ios and iostream, what's the difference?

[cpthk]

I have these three codes:

fstream file1;
file1.open("text", iostream:in);

fstream file2;
file2.open("text", ios:in);

fstream file3;
file3.open("text", ios_base:in);

I have seen all three. What's the difference between these three?
Thanks.

[Speedo]

It's the same object, available in any of the classes because of inheritance.

ios_base -> ios -> iostream -> fstream