Static (keyword)

static is a reserved word in programming languages such as C, C++, Objective-C, C#, Java, PHP, Javascript, Dart, etc., to modify a declaration. The effect of the keyword varies depending on the details of the specific programming language, most commonly used to modify the lifetime (as a static variable) and visibility (depending on linkage), or to specify a class member instead of an instance member in classes.

C

In C, the static keyword declares a static variable, which is a variable with the same lifetime as the program. In addition, if the static keyword is used for a top-level variable, it makes it to have internal linkage only, instead of external linkage by default.

static is a storage class specifier (not to be confused with classes in object-oriented programming), with extern, auto and register (which are also reserved words) as well. 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 C, the term "static variable" has two meanings which are easy to confuse:

1. A variable with the same lifetime as the program, as described above (language-independent); or
2. (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". It is statically allocated with the lifetime being the program execution.

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").

Static local variables

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.

C++

In C++, the keyword static extends to member variables and functions. In the case of static member variable.

Static member variables

Member variables declared as static inside class definitions are class variables (shared between all class instances, as opposed to instance variables). They are also statically allocated with the lifetime being the execution of the program, the same with static top-level variables and static local variables.

Static member function

Similarly, a static member function – a member function declared as static inside a class definition – is 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, and it cannot access the this pointer or access non-static members of the class.

Java

This keyword static means that this method is 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().