fstream file("b.txt", ios::in|ios::out|ios::app);

mode

• ate - seek to the end of stream immediately after open，不会导致create属性
• app - seek to the end of stream before each write，Every output is appended at the end of file, 可造成out属性
• in 表示只读属性
• out 表可写属性+create属性
• in+out 表只读+可写属性（没有create属性）
• trunc

class default mode to parameter
ofstream ios::out | ios::trunc
ifstream ios::in
fstream ios::in | ios::out

//fstream file("b1.txt", fstream::in|fstream::out); do NOT create if no exist
fstream file("b1.txt", fstream::out);   // create
file<<"s";
cout<<"s";
file.close();
}