Question

I have 2 questions:

1. What happens to s if I again do a “getline(in,line)” after the while loop ends?

   ifstream in("string.txt");
   string s, line;
   s = "";
   while(getline(in,line))
   {
   
       s = s + line + "\n";
   
   }
   
   cout<<s<<endl<<"******************************************"<<endl;
   

2. The getline() function: everytime it is called, does it goes to the “next” line of the ifstream “in” object in the above code? If so what happens when the while loop ends and I call the same function again? (almost the same as first question, just a subtle difference)

Answer 1

What happens to s if I again do a “getline(in,line)” after the while loop ends?

The loop ends when std::getline() returns something that resembles a boolean false. When you look at the signature of the function you will see that it returns the stream. Streams have an implicit conversion to something boolean-like that resembles their state. If if(stream) evaluates the stream to false, then that means that the stream is in a bad state. This could be the EOF flag set or one of the error flags, or both.
Any attempt to use a stream that is in a bad state will fail. Nothing will be read.

The getline() function: everytime it is called, does it goes to the “next” line of the ifstream “in” object in the above code? If so what happens when the while loop ends and I call the same function again? (almost the same as first question, just a subtle difference)

This has nothing to do with std::getline(). The position in the opened file is a property of the (file) stream. Every time you (re-)open the file for reading (without passing extra parameter for setting the position), the position is set to the beginning of the stream (file).