Inserting a line in a .txt in C++

#include <time.h>
#include <stdio.h>
#include <iostream>
#include <fstream>
using namespace std;

int main()
{  char key;
int size;
char* buffer;
char line_insert;

    ofstream fout;
    ifstream fin;

fin.open ("source.txt");
fout.open ("new_text.txt");
    while(1){

           
             cin>>key;
             if (key=='a')
             {cin>>line_insert;
  
               fout << line_insert;
               
                 
                 key=='0';
                 
                 
                             fin.seekg(0,ifstream::end); // get size of file

                             size=fin.tellg();
                             fin.seekg(0);

                             buffer = new char [size];// allocate memory for file content


                             fin.read (buffer,size);// read content of infile


                             fout.write (buffer,size);// write to outfile

                            delete[] buffer;// release dynamically-allocated memory

                       
                             fout.close();
                             fin.close();
                             
                             
                             fin.open ("new_text.txt");
                             fout.open ("source.txt");
                 fin.seekg(0,ifstream::end); // get size of file

                             size=fin.tellg();
                             fin.seekg(0);

                             buffer = new char [size];// allocate memory for file content


                             fin.read (buffer,size);// read content of infile


                             fout.write (buffer,size);// write to outfile

                            delete[] buffer;// release dynamically-allocated memory
                       if (key =='b')
                      { break;}
                       }
                       fout.close();
system("PAUSE");
  return 0;


}