C++ Basics: Objects & Classes - CSU1287P - Shoolini U

Object and Clases in C++

1. Introduction to Objects and Classes in C++

In the realm of object-oriented programming (OOP), C++ is one of the most powerful and versatile languages. This article aims to provide a comprehensive understanding of object and class concepts in C++ from the basics to the level of computer science students. Topics covered include core object concepts, implementation of classes in C++, C++ objects as physical objects and data types, constructors, object usage as function arguments, default copy constructors, returning objects from functions, structures and classes, class objects and memory, and static class data, as well as the const keyword in relation to classes.

2. Making Sense of Core Object Concepts

Before diving into the implementation of classes and objects in C++, it's essential to understand the core concepts of object-oriented programming. These concepts are Encapsulation, Abstraction, Polymorphism, Classes, Messages, Association, and Interfaces.

2.1 Encapsulation

Encapsulation is the process of bundling data and methods that operate on that data within a single unit or class. This prevents unauthorized access and modification of data, thus ensuring data integrity and security.

2.2 Abstraction

Abstraction is the process of simplifying complex systems by breaking them down into smaller, more manageable components. It allows programmers to focus on the essential features of a component without worrying about its underlying implementation details. In OOP, abstraction is achieved through classes and interfaces.

2.3 Polymorphism

Polymorphism is the ability of a single function or method to work with different data types, objects, or classes. This allows for more flexible and reusable code by reducing the need for duplicate code for each specific type or object.

2.4 Classes

A class is a blueprint for creating objects in C++. It defines the data members and member functions that make up an object, as well as any access restrictions or rules for inheritance. Objects are instances of a class and contain the state and behavior defined by the class.

2.5 Messages

Messages are a means of communication between objects in OOP. When an object sends a message to another object, it is essentially requesting that the receiving object perform a specific action or return specific information. Messages are implemented as function calls in C++.

2.6 Association

Association is a relationship between two or more classes, which allows them to communicate with each other. The relationship can be one-to-one, one-to-many, or many-to-many, and it can be unidirectional or bidirectional.

2.7 Interfaces

Interfaces define a contract or set of rules that classes must adhere to when implementing the interface. They provide a way to enforce consistent behavior among classes without imposing implementation details. In C++, interfaces are typically implemented using abstract base classes and pure virtual functions.

3. Implementation of Class in C++

Classes are the foundation of object-oriented programming in C++. They define the structure and behavior of objects and are essential for creating reusable and maintainable code. In this section, we will discuss how to define and use classes in C++.

3.1 Defining a Class

To define a class in C++, you use the "class" keyword followed by the name of the class and a pair of curly braces that enclose the class members. The class members include data members (variables) and member functions (methods). Access specifiers such as "public", "private", and "protected" control the visibility and accessibility of class members. Here's a simple example:

class MyClass {
public:
  int myVar;
  void myFunction() {
    // Function implementation
  }
}; // Don't forget the semicolon at the end

3.2 Creating Objects

To create an object (instance) of a class, simply declare a variable of the class type, followed by the object name. You can then access the public data members and member functions using the dot (.) operator:

MyClass obj;
obj.myVar = 42;
obj.myFunction();

3.3 Constructors

Constructors are special member functions that are called automatically when an object is created. They are used to initialize the object's data members with default or user-specified values. A constructor has the same name as the class and does not return any value. You can define multiple constructors with different parameter lists using constructor overloading:

class MyClass {
public:
    MyClass() { // Default constructor
        myVar = 0;
    }

    MyClass(int x) { // Parameterized constructor
        myVar = x;
    }

    int myVar;
};

4. C++ Objects as Physical Objects and Data Types

In C++, objects can be thought of as physical objects and data types. As physical objects, they represent instances of a class and have a specific location in memory. As data types, they can be used to declare variables, passed as arguments to functions, and returned from functions.

4.1 Objects as Function Arguments

Objects can be passed as arguments to functions in two ways: by value and by reference. When passing by value, a copy of the object is made, and any modifications to the object within the function do not affect the original object. When passing by reference, the function works with the original object, and any modifications made within the function are reflected in the original object.

void funcByValue(MyClass obj) {
    obj.myVar = 10; // This will not affect the original object
}

void funcByReference(MyClass& obj) {
    obj.myVar = 10; // This will affect the original object
}

4.2 The Default Copy Constructor

When an object is passed by value or returned from a function, the default copy constructor is used to create a new object as a copy of the original object. The default copy constructor performs a shallow copy, which means that it copies the object's data members but does not create new instances of any objects referenced by pointers. If your class contains pointers or other resources that require deep copying, you should define a custom copy constructor:

class MyClass {
public:
    MyClass(const MyClass& other) {
        myVar = other.myVar;
        // Perform deep copy for pointers or other resources
    }

    int myVar;
};

4.3 Returning Objects from Functions

Objects can be returned from functions, either by value or by reference. When returning by value, a copy of the object is created using the copy constructor. When returning by reference, the original object is returned, and no copy is made. It is important to be cautious when returning objects by reference, as you should avoid returning references to local objects, which will be destroyed when the function exits, resulting in undefined behavior.

MyClass funcReturnByValue() {
    MyClass obj;
    obj.myVar = 42;
    return obj; // A copy of obj is returned
}

MyClass& funcReturnByReference(MyClass& obj) {
    obj.myVar = 42;
    return obj; // The original obj is returned
}

5. Structures and Classes

Structures and classes in C++ are very similar, with the primary difference being the default access specifier. In a class, members are private by default, whereas, in a structure, they are public. Other than this difference, structures can have constructors, destructors, and member functions just like classes:

struct MyStruct {
    int myVar;
    void myFunction() {
        // Function implementation
    }
};

6. Classes, Objects, and Memory

When working with classes and objects, it is important to understand how memory is allocated and managed in C++. Objects can be stored in either the stack or the heap, and their lifetime and accessibility can be controlled using pointers and references.

6.1 Stack Allocation

When an object is declared as a local variable, it is allocated on the stack. The object's lifetime is limited to the scope in which it is declared, and it is automatically destroyed when the scope is exited:

void myFunction() {
    MyClass obj; // obj is allocated on the stack
} // obj is destroyed when myFunction exits

6.2 Heap Allocation

Objects can also be allocated on the heap using the "new" keyword. This allows the object to have a longer lifetime, as it is not automatically destroyed when the scope is exited. However, it is the programmer's responsibility to delete the object using the "delete" keyword when it is no longer needed to avoid memory leaks:

MyClass* myFunction() {
    MyClass* obj = new MyClass(); // obj is allocated on the heap
    return obj;
}

int main() {
    MyClass* pObj = myFunction();
    // Do something with pObj
    delete pObj; // It's important to delete pObj to free memory
}

7. Static Class Data

Static class data members and member functions have a unique property: they belong to the class itself, not to any specific instance of the class. This means that there is only one copy of the static data member for all objects of the class, and static member functions can be called without an object of the class.

7.1 Static Data Members

To declare a static data member, use the "static" keyword in the class definition. The static data member must be initialized outside the class definition:

class MyClass {
public:
    static int myStaticVar;
};

int MyClass::myStaticVar = 0; // Initialize the static data member

7.2 Static Member Functions

Static member functions can be declared using the "static" keyword in the class definition. They can only access static data members and call other static member functions, as they do not have access to any specific instance of the class:

class MyClass {
public:
    static void myStaticFunction() {
        // Access static data members and call other static functions
    }
};

8. Const and Classes

The "const" keyword can be used in classes to enforce read-only access to data members and ensure that member functions do not modify the state of the object. This can help create more robust and maintainable code by preventing accidental modifications to an object's state.

8.1 Const Data Members

To declare a const data member, use the "const" keyword in the class definition. Const data members must be initialized at the time of object creation, typically using constructor initializer lists:

class MyClass {
public:
    MyClass(int x) : myConstVar(x) {}

    const int myConstVar;
};

8.2 Const Member Functions

Const member functions are member functions that do not modify the state of the object. They are declared using the "const" keyword after the function signature. Const member functions can only call other const member functions and cannot modify any data members:

class MyClass {
public:
    int getVar() const {
        return myVar;
    }

    int myVar;
};

8.3 Const Objects

Const objects are objects that cannot be modified after they are created. To declare a const object, use the "const" keyword before the class type. Const objects can only call const member functions:

const MyClass obj(42);
int x = obj.getVar(); // Only const member functions can be called

9. Conclusion

In this article, we have covered the essential concepts and features of objects and classes in C++, from basic to advanced levels. We have discussed core object concepts such as Encapsulation, Abstraction, Polymorphism, Classes, Messages, Association, and Interfaces. We have also delved into the implementation of classes in C++, including constructors, object usage as function arguments, default copy constructors, returning objects from functions, structures and classes, and class objects and memory. Lastly, we have explored the use of static class data and the const keyword in relation to classes.

With a thorough understanding of these concepts, you will be well-equipped to create robust, maintainable, and reusable code using object-oriented programming principles in C++.