Why c++ has iostream when it has istream and stream?

Question

According to my test in vs2019, ostream takes 72 bytes, istream takes 80 bytes and iostream takes 88 bytes. There is no discernible difference in their size, so I don't think 'it's aimed at saving resource' is right.

Another confusing problem is why istringstream enables construction with std::ios::out?

How else do you propose implementing a stream that supports both input and output? — Sam Varshavchik, Mar 30 '20 at 12:30
Basically to prevent simple errors like `std::cout >> x` and `std::cin << y`. Same reason you limit the interface on any class when you can, to avoid mistakes. — François Andrieux, Mar 30 '20 at 12:41

score 2 · Answer 1 · answered Mar 30 '20 at 12:36

The openmode for a stream is a bitmask type. I.e. it's a set of bits that you can use bitwise OR to add into a more complex set of flags.

The openmode flags for the string-streams are passed on to the underlying string buffer which is bidirectional.

Passing the flag std::ios::out when creating or opening an input-only stream is just silently ignored.

And the size of an object or a class is in almost all cases irrelevant, and comparing the sizes of unrelated objects (even in the same inheritance hierarchy) often have no meaning.

score 0 · Answer 2 · answered Mar 30 '20 at 12:35

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You need to be able to read and write to the standard input/output streams, and it is also a standard.

Why would you #include <istream> and #include <ostream>. It makes sense to just #include <iostream>.

answered Mar 30 '20 at 12:35

appledoes

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I _think_ the question talks about `sizeof(std::istream)`, not `` – MSalters Mar 30 '20 at 12:58
Oh, fair point. – appledoes Mar 30 '20 at 13:06

Why c++ has iostream when it has istream and stream?

2 Answers2