Object lifetime

In object-oriented programming (OOP), the object lifetime (or life cycle) of an object is the time between an object's creation and its destruction. Rules for object lifetime vary significantly between languages, in some cases between implementations of a given language, and lifetime of a particular object may vary from one run of the program to another.

In some cases object lifetime coincides with variable lifetime of a variable with that object as value (both for static variables and automatic variables), but in general object lifetime is not tied to the lifetime of any one variable. In many cases – and by default in many object-oriented languages (OOLs), particularly those that use garbage collection (GC) – objects are allocated on the heap, and object lifetime is not determined by the lifetime of a given variable: the value of a variable holding an object actually corresponds to a reference to the object, not the object itself, and destruction of the variable just destroys the reference, not the underlying object.

While the basic idea of object lifetime is simple – an object is created, used, then destroyed – details vary substantially between languages, and within implementations of a given language, and is intimately tied to how memory management is implemented. Further, many fine distinctions are drawn between the steps, and between language-level concepts and implementation-level concepts. Terminology is relatively standard, but which steps correspond to a given term varies significantly between languages.

Terms generally come in antonym pairs, one for a creation concept, one for the corresponding destruction concept, like initialize/finalize or constructor/destructor. The creation/destruction pair is also known as initiation/termination, among other terms. The terms allocation and deallocation or freeing are also used, by analogy with memory management, though object creation and destruction can involve significantly more than simply memory allocation and deallocation, and allocation/deallocation are more properly considered steps in creation and destruction, respectively.

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