Question

Suppose that the classes Dog and cat derive from Animal, which in turn derives from Creature. Suppose further that pDog, pCat, pAnimal, and pCreature are pointers to the respective classes. Suppose that Animal and creature are both abstract classes. Will the statement pCreature \(=\) new Dog; compile?

Short answer

Based on the given class hierarchy and principles of class inheritance in C++, the statement "pCreature = new Dog;" is valid, and it will compile. However, if you want to access any Dog-specific member through the pCreature pointer, you would need to perform a downcast first.

Step-by-step solution

We are given the following class hierarchy: 1. Creature (abstract class) - Animal (abstract class) - Dog - Cat Here, Creature is the base class, and Animal derives from Creature. In turn, Dog and Cat both derive from Animal. Since both Creature and Animal are abstract classes, they cannot be instantiated. #Step 2: Understanding pointers#

In the given statement, pDog, pCat, pAnimal, and pCreature are pointers to the respective classes. This means they can be assigned the memory address of an object of their corresponding class (or a derived class). #Step 3: Analyze the statement#

The statement to analyze is: pCreature = new Dog; In this statement, we are trying to instantiate a Dog object using the "new" keyword and assigning its memory address to the pCreature pointer, which is a pointer to the Creature class. #Step 4: Determine the validity of the statement#

In C++, a pointer to a base class can be assigned the memory address of an object of a derived class. Since Dog is a derived class of Creature (through Animal), the statement "pCreature = new Dog;" is valid and will compile. Please, remember that if you want to access any Dog-specific member through pCreature pointer, you will have to perform a downcast first. In conclusion, the statement "pCreature = new Dog;" will compile.

Key concepts

Class Hierarchy

In programming, a class hierarchy is a way to organize classes in an orderly manner based on inheritance. This hierarchy starts with a base class and progressively extends to more specific classes. For example, in the provided exercise, the class hierarchy is structured as follows:

• Creature - This is the base class and the most general one.
• Animal - Derived from Creature, it narrows down the classification to creatures that are animals.
• Dog & Cat - These are derived classes of Animal, further specifying the type of animal.

This hierarchy allows for a structured and logical organization of classes where a derived class inherits properties and methods from its parent. It helps in code reusability and facilitates a clearer understanding of relationships between classes. By defining common behaviors at higher levels, you ensure that specific classes like Dog and Cat automatically have access to those behaviors without extra code duplication.

Abstract Classes

Abstract classes play a crucial role in object-oriented programming. They are classes that cannot be instantiated on their own and are designed to be inherited by other classes. The existence of abstract classes is primarily to provide a template or blueprint for more specific classes.

• In the exercise, Creature and Animal are both abstract classes.
• An abstract class typically contains one or more pure virtual functions - functions that are declared but not defined by the abstract class itself. These must be implemented by any non-abstract class that inherits from it.

Their main purpose is to define a common interface for all its derived classes. This ensures that all derived classes adhere to a specific structure or contract, promoting consistency and reducing errors. Abstract classes enable programmers to define base behaviors and enforce that those behaviors are not accidentally overlooked by the derived classes.

Polymorphism

Polymorphism is a core concept in object-oriented programming that allows for treating objects of different classes through the same interface. It enables methods to do the right thing based on the object they are acting upon, even if it is being referred to by a pointer or reference to the base class.

• The statement pCreature = new Dog; exemplifies polymorphism. Here, though pCreature is a pointer to the base class Creature, it is assigned an object of a derived class, Dog.
• In C++, through polymorphism, you can call derived class functions using base class pointers. However, base class pointers pointing to derived class objects need to use virtual functions to achieve runtime polymorphism.

Thus, polymorphism enhances the flexibility and maintainability of the code by allowing the same function or operation to behave differently on different classes of objects.

Pointers in C++

Pointers are a powerful feature in C++ that allows for direct memory management. They store the address of variables or objects, providing a mechanism to access and manipulate data directly.

• In the context of the exercise, pCreature is a pointer to the Creature class, and assigning it with new Dog indicates it points to the start of the memory address allocated for a Dog object.
• Pointers enable dynamic memory allocation as seen with the new operator, allocating memory for a specified class type and returning its address.
• Using pointers, you can achieve polymorphism, allowing base class pointers to refer to derived class objects as demonstrated in the exercise. This feature paves the way for dynamic dispatch, where the function to execute is decided at runtime.

Understanding pointers is essential for dealing with dynamic memory, achieving efficient code, and implementing advanced aspects like polymorphism and inheritance in C++.