferable for dispose to be idempotent (meaning "calling multiple times is the same as calling once"). This is easily implemented by using the same boolean disposed field and checking it in a guard clause at the start of dispose , in that case returning immediately, rather than raising an exception. Java distinguishes disposable types (those that implement AutoCloseable ) from disposable types where dispose

915-449: A resource just adds another possible failure. A standard way to implement this is to add a boolean field to the object, called disposed , which is set to true by dispose , and checked by a guard clause to all methods (that use the resource), raising an exception (such as ObjectDisposedException in .NET) if the object has been disposed. Further, it is possible to call dispose on an object more than once. While this may indicate

976-484: A runtime system may also perform support services such as type checking , debugging , or code generation and optimization . The runtime system is also the gateway through which a running program interacts with the runtime environment . The runtime environment includes not only accessible state values, but also active entities with which the program can interact during execution. For example, environment variables are features of many operating systems, and are part of

1037-406: A runtime system that implements the execution model's behavior. Most scholarly papers on runtime systems focus on the implementation details of parallel runtime systems. A notable example of a parallel runtime system is Cilk , a popular parallel programming model. The proto-runtime toolkit was created to simplify the creation of parallel runtime systems. In addition to execution model behavior,

1098-471: Is "any behavior not directly attributable to the program itself". This definition includes putting parameters onto the stack before function calls, parallel execution of related behaviors, and disk I/O . By this definition, essentially every language has a runtime system, including compiled languages , interpreted languages , and embedded domain-specific languages . Even API -invoked standalone execution models, such as Pthreads ( POSIX threads ), have

1159-532: Is actually written. If a resource is unlimited or effectively unlimited, and no explicit finalization is necessary, it is not important to release it, and in fact short-lived programs often do not explicitly release resources: due to short run time, they are unlikely to exhaust resources, and they rely on the runtime system or operating system to do any finalization. However, in general resources must be managed (particularly for long-lived programs, programs that use many resources, or for safety, to ensure that data

1220-416: Is alive, but may not be in a consistent state. The period between when initialization completes to when finalization starts is when the object is both alive and in a consistent state. If creation or destruction fail, error reporting (for example, raising an exception) can be complicated since the object or related objects may be in an inconsistent state. For a static variable , with lifespan coinciding with

1281-499: Is called the Resource Acquisition Is Initialization (RAII) idiom. Resources are acquired during initialization, and released during finalization. In languages with non-deterministic lifetime objects (i.e. garbage collected), the management of memory is generally kept separate from management of other resources. A C++ class can be declared with defaults as: When declared in an automatic context,

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1342-409: Is idempotent (the subtype Closeable ). Disposal in the presence of inheritance and composition of objects that hold resources have analogous problems to destruction/finalization (via destructors or finalizers). Further, since the dispose pattern usually does not have language support for this, boilerplate code is necessary. Firstly, if a derived class overrides a dispose method in the base class,

1403-552: Is implemented by the Pthreads runtime system (this runtime system is often the OS kernel). As an extreme example, the physical CPU itself can be viewed as an implementation of the runtime system of a specific assembly language. In this view, the execution model is implemented by the physical CPU and memory systems. As an analogy, runtime systems for higher-level languages are themselves implemented using some other languages. This creates

1464-451: 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 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

1525-399: 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. A key problem with the dispose pattern is that if

1586-442: Is the case of using an application programming interface (API) to interact with a runtime system. The calls to that API look the same as calls to a regular software library , however at some point during the call the execution model changes. The runtime system implements an execution model different from that of the language the library is written in terms of. A person reading the code of a normal library would be able to understand

1647-429: 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 f is exited, the file object that f 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

1708-439: Is where the object is destroyed when code decrements the object's reference count to zero. With an object pool , where objects may be created ahead of time and reused, the apparent creation and destruction of an object may not correspond to actual. The pool provides reinitialization for creation and finalization for destruction. Both creation and destruction may be non-deterministic. Objects with static memory allocation have

1769-451: Is written out). Explicit disposal means that resource finalization and release is deterministic and prompt: the dispose method does not complete until these are done. 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

1830-412: The FILE type (confusingly called " file handles ": these are a language-level abstraction), which stores an (operating system) handle to the file (such as a file descriptor ), together with auxiliary information like I/O mode (reading, writing) and position in the stream. These objects are created by calling fopen (in object-oriented terms, a constructor ), which acquires the resource and returns

1891-514: The IDisposable interface : which is equal to: Similarly, the Python language has a with statement that can be used to similar effect with a context manager object. The context manager protocol requires implementing __enter__ and __exit__ methods which get automatically called by the with statement construct, to prevent duplication of code that would otherwise occur with

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1952-430: The dispose method is not called, the resource is leaked. A common cause of this is early exit from a function, due to an early return or exception. For example: If the function returns at the first return, the file is never closed and the resource is leaked. If the intervening code raises an exception, the function exits early and the file is never closed, so the resource is leaked. Both of these can be handled by

2013-597: The try / finally pattern. The Java language introduced a new syntax called try -with-resources in Java version 7. It can be used on objects that implement the AutoCloseable interface (that defines method close()): Beyond the key problem of correct resource management in the presence of returns and exceptions, and heap-based resource management (disposing objects in a different scope from where they are created), there are many further complexities associated with

2074-464: The C language is a particular set of instructions inserted by the compiler into the executable image. Among other things, these instructions manage the process stack, create space for local variables, and copy function call parameters onto the top of the stack. There are often no clear criteria for determining which language behaviors are part of the runtime system itself and which can be determined by any particular source program. For example, in C,

2135-475: The lifetime of a variable that represents the object. In other contexts – where the object is accessed by reference – object lifetime is not determined by the lifetime of a variable. For example, destruction of the variable may only destroy the reference; not the referenced object. Aspects of object lifetime vary between programming languages and within implementations of a language. The core concepts are relatively common, but terminology varies. For example,

2196-459: The application(s) running within that runtime environment. Any other code that tries to run, or any failures in the application(s), will break the runtime environment. Breaking the runtime environment in turn breaks the OS, stopping all processing and requiring a reboot. If the boot is from read-only memory, an extremely secure, simple, single-mission system is created. Examples of such directly bundled runtime systems include: The runtime system of

2257-407: The concepts of create and destroy are sometimes termed construct and destruct and the language elements are termed constructor (ctor) and destructor (dtor). Creation of an object is generally deterministic , but destruction varies by programming context. Some contexts allow for deterministic destruction, but some do not. Notably, in a garbage-collection environment, objects are destroyed when

2318-400: The dispose pattern. These problems are largely avoided by RAII . However, in common simple use these complexities do not arise: acquire a single resource, do something with it, automatically release it. A fundamental problem is that having a resource is no longer a class invariant (the resource is held from object creation until it is disposed, but the object is still live at this point), so

2379-482: The execution model implemented by the runtime system. For example, the trap instruction is one method of switching execution models. This difference is what distinguishes an API-invoked execution model, such as Pthreads, from a usual software library. Both Pthreads calls and software library calls are invoked via an API, but Pthreads behavior cannot be understood in terms of the language of the call. Rather, Pthreads calls bring into play an outside execution model, which

2440-526: The garbage collector chooses. The syntax for creation and destruction varies by programming context. In many contexts, including C++, C# and Java, an object is created via special syntax like new typename() . In C++, that provides manual memory management , an object is destroyed via the delete keyword. In C# and Java, with no explicit destruction syntax, the garbage collector destroys unused objects automatically and non-deterministically. An alternative and deterministic approach to automatic destruction

2501-432: The handle itself (for example, if different operating systems represent files differently), and to store additional auxiliary data with the handle, so handles can be stored as a field in a 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. For example, in C file input/output , files are represented by objects of

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2562-427: The invocation of the runtime system into an invocation of the operating system. This hides the complexity or variations in the services offered by different operating systems. This also implies that the OS kernel can itself be viewed as a runtime system, and that the set of OS calls that invoke OS behaviors may be viewed as interactions with a runtime system. In the limit, the runtime system may provide services such as

2623-458: The library's behavior by just knowing the language the library was written in. However, a person reading the code of the API that invokes a runtime system would not be able to understand the behavior of the API call just by knowing the language the call was written in. At some point, via some mechanism, the execution model stops being that of the language the call is written in and switches over to being

2684-444: The object is a plain old data structure . In other cases, cleanup 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). 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++,

2745-523: The object that is used needs to be tracked), but at the cost of considerable complexity when there are further relationships between objects, while aggregation (viewing) is considerably simpler, at the cost of lacking encapsulation. In .NET , convention is to only have direct user of resources be responsible: "You should implement IDisposable only if your type uses unmanaged resources directly." See resource management for details, and further examples. Runtime environment In computer programming ,

2806-437: The overriding method in the derived class generally needs to call the dispose method in the base class, in order to properly release resources held in the base. Secondly, if an object has a "has a" relationship with another object that holds a resource (i.e., if an object indirectly uses a resource through another object that directly uses a resource), should the indirectly using object be disposable? This corresponds to whether

2867-406: The program run, a creation or destruction error is problematic since program execution is before or after normal execution. In class-based programming, object creation is also known as instantiation (creating an instance of a class ). Creation and destruction can be customized via a constructor and a destructor and sometimes with separate initializer and finalizer methods. Notably,

2928-497: The proto-runtime approach. Notable early examples of runtime systems are the interpreters for BASIC and Lisp . These environments also included a garbage collector . Forth is an early example of a language designed to be compiled into intermediate representation code; its runtime system was a virtual machine that interpreted that code. Another popular, if theoretical, example is Donald Knuth 's MIX computer. In C and later languages that supported dynamic memory allocation,

2989-471: The relationship is owning ( object composition ) or viewing ( object aggregation ), or even just communicating ( association ), and both conventions are found (indirect user is responsible for the resource or is not responsible). If the indirect use is responsible for the resource, it must be disposable, and dispose the owned objects when it is disposed (analogous to destroying or finalizing owned objects). Composition (owning) provides encapsulation (only

3050-457: The resource may not be available when the object tries to use it, for example trying to read from a closed file. This means that all methods on the object that use the resource potentially fail, concretely usually by returning an error or raising an exception. In practice this is minor, as use of resources can usually fail for other reasons as well (for example, trying to read past the end of a file), so these methods already might fail, and not having

3111-399: The resource. For example, files are provided by the operating system (specifically the file system ), which in many systems represents open files with a file descriptor (an integer representing the file). These handles can be used directly, by storing the value in a variable and passing it as an argument to functions that use the resource. However, it is frequently useful to abstract from

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3172-426: The runtime environment. The object file, on one hand, may be missing information from the runtime environment that will be resolved by linking . On the other hand, the code in the object file still depends on assumptions in the runtime system; for example, a function may read parameters from a particular register or stack location, depending on the calling convention used by the runtime environment. Another example

3233-410: The runtime environment; a running program can access them via the runtime system. Likewise, hardware devices such as disks or DVD drives are active entities that a program can interact with via a runtime system. One unique application of a runtime environment is its use within an operating system that only allows it to run. In other words, from boot until power-down, the entire OS is dedicated to only

3294-459: The runtime system also included a library that managed the program's memory pool. In the object-oriented programming languages , the runtime system was often also responsible for dynamic type checking and resolving method references. Object lifetime In object-oriented programming (OOP), object lifetime is the period of time between an object's creation and its destruction. In some programming contexts, object lifetime coincides with

3355-711: The same program to be executed on any machine without an explicit recompiling step, a feature that has become very important since the proliferation of the World Wide Web . To speed up execution, some runtime systems feature just-in-time compilation to machine code. A modern aspect of runtime systems is parallel execution behaviors, such as the behaviors exhibited by mutex constructs in Pthreads and parallel section constructs in OpenMP . A runtime system with such parallel execution behaviors may be modularized according to

3416-440: The same way: being acquired in a constructor or factory, and released by an explicit close or dispose method. The fundamental problem that freeing resources aims to solve is that resources are expensive (for example, there may be a limit on the number of open files), and thus should be released promptly. Further, some finalization work is sometimes needed, particularly for I/O, such as flushing buffers to ensure that all data

3477-416: The semantics of a particular program and the runtime environment is reflected by the different ways of compiling a program: compiling source code to an object file that contains all the functions versus compiling an entire program to an executable binary. The object file will only contain assembly code relevant to the included functions, while the executable binary will contain additional code that implements

3538-428: The setup of the stack is part of the runtime system. It is not determined by the semantics of an individual program because the behavior is globally invariant: it holds over all executions. This systematic behavior implements the execution model of the language, as opposed to implementing semantics of the particular program (in which text is directly translated into code that computes results). This separation between

3599-478: The specific runtime system to generate correct code. Typically the runtime system will have some responsibility for setting up and managing the stack and heap , and may include features such as garbage collection , threads or other dynamic features built into the language. Every programming language specifies an execution model , and many implement at least part of that model in a runtime system. One possible definition of runtime system behavior, among others,

3660-463: The steps can be customized. The frequency of customization tends to vary by step and programming context. Initialization is the most commonly customized step. Finalization is common in languages with deterministic destruction, notably C++, but rare in garbage-collected languages. Allocation is rarely customized, and deallocation generally cannot be customized. The way to create objects varies across programming contexts. In some class-based languages,

3721-450: The target machine (the runtime). Most programming languages have some form of runtime system that provides an environment in which programs run. This environment may address a number of issues including the management of application memory , how the program accesses variables , mechanisms for passing parameters between procedures , interfacing with the operating system (OS), among others. The compiler makes assumptions depending on

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