void check_file()
{
char cid[255], line[200];


sprintf(cid,"test.txt");
ifstream fin1(cid);
ofstream fout(cid,ios::app);
if(fin1.fail()) {
  cout<<"file does not exist"<<endl;
  fout<<"this is top line"<<endl;
} else {
  cout<<"file exist, 1st letter is "<<cid<<endl;
}
fout<<"2nd lines"<<endl;


return 0;
}

=============================================
最初は
> root -l -q -b check_file.C

Applying ATLAS style settings...

root [0]
Processing check_file.C...
file does not exist　　<-------- ないので作る
次回は
> root -l -q -b check_file.C

Applying ATLAS style settings...

root [0]
Processing check_file.C...
file exist, 1st letter is test.txt  <--------ある