Question

Find the errors in the following class and explain how to correct them: class Example { public: Example( int y = 10 ) : data( y ) { // empty body } // end Example constructor int getIncrementedData() const { return data++; } // end function getIncrementedData [Page 569] static int getCount() { cout << "Data is " << data << endl; return count; } // end function getCount private: int data; static int count; }; // end class Example

Short answer

Remove 'const' from `getIncrementedData` and fix `getCount` by not accessing `data`. Static methods can only use static members.

Step-by-step solution

Identifying the Error in getIncrementedData

The method `getIncrementedData()` is marked as `const`, meaning it should not modify any member variables. However, it is incrementing `data` which violates the `const` modifier. A `const` method must only use member variables without altering them.

Correcting getIncrementedData

Remove the `const` qualifier from the `getIncrementedData` method if you intend to modify the member variable `data`. Alternatively, if the increment is not required, ensure the method does not alter any class members.

Identifying the Error in getCount

In the method `getCount()`, it attempts to access `data`, which is a non-static member variable, from a `static` method. A static method can only access static members of the class.

Correcting getCount

Remove the reference to `data` with the `cout` statement in `getCount()`. Ensure that any interaction with member variables is done with static members only, or convert non-static to static if logic permits.

Key concepts

const keyword in C++

In C++, the `const` keyword is used to indicate that a variable's value is constant, or that a function should not modify class variables. It is often used for objects passed as parameters to functions, ensuring their state is not altered during the process. This keyword can also be applied to class member functions.

• Const Member Functions: When you declare a member function as `const`, it promises not to change any member variables of the class. If a `const` function attempts to change a member variable, the compiler will throw an error.

In our exercise, `getIncrementedData()` was marked as `const` but attempted to increment the `data` variable, which violates the `const` promise. The solution is either to remove the `const` qualifier or to ensure that the function does not modify any state.

static member functions

Static member functions in C++ belong to the class rather than any particular object instance. This means they can be called on the class itself, without needing an object of the class. Static functions are particularly useful for implementing utility functions that do not need any object data.

• Access Restrictions: Because static member functions do not operate on a specific object instance, they cannot access instance (or non-static) variables or methods directly. They can only interact with static members of the class.

In the provided class, the `getCount()` function tried to access `data`, a non-static member variable, which is not allowed in static functions. The solution was to either remove such access or ensure the function only interacts with static members, fixing the class design accordingly.

C++ constructors

In C++, constructors are special member functions of a class that initialize objects. Their main purpose is to set initial values and allocate resources when an instance of a class is created.

• Types of Constructors: Constructors can have different forms, such as default constructors, parameterized constructors, and copy constructors. The absence of parameters or their presence defines them.
• Constructor Initialization List: Initialization lists can be used in constructors to efficiently assign values to member variables.

The given example used a parameterized constructor with an initialization list, setting `data` to a default of 10. This is a typical practice to ensure all members are initialized with certain values at the time of object creation.

object-oriented programming in C++

Object-oriented programming, or OOP, is a paradigm based on the concept of "objects", which can contain data and methods. C++ is one of the languages that supports OOP, allowing for encapsulation, inheritance, and polymorphism.

• Class and Objects: In OOP, a class acts as a blueprint for creating objects. Each object created is an instance of a class and has its own set of data and functions that operate on this data.
• Encapsulation: It refers to bundling the data and the functions that operate on the data into a single unit or class, and controlling the access to this data via access specifiers like `public`, `private`, and `protected`.
• Polymorphism and Inheritance: These features allow the reuse and extension of existing code, making C++ a powerful tool for designing complex software systems efficiently.

In the exercise, the Example class demonstrates encapsulation by keeping data private and providing public interfaces for interaction, while the misuse of certain keywords highlights the importance of understanding C++'s OOP features fully.