In
class-based,
object-oriented programming
Object-oriented programming (OOP) is a programming paradigm based on the concept of '' objects''. Objects can contain data (called fields, attributes or properties) and have actions they can perform (called procedures or methods and impl ...
, an instance variable is a
variable defined in a
class
Class, Classes, or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used d ...
(i.e., a
member variable
In object-oriented programming
Object-oriented programming (OOP) is a programming paradigm based on the concept of '' objects''. Objects can contain data (called fields, attributes or properties) and have actions they can perform (called ...
), for which each instantiated
object of the class has a separate copy, or
instance. An instance variable has similarities with a
class variable, but is non-
static. An instance variable is a variable which is declared in a class but outside of
constructors,
methods, 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. Both are a type of
class attribute (or class property,
field, or data member). While an instance variable's value may differ between instances of a class, a
class variable can only have one value at any one time, shared between all instances. The same dichotomy between ''instance'' and ''class'' members applies to methods ("member functions") as well.
Each instance variable lives in
memory
Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembe ...
for the
lifetime of the object it is owned by.
Instance variables are properties of that object. All instances of a class have their own copies of instance variables, even if the value is the same from one object to another. One class instance can change values of its instance variables without affecting all other instances. A class may have both instance variables and
class variables.
Instance variables can be used by all instance methods of an object, but may not be used by class methods. An instance variable may also be changed directly, provided
access restrictions are set.
Examples
C++
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 class instances with the same value.
Java
//Example.java
class Example
//Main.java
class Main
In this
Java
Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...
example, we can see how instance variables can be modified in one instance without affecting another.
Python
class Dog:
def __init__(self, breed):
self.breed = breed # instance variable
# dog_1 is an object
# which is also an instance of the Dog class
dog_1 = Dog("Border Collie")
In the above
Python code, the instance variable is created when an argument is parsed into the instance, with the specification of the breed positional argument.
References
Object-oriented programming
Variable (computer science)
{{Compu-prog-stub