ios and iostream, what's the difference?

I have these three codes:

Code:

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

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Code:

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

---

Code:

---

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

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

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

ios_base -> ios -> iostream -> fstream