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Object-Oriented Programming in C++

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Classes and Objects

A C++ class is a user-defined data type that encapsulates information and behavior about an object.

A class can have two types of class members:

  • Attributes, also known as member data, consist of information about an instance of the class.
  • Methods, also known as member functions, are functions that can be used with an instance of the class.

An object is an instance of a class and can be created by specifying the class name.

#include <iostream>
class Dog {
public:
int age;
void sound() {
std::cout << "woof\n";
}
};
int main() {
Dog buddy;
buddy.age = 5;
buddy.sound(); // Outputs: woof
}

Access Specifiers

Access specifiers are C++ keywords that determine the scope of class components:

  • public: Class members are accessible from anywhere in the program.
  • private: Class members are only accessible from inside the class.

Encapsulation is achieved by declaring class attributes as private:

  • Accessor functions: return the value of private member variables.
  • Mutator functions: change the value of private member variables.
#include <iostream>
class Computer {
private:
int password;
public:
int getPassword() {
return password;
}
void setPassword(int new_password) {
password = new_password;
}
};
int main()
{
Computer dell;
dell.setPassword(12345);
std::cout << dell.getPassword();
return 0;
}

Constructors

For a C++ class, a constructor is a special kind of method that enables control regarding how the objects of a class should be created. Different class constructors can be specified for the same class, but each constructor signature must be unique.

A constructor can have multiple parameters as well as default parameter values.

In order to initialize const or reference type attributes, use member initializer lists instead of normal constructors.

#include <iostream>
using namespace std;
class House {
private:
std::string location;
int rooms;
public:
// Constructor with default parameters
House(std::string loc = "New York", int num = 5) {
location = loc;
rooms = num;
}
// Destructor
~House() {
std::cout << "Moved away from " << location << "\n";
}
};
int main()
{
House default_house; // Calls House("New York", 5)
House texas_house("Texas"); // Calls House("Texas", 5)
House big_florida_house("Florida", 10); // Calls House("Florida", 10)
return 0;
}

Inheritance

In C++, a class can inherit attributes and methods from another class. In an inheritance relationship, there are two categories of classes:

  • Base class: The class being inherited from.
  • Derived class: The class that inherits from the base class.

It’s possible to have multi-level inheritance where classes are constructed in order from the “most base” class to the “most derived” class.

#include <iostream>
class Base {
public:
int base_id;
Base(int new_base) : base_id(new_base) {}
};
class Derived: public Base {
public:
int derived_id;
Derived(int new_base, int new_derived)
: Base(new_base), derived_id(new_derived) {}
void show() {
std::cout << base_id << " " << derived_id;
}
};
int main() {
Derived temp(1, 2);
temp.show(); // Outputs: 1 2
return 0;
}

Polymorphism

In C++, polymorphism occurs when a derived class overrides a method inherited from its base class with the same function signature.

Polymorphism gives a method many “forms”. Which form is executed depends on the type of the caller object.

#include <iostream>
class Employee {
public:
void salary() {
std::cout << "Normal salary.\n";
}
};
class Manager: public Employee {
public:
void salary() {
std::cout << "Normal salary and bonus.\n";
}
};
int main() {
Employee newbie;
Manager boss;
newbie.salary(); // Outputs: Normal salary.
boss.salary(); // Outputs: Normal salary and bonus.
return 0;
}

Class Members

A class is comprised of class members:

  • Attributes, also known as member data, consist of information about an instance of the class.
  • Methods, also known as member functions, are functions that can be used with an instance of the class.
class City {
// Attribute
int population;
public:
// Method
void add_resident() {
population++;
}
};

Constructor

For a C++ class, a constructor is a special kind of method that enables control regarding how the objects of a class should be created. Different class constructors can be specified for the same class, but each constructor signature must be unique.

#include "city.hpp"
class City {
std::string name;
int population;
public:
City(std::string new_name, int new_pop);
};

Objects

In C++, an object is an instance of a class that encapsulates data and functionality pertaining to that data.

City nyc;

Class

A C++ class is a user-defined data type that encapsulates information and behavior about an object. It serves as a blueprint for future inherited classes.

class Person {
};

Access Control Operators

C++ classes have access control operators that designate the scope of class members:

  • public
  • private

public members are accessible everywhere; private members can only be accessed from within the same instance of the class or from friends classes.

class City {
int population;
public:
void add_resident() {
population++;
}
private:
bool is_capital;
};