Related fstream lib

Question

#include<iostream>
#include<fstream>
Int main()
{
stu s; //stu is a class
std::ftream f ("t.txt",std::ios::in|std::ios::out|std::ios::nocreate);
f.read((char *),sizeof(s));
s.show;
return 0;
}
Using above code when t.txt is not existed or created,
It doesn't work;
But when the file is created , then this code works with no error;
I want some reason how fstream works ;please help me.thanks