Question

If a member variable is declared ______________, all objects of that class share that variable.

Short answer

Answer: The keyword used to declare a member variable shared by all objects of a class is `static`.

Step-by-step solution

Understanding class variables vs. instance variables

In object-oriented programming, there are two types of variables: class variables and instance variables. Instance variables are specific to each object created from the class and have independent values for different objects. On the other hand, class variables are shared among all objects of the class, meaning that there is only one copy of the variable that is common to all instances of the class.

Identifying the keyword for class variables

The keyword used to declare class variables varies depending on the programming language being used. In most languages, class variables are declared using one of the following keywords: `static`, `class`, or `shared`.

Answering the given question

According to the given exercise, we need to fill the blank. "If a member variable is declared ______________, all objects of that class share that variable." The correct keyword to fill in the blank is 'static'. Therefore, the complete statement is: If a member variable is declared `static`, all objects of that class share that variable.

Key concepts

Understanding Class Variables

Class variables are unique in the way they differ from typical member variables in object-oriented programming. They are often seen as a shared resource among all instances of a class. This means that while each object of a class usually has its own individual data, class variables are common to the class itself.
Here's how they work:

• A class variable is defined once for the entire class. There is no separate copy for each object instance, making it shared by all objects of the class.
• Changes to class variables reflect across all objects, as all instances access the same single copy.
• They are typically used for constants or data that needs to be consistent across all instances.

Think of class variables like a communal chalkboard. Any class instance can read from or write to this chalkboard, and those changes are visible to every other instance.

Distinguishing Instance Variables

Instance variables provide each object with its own set of data, offering a way to store information that varies from one object to another. These variables are crucial for capturing the unique state of each object.

• Each time an object is created, a new set of instance variables is allocated, and these are unique to the object.
• Changing an instance variable in one object does not affect the same variable in another object.
• They provide customized data storage that reflects the state or behavior specific to that particular object.

Imagine each object as a backpack. Each backpack can carry different marks of a hiking trail, symbolizing the distinct attributes of each hiker. The data they carry (like these marks) is not shared with others but is stored individually in each backpack.

The Role of the Static Keyword

The `static` keyword plays a vital role in distinguishing class variables from instance variables in many programming languages, such as Java and C++. It indicates that a particular member belongs to the class itself, rather than instances of the class.

• The static keyword applies to variables and methods, denoting that they are associated with the class and not with any specific object.
• For static variables, only one instance exists for the class. Regardless of how many objects of the class are instantiated, they all share the same static variables.
• The static keyword ensures memory efficiency since static variables are stored in a specific area of memory called the 'static area'.

Using our chalkboard analogy once again, when you declare a variable static, it means you are using a single communal chalkboard that every instance can read from or contribute to—ensuring consistent information for all members of the class.