In
class-based
Class-based programming, or more commonly class-orientation, is a style of object-oriented programming (OOP) in which inheritance occurs via defining '' classes'' of objects, instead of inheritance occurring via the objects alone (compare proto ...
,
object-oriented programming
Object-oriented programming (OOP) is a programming paradigm based on the concept of " objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of pr ...
, an instance variable is a
variable defined in a
class (i.e. a
member variable
In object-oriented programming, a member variable (sometimes called a member field) is a variable that is associated with a specific object, and accessible for all its methods (''member functions'').
In class-based programming languages, these a ...
), for which each instantiated
object
Object may refer to:
General meanings
* Object (philosophy), a thing, being, or concept
** Object (abstract), an object which does not exist at any particular time or place
** Physical object, an identifiable collection of matter
* Goal, an ai ...
of the class has a separate copy, or
instance. An instance variable has similarities with a
class variable, but is non-
static
Static may refer to:
Places
*Static Nunatak, a nunatak in Antarctica
United States
* Static, Kentucky and Tennessee
* Static Peak, a mountain in Wyoming
** Static Peak Divide, a mountain pass near the peak
Science and technology Physics
*Static ...
.
An instance variable is a variable which is declared in a class but outside of
constructors,
method
Method ( grc, μέθοδος, methodos) literally means a pursuit of knowledge, investigation, mode of prosecuting such inquiry, or system. In recent centuries it more often means a prescribed process for completing a task. It may refer to:
*Scien ...
s, or
blocks. Instance variables are created when an object is instantiated, and are accessible to all the constructors, methods, or blocks in the class.
Access modifiers can be given to the instance variable.
An instance variable is not a
class variable although there are similarities. It is a type of
class attribute (or class property,
field
Field may refer to:
Expanses of open ground
* Field (agriculture), an area of land used for agricultural purposes
* Airfield, an aerodrome that lacks the infrastructure of an airport
* Battlefield
* Lawn, an area of mowed grass
* Meadow, a grass ...
, or data member). The same dichotomy between ''instance'' and ''class'' members applies to methods ("member functions") as well; a class may have both
instance method
A method in object-oriented programming (OOP) is a procedure associated with a message and an object. An object consists of ''state data'' and ''behavior''; these compose an ''interface'', which specifies how the object may be utilized by any o ...
s and
class methods.
Each instance variable lives in memory for the
lifetime
Lifetime may refer to:
* Life expectancy, the length of time a person is expected to remain alive
Arts, entertainment, and media
Music
* Lifetime (band), a rock band from New Jersey
* ''Life Time'' (Rollins Band album), by Rollins Band
* ...
of the object it is owned by.
Variables are properties an object knows about itself. All instances of an object have their own copies of instance variables, even if the value is the same from one object to another. One object instance can change values of its instance variables without affecting all other instances. Instance variables can be used by all methods of a class unless the method is declared as static.
Example
struct Request ;
int Request::count1 = 0;
In this C++ example, the instance variable
Request::number is a copy of the class variable
Request::count1 where each instance constructed is assigned a sequential value of
count1 before it is
incremented. Since
number is an instance variable, each
Request object contains its own distinct value; in contrast, there is only one object
Request::count1 available to all instances with the same value.
References
Object-oriented programming
Variable (computer science)
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