When we consider a C++ program, it can be defined as a collection of objects that communicate via invoking each other's methods. Let us now briefly look into what a class, object, methods, and instant variables mean.

·      Object − Objects have states and behaviors. Example: A dog has states - color, name, breed as well as behaviors - wagging, barking, eating. An object is an instance of a class.

·      Class − A class can be defined as a template/blueprint that describes the behaviors/states that object of its type support.

·      Methods − A method is basically a behavior. A class can contain many methods. It is in methods where the logics are written, data is manipulated and all the actions are executed.

·      Instance Variables − Each object has its unique set of instance variables. An object's state is created by the values assigned to these instance variables.

C++ Program Structure

Let us look at a simple code that would print the words Hello World.

#include <iostream>

using namespace std;

 

// main() is where program execution begins.

int main() {

   cout << "Hello World"; // prints Hello World

   return 0;

}

 

Hello world using c++:-

#include <iostream>
using namespace std;

// main() is where program execution begins.
int main() {
   cout << "Hello World"; // prints Hello World
   return 0;
}

Let us look at the various parts of the above program −

• The C++ language defines several headers, which contain information that is either necessary or useful to your program. For this program, the header <iostream> is needed.

• The line using namespace std; tells the compiler to use the std namespace. Namespaces are a relatively recent addition to C++.

• The next line '// main() is where program execution begins.' is a single-line comment available in C++. Single-line comments begin with // and stop at the end of the line.

• The line int main() is the main function where program execution begins.

• The next line cout << "Hello World"; causes the message "Hello World" to be displayed on the screen.

• The next line return 0; terminates main( )function and causes it to return the value 0 to the calling process.

Compile and Execute C++ Program

Let's look at how to save the file, compile and run the program. Please follow the steps given below −

• Open a text editor and add the code as above.

• Save the file as: hello.cpp

• Open a command prompt and go to the directory where you saved the file.

• Type 'g++ hello.cpp' and press enter to compile your code. If there are no errors in your code the command prompt will take you to the next line and would generate a.out executable file.

• Now, type 'a.out' to run your program.

• You will be able to see ' Hello World ' printed on the window.

Classes and Objects example :-

#include <iostream>

using namespace std;

class Box {
   public:
      double length;         // Length of a box
      double breadth;        // Breadth of a box
      double height;         // Height of a box

      // Member functions declaration
      double getVolume(void);
      void setLength( double len );
      void setBreadth( double bre );
      void setHeight( double hei );
};

// Member functions definitions
double Box::getVolume(void) {
   return length * breadth * height;
}

void Box::setLength( double len ) {
   length = len;
}
void Box::setBreadth( double bre ) {
   breadth = bre;
}
void Box::setHeight( double hei ) {
   height = hei;
}

// Main function for the program
int main() {
   Box Box1;                // Declare Box1 of type Box
   Box Box2;                // Declare Box2 of type Box
   double volume = 0.0;     // Store the volume of a box here
 
   // box 1 specification
   Box1.setLength(6.0); 
   Box1.setBreadth(7.0); 
   Box1.setHeight(5.0);

   // box 2 specification
   Box2.setLength(12.0); 
   Box2.setBreadth(13.0); 
   Box2.setHeight(10.0);

   // volume of box 1
   volume = Box1.getVolume();
   cout << "Volume of Box1 : " << volume <<endl;

   // volume of box 2
   volume = Box2.getVolume();
   cout << "Volume of Box2 : " << volume <<endl;
   return 0;
}