Question

Consider the following definition of the class myClass: class myClass { public: void setX(int a); //Function to set the value of x. //Postcondition: x = a; void printX() const; //Function to output x. static void printCount(); //Function to output count. static void incrementCount(); //Function to increment count. //Postcondition: count++; myClass(int a = 0); //constructor with default parameters //Postcondition x = a; //If no value is specified for a, x = 0; private: int x; static int count; }; a. Write a C++ statement that initializes the member variable count to 0. b. Write a C++ statement that increments the value of count by 1. c. Write a C++ statement that outputs the value of count. d. Write the definitions of the functions of the class myClass as described in its definition. e. Write a C++ statement that declares myObject1 to be a myClass object and initializes its member variable x to 5. f. Write a C++ statement that declares myObject2 to be a myClass object and initializes its member variable x to 7. g. Which of the following statements are valid? (Assume that myObject1 and myObject2 are as declared in Parts e and f.) myObject1.printCount(); //Line 1 myObject1.printX(); //Line 2 myClass.printCount(); //Line 3 myClass.printX(); //Line 4 myClass::count++; //Line 5 h. Assume that myObject1 and myObject2 are as declared in Parts e and f. What is the output of the following C++ code? myObject1.printX(); cout << endl; myObject1.incrementCount(); 1 2 Exercises | 717 myClass::incrementCount(); myObject1.printCount(); cout << endl; myObject2.printCount(); cout << endl; myObject2.printX(); cout << endl; myObject1.setX(14); myObject1.incrementCount(); myObject1.printX(); cout << endl; myObject1.printCount(); cout << endl; myObject2.printCount(); cout << endl;

Short answer

Initialize `count` as `int myClass::count = 0;`. Increment and print using `incrementCount()` and `printCount()`. Valid: Lines 1, 2. Output: 5, 2, 7, 14, 3.

Step-by-step solution

Initialize static variable `count`

To initialize the static member variable `count` to 0, add the statement `int myClass::count = 0;` outside of the class definition. Static variables are shared across all instances of a class and must be initialized outside the class.

Increment static variable `count`

Inside the class member function `incrementCount`, write the statement `count++;`. This increments the static variable `count` by 1 whenever the function is called.

Output static variable `count`

Inside the static class member function `printCount`, write the statement `cout << count;`. This will output the current value of the static variable `count`.

Define member functions of `myClass`

Define the member functions as follows: - `void myClass::setX(int a) { x = a; }` to set the member variable `x`. - `void myClass::printX() const { cout << x; }` to print the value of `x`. - Implement `myClass(int a) : x(a) {}` as the constructor to set `x`.

Declare and initialize `myClass` objects

For `myObject1`, use `myClass myObject1(5);` to declare the object and initialize `x` to 5. For `myObject2`, use `myClass myObject2(7);` to initialize `x` to 7.

Validate given statements

Check which statements are valid: - `myObject1.printCount();` is valid as it accesses a static method. - `myObject1.printX();` is valid. - `myClass.printCount();` is invalid; should use `myClass::printCount();`. - `myClass.printX();` is invalid because `printX` is not a static function. - `myClass::count++;` is invalid as `count` is private.

Understand the code's output behavior

Analyze the provided code segment: - `myObject1.printX();` prints 5. - Two `incrementCount();` calls make `count` equal to 2. - `myObject1.printCount();` and `myObject2.printCount();` print 2, since `count` is shared. - `myObject2.printX();` prints 7. - After setting `myObject1.setX(14);` and incrementing count, `myObject1.printX();` prints 14. - `printCount();` again outputs 3 for both `myObject1` and `myObject2`.

Key concepts

Static Member Variables

Static member variables in C++ are unique because they are shared across all instances of a class. This means that no matter how many objects you create from a class, there will be one common static member for all objects.

• Static variables are defined with the keyword `static` within a class definition, but they must be initialized outside the class.
• Initialization is done once, using a statement like `int myClass::count = 0;`, where `myClass` is the class name and `count` is the static variable.
• Because `count` is shared, changes made by any object of the class affect all other objects when accessing this variable.

In our example, the variable `count` is used to keep track of how many times the `incrementCount()` function is called, showing a common use case for static variables.

Member Function Definitions

Member functions in C++ are defined to perform specific tasks related to class objects. For proper encapsulation and functionality, these functions are typically declared inside the class and defined outside.

• Function definitions use the scope resolution operator `::` to associate the function definition with the class.
• For example, `void myClass::setX(int a) { x = a; }` sets the value of `x` for an object of `myClass`.
• Similarly, `void myClass::printX() const { cout << x; }` is defined to output the value of `x`. Note the use of `const`, indicating the function does not modify the object.

By managing different functionalities through member functions, a class allows objects to interact and perform operations in a controlled manner. For instance, functions like `incrementCount()` or `printCount()` can modify and access the static variable `count` safely.

Object Initialization

Object initialization in C++ involves providing initial values to member variables during the creation of a class object. This can be done via constructors, which are special member functions of the class.

• Constructors have the same name as the class and do not have a return type.
• They can have parameters, allowing for passing of initial values, like `myClass(int a = 0)` sets member variable `x` to `a`, or defaults to 0 if no value is provided.
• For example, `myClass myObject1(5);` declares an object `myObject1` with `x` initialized to 5.

This approach ensures that objects are properly initialized before use, minimizing errors related to uninitialized variables. Properly designed constructors improve both the safety and readability of code.

Access Modifiers

Access modifiers in C++ are keywords that set the accessibility of classes and class members. They control the visibility and accessibility of the members from outside the class.

• `public`: Members are accessible from outside the class. For example, both `setX()` and `printX()` functions are declared public, allowing external code to call them.
• `private`: Members are accessible only within the class itself, not from outside code. In `myClass`, `count` and `x` are private, meaning they can only be accessed and modified through class functions.
• `protected` (not used in the example): Members are accessible within the class and by derived class members.

Proper use of access modifiers is a core principle in encapsulation, one of the fundamental concepts in object-oriented programming, ensuring data hiding and security.