What is the difference between an instance variable and a class variable?

In object-oriented programming, both instance variables and class variables are used to store data within a class. However, there are some key differences between the two.

1. Definition:
- Instance Variable: An instance variable is a variable that is unique to each instance or object of a class. It is declared within a class but outside any method or constructor. Each object of the class has its own copy of instance variables.
- Class Variable: A class variable, also known as a static variable, is a variable that is shared among all instances or objects of a class. It is declared within a class but outside any method or constructor, and it is prefixed with the keyword "static". Class variables are associated with the class itself rather than with any specific instance.

2. Scope:
- Instance Variable: The scope of an instance variable is limited to the object or instance of the class in which it is defined. Each object has its own set of instance variables, and changes made to one object's instance variable do not affect the values of instance variables in other objects.
- Class Variable: The scope of a class variable is global within the class and accessible to all instances of the class. Any changes made to a class variable are reflected in all instances of the class.

3. Memory Allocation:
- Instance Variable: Each instance variable is allocated memory separately for each object or instance of the class. This means that if there are multiple objects of the class, each object will have its own copy of the instance variables.
- Class Variable: Class variables are allocated memory only once, regardless of the number of instances created. All instances of the class share the same memory location for class variables.

4. Usage:
- Instance Variable: Instance variables are used to store data that is unique to each object or instance of a class. They represent the state or characteristics of individual objects and are accessed using the object reference.
- Class Variable: Class variables are used to store data that is common to all instances of a class. They represent the behavior or characteristics that are shared among all objects of the class and are accessed using the class name.

5. Access:
- Instance Variable: Instance variables are accessed using the object reference or instance of the class. Each object has its own set of instance variables, and they can have different values for each object.
- Class Variable: Class variables are accessed using the class name itself, without the need for any object reference. They are shared among all instances of the class and have the same value for all objects.

In summary, the main difference between instance variables and class variables lies in their scope, memory allocation, and usage. Instance variables are unique to each object and have separate memory allocation, while class variables are shared among all instances and have memory allocated only once. Instance variables represent the state of individual objects, while class variables represent shared behavior or characteristics.