In computer programming, unreachable memory is a block of dynamically allocated memory where the

program Program, programme, programmer, or programming may refer to: Business and management * Program management, the process of managing several related projects * Time management * Program, a part of planning Arts and entertainment Audio * Progra ...

that allocated the memory no longer has any reachable pointer that refers to it. Similarly, an unreachable object is a dynamically allocated

object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ai ...

that has no reachable reference to it. Informally, unreachable memory is

dynamic memory Memory management is a form of resource management applied to computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and free it for reuse when ...

that the program cannot reach directly, nor get to by starting at an object it can reach directly, and then following a chain of pointer references. In dynamic memory allocation implementations that employ a

garbage collector A waste collector, also known as a garbageman, garbage collector, trashman (in the US), binman or (rarely) dustman (in the UK), is a person employed by a public or private enterprise to collect and dispose of municipal solid waste (refuse) and r ...

, objects are reclaimed after they become unreachable. 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, Julia) 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. Some garbage collectors implement weak references. If an object is reachable only through either weak references or chains of references that include a weak reference, then the object is said to be . The garbage collector can treat a weakly reachable object graph as unreachable and deallocate it. (Conversely, references that prevent an object from being garbage collected are called ''strong references''; a weakly reachable object is unreachable by any chain consisting only of strong references.) Some garbage-collected object-oriented languages, such as Java and Python, feature weak references. The Java package

java.lang.ref 


Java Platform, Standard Edition (Java SE) is a computing platform for development and deployment of  portable code for  desktop and server environments. Java SE was formerly known as Java 2 Platform, Standard Edition (J2SE).

The platform uses  J ...

supports soft, weak and phantom references, resulting in the additional object reachability states softly reachable and phantom reachable. Unreachable memory (in languages, like C, that do not reclaim) is often associated with software aging.

External links

* {{Memory management navbox Automatic memory management