Converting an ifstream to an istream

Boy, am I beginning to hate C++, and here's one of the reasons why:

Me (to compiler):

Code:

---

#include <fstream>

int important_function(std::istream ){
        return 0;
}

int main() {
        std::ifstream f("dummy_file");
        important_function(f);
        return 0;
}

---

Compiler (to me): 'important_function' : cannot convert parameter 1 from 'std::ifstream' to 'std::istream'

Me (to compiler): How about this?

Code:

---

#include <fstream>

int important_function(std::istream ){
        return 0;
}

int main() {
        std::ifstream f("dummy_file");
        important_function(*(reinterpret_cast<istream *>(f)));
        return 0;
}

---

Compiler (to me): 'important_function' : cannot convert parameter 1 from 'std::istream' to 'std::istream'

I ended up using a reference, which I don't need. I only need the value. Why can't it just copy the damn structure? Is there any way to make it to?

Thank you for your time :-)

As far as I can tell from the C++ standard, the copy constructor and assignment operator for std::istream is disabled (i.e. private).

Thanks. So there's no way to copy one (save for modifying the header)?
I'll use memcpy(), just to spite it.
As I said, I hate C++.

Quote:

---

So there's no way to copy one

---

Why do you need to copy? The use of references here is more efficient anyway.

Quote:

---

I'll use memcpy(), just to spite it.

---

Dont, a std::istream is not a POD.

Quote:

---

As I said, I hate C++.

---

Then dont use it.

Quote:

---

Originally Posted by manannan

Thanks. So there's no way to copy one (save for modifying the header)?
I'll use memcpy(), just to spite it.
As I said, I hate C++.

---

Why work against it? A const reference will give you the same thing minus the overhead of copying. Further, using memcpy just sounds like a bad idea.

Quote:

---

Originally Posted by laserlight

Why do you need to copy? The use of references here is more efficient anyway.

---

I don't know... it's just that if my program uses some data that I can't tamper with, I feel uncomfortable :-)

Quote:

---

Originally Posted by laserlight

Then dont use it.

---

Perhaps I was too rash in saying this. C++ implements C as a subset, and thus is no worse than C. Perhaps someday I'll grok all these fancy new features, too.

Using a const reference is a much better idea.

a) You are using polymorphism -- the reason why ifstream derives from istream in the first place.
b) It is type safe.
c) You are in no danger of modifying something you don't want to.
d) It is more efficient.
e) Reinterpret cast should only be used if you know what you are doing, and if there is no other way (read, "very rarely").

No Todo, we're not on the C Board anymore.

Cheers

Quote:

---

So there's no way to copy one (save for modifying the header)?

---

There is, but you don't want to do it. A much easier solution would be to just pass references when working with stream objects. That's how they're designed, and that's how you should use them.

Quote:

---

I'll use memcpy(), just to spite it.

---

Good luck. Doing a byte by byte copy of a non-POD type is dangerous to the max.

Quote:

---

it's just that if my program uses some data that I can't tamper with, I feel uncomfortable :-)

---

C does this in the standard libraries. Every used strtok? You're not allowed to fiddle with the contents of a FILE *, so why should you think it's okay with iostreams?

Quote:

---

A const reference will give you the same thing minus the overhead of copying.

---

Quote:

---

Using a const reference is a much better idea.

---

hmm... I had the impression that using a std::istream (or std::ostream, for that matter) means changing it. Under what circumstances would we pass a std::istream as a const reference?

That's true; as far as I can tell, there aren't any const functions for istreams.

As a general rule, if you're passing an istream (or an ifstream) as an argument, you want to read data from it. Hence one typically passes the stream by reference, so all attempts to read come from the same actual stream (eg the same file, opened with the same file handle, etc).