Question

Static member variables are defined _________the class.

Short answer

Answer: Static member variables are defined "outside" the class, but within the same source file (typically in the implementation file). The actual memory allocation and initialization happens outside the class definition.

Step-by-step solution

Understand Static Member Variables

Static member variables are variables that belong to the class, instead of any particular object of the class. They have a single value/copy shared across all instances of the class.

Where are Static Member Variables Defined?

Static member variables are defined "outside" the class, but within the same source file (typically in the implementation file). The actual memory allocation and initialization happens outside the class definition.

Example of Defining a Static Member Variable

Consider a class called 'Counter' with a static member variable 'count'. Here's an example of how to define the static member variable in C++: // Counter.h file class Counter { public: static int count; }; // Counter.cpp file int Counter::count = 0; // This is the definition and initialization of the static member variable.

Key concepts

C++ Class Concepts

In C++, a class is a blueprint for creating objects. A class defines attributes and behaviors that the created objects will possess. Think of it as a template for constructing similar entities, where each object is an instance of a class.

For example, if you have a class named 'Vehicle', it might have attributes such as 'wheels' and 'engine', and behaviors like 'drive' and 'stop'. Each object created from the 'Vehicle' class, like a 'car' or a 'motorcycle', has these attributes and behaviors, which can be independently modified.

Classes are defined using the class keyword followed by the class name and a body containing declarations of fields, methods, and sometimes other classes. Here's the syntax for defining a simple class:

class Vehicle {public:    int wheels;    void drive() {        // Code to drive the vehicle    }};

Within a class, 'members' refer to its variables and functions. Members can be either 'static' or non-static, which determines how they behave and how they're accessed.

Static Member Definition

A static member variable in C++ is associated with the class itself rather than any particular instance of the class. This means that there is only one copy of the variable for the entire class, and all instances of the class share this single copy.

Using static members can be very useful. For instance, if you want to count the number of instances of a class that have been created, or if you have a constant value or utility function that doesn't vary between instances, static members can be the right choice.

The keyword static is used to declare a static member within a class definition like this:

class ClassName {public:    static int sharedVariable;};

Note that the declaration within the class body is not sufficient to create the variable. The static member also needs to be defined outside of the class to allocate memory for it, which is typically done in the implementation file (.cpp).

Static Members Initialization

Initialization of static member variables in C++ is a crucial step that actually allocates the single copy of the variable and sets its initial value. This is done outside of the class definition but in the same file, often the implementation (.cpp) file.

For example, let's consider our 'Counter' class example from the exercise. The class declares a static member variable 'count'. To initialize this variable, you write the following in the implementation file:

int Counter::count = 0;

Here, int Counter::count specifies that we're defining the 'count' variable belonging to the 'Counter' class, and we're initializing it with the value of 0. This definition ensures that 'count' exists and provides an initial value that all instances will see unless it's changed.

Remember, you only need to define and initialize a static member variable once, and you must do so outside of the class body. The initialization syntax matches the declaration in the class and uses the scope resolution operator :: to link the variable with the class.