Static (keyword)
This article is missing information about the use of the static keyword to declare class methods in C++ and Java.(April 2014) |
In some programming languages such as C (and its close descendants like 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.
Common C/C++ behavior
In C and C++, the effect of the static
keyword in C depends on where the declaration occurs.
static
may act as a storage class (not to be confused with classes in object-oriented programming), as are extern
, auto
and 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:
- A variable with the same lifetime as the program, as described above (language-independent); or
- (C-family-specific) A variable declared with storage class
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.
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".
Static function
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").
This usage is deprecated in C++ (Annex D.2) and an unnamed namespace should be used instead to achieve the same result.
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 (auto
and 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.
C++ specific
Static member variables
In C++, member variables declared as static
inside class definitions are class variables (shared between all class instances, as opposed to instance variables).
Static method
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. A method declared as static
can be called without intanstiating the class.
Java
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