In programming languages such as C, C++, Objective-C, and Java, static is a reserved word controlling both lifetime (as a static variable) and visibility (depending on linkage). The effect of the keyword varies depending on the details of the specific programming language.
In C and C++, the effect of the static keyword depends on where the declaration occurs.
static may act as a storage class (not to be confused with classes in object-oriented programming), as can [[External variable|extern]], [[Automatic variable#C, C++|auto]] and [[Register (keyword)|register]] (which are also reserved words). Every variable and function has one of these storage classes; if a declaration does not specify the storage class, a context-dependent default is used:
extern for all top-level declarations in a source file,auto for variables declared in function bodies.| Storage class | Lifetime | Visibility | |
|---|---|---|---|
extern | program execution | external (whole program) | |
static | program execution | internal (translation unit only) | |
auto, register | function execution | (none) |
In these languages, the term "static variable" has two meanings which are easy to confuse:
static.Variables with storage class extern, which include variables declared at top level without an explicit storage class, are static in the first meaning but not the second.
A variable declared as static at the top level of a source file (outside any function definitions) is only visible throughout that file ("file scope", also known as "internal linkage"). In this usage, the keyword static is known as an "access specifier".
Similarly, a static functiona function declared as static at the top level of a source file (outside any class definitions)is only visible throughout that file ("file scope", also known as "internal linkage").
Variables declared as static inside a function are statically allocated, thus keep their memory location throughout all program execution, while having the same scope of visibility as automatic local variables (auto and register), meaning they remain local to the function. Hence whatever values the function puts into its static local variables during one call will still be present when the function is called again.
In C++, member variables declared as static inside class definitions are class variables (shared between all class instances, as opposed to instance variables).
Similarly, a static member functiona member function declared as static inside a class definitionis meant to be relevant to all instances of a class rather than any specific instance. A member function declared as static can be called without instantiating the class.
This keyword static means that this method is now a class method; it will be called through class name rather than through an object.
A static method is normally called as <classname>.methodname, whereas an instance method is normally called as <objectname>.methodname.