In the C programming language (and its close descendants such as C++ and Objective-C),
static is a reserved word controlling both lifetime (as a static variable) and visibility (depending on linkage). The word
static is also used in languages influenced by C, such as Java.
static is a storage class (not to be confused with classes in object-oriented programming), as are
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:
externfor all top-level declarations in a source file,
autofor variables declared in function bodies.
||program execution||external (whole program)|
||program execution||internal (translation unit only)|
In these languages, the term "static variable" has two meanings which are easy to confuse:
- A variable with the same lifetime as the program, as described above (language-independent); or
- (C-family-specific) A variable declared with storage class
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.
static keyword when prefixed while declaring a variable or a function can have other effects depending on where the declaration occurs.
Static global variable
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 function – a 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").
Static local variables
Variables declared as
static inside a function are statically allocated, thus keep their memory cell throughout all program execution, while having the same scope of visibility as automatic local variables (
register), meaning 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.
Static member variables
Similarly, a static method – a method declared as
static inside a class definition – is meant to be relevant to all instances of a class rather than any specific instance.