Question

Describe the difference between an instance member variable and a static member variable.

Short answer

Answer: Instance member variables are variables that belong to a specific instance of an object, with each object having its own separate copy of the variable. These variables are used when you want to store information specific to each object instance. Static member variables, on the other hand, belong to the class itself and are shared among all instances of the class, making them suitable for storing information common to all objects of a class.

Step-by-step solution

Definition: Instance Member Variables

An instance member variable is a variable that belongs to a specific instance of an object. Each object of the same class has its own separate copy of the instance member variable. The value of this variable is distinct and independent for each object instance.

Definition: Static Member Variables

A static member variable, also known as a class variable, is a variable that belongs to the class itself, rather than the instances of the class. This means that there is only one copy of the static member variable, which is shared among all instances of the class.

Comparison: Instance vs Static Member Variables

The primary difference between an instance member variable and a static member variable is how they are associated with the class and instances. While an instance member variable has its own unique value for each object of a class, a static member variable is shared among all objects of the same class.

Usage: Instance Member Variables

Instance member variables are used when you want to store information or a state that is specific to each object instance. For example, in a class representing a car, you might have instance member variables for the current speed, make, and model of the car. As each car will have different values for these variables, it makes sense to use instance member variables.

Usage: Static Member Variables

Static member variables are used when you want to store information that is common to all objects of a class. For instance, if you have a class representing a counter, you might use a static member variable to keep track of how many counter objects have been created. Since this count is relevant to all instances of the class, it's better to use a static member variable.

Key concepts

Instance Member Variables

Instance member variables are crucial in object-oriented programming because they allow each object created from a class to maintain its own distinct state. When a class is instantiated, each object has its own memory allocation for its instance member variables. This means that changes made to one object's instance member variable do not impact other objects created from the same class.
For example:

• If you have a `Car` class, each car object can have its own color, model, and speed. These properties are stored as instance member variables.
• Every `Car` object can have different values assigned to its instance member variables like `color` and `speed`, making each unique.

Thus, instance member variables are perfect when each object needs to keep its own distinct data.

Static Member Variables

Static member variables, often referred to as class variables, are shared among all instances of a class. Instead of being part of the object, these variables belong to the class itself and thus only occupy one memory location. This shared nature makes them ideal for storing data or states that are common across all instances.
Consider this:

• In a `Car` class, imagine we want to keep track of the number of cars created. A static member variable `totalCars` would be appropriate.
• By updating `totalCars` each time a new car is instantiated, all cars share this counter.

Unlike instance variables, changes in a static member variable are reflected across all instances of the class, which can be particularly useful for counting or configuration purposes.

Class Variables

Class variables are essentially synonymous with static member variables in most programming languages. They are defined within a class and are marked with the `static` keyword. This guides the program to handle them at the class level rather than at the object level.
Use class variables when:

• You want to apply a single value to all instances.
• You're managing global data across all objects of the class.

For instance, if you're setting a default tax rate for all `Invoice` objects, a class variable would be apt. This setup ensures consistency across all instances but requires careful use to avoid unintended side effects from changes affecting all objects derived from the class.