What is fstream in c++ code example

Example 1: fstream in c++

#include<fstream>
int main()
{
  fstream file; 
  /*if we use fstream then we need to specify at least one 
  parameter mode like ios::out or ios::in else the file will not open */  
  file.open("filename.txt", ios::out|ios::in);
  /*all work with file*/
  file.close();
  return 0;
}

Example 2: pionter in c++

#include <iostream>
using namespace std;
int main(){
   //Pointer declaration
   int *p, var=101;
 
   //Assignment
   p = &var;

   cout<<"Address of var: "<<&var<<endl;
   cout<<"Address of var: "<<p<<endl;
   cout<<"Address of p: "<<&p<<endl;
   cout<<"Value of var: "<<*p;
   return 0;
}

Example 3: what is stdin in c

//stdin accesses the keyboard, it's a file pointer