CODEWITHSUNDEEP
c++iostream
Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385274

Why is this non-open ifstream still "good" after I try to extract from it?

In GCC 4.7.0 20111217, GCC 4.1.2, GCC 4.3.4 and GCC 4.5.1:

#include <iostream>
#include <fstream>

int main() {
   std::ifstream f;
   std::cout << f.get() << ", " << f.good() << ", " << f.bad();
}

// Output: -1, 1, 0

I'd have expected -1, 0, 1 (which Clang 3.1 gives me), because of these paragraphs:

[C++11: 27.7.2.3]:

int_type get();

4 Effects: Behaves as an unformatted input function (as described in 27.7.2.3, paragraph 1). After constructing a sentry object, extracts a character c, if one is available. Otherwise, the function calls setstate(failbit), which may throw ios_base::failure (27.5.5.4),
5 Returns: c if available, otherwise traits::eof().

and:

[C++11: 27.9.1.1/3]: In particular:

  • If the file is not open for reading the input sequence cannot be read.
  • If the file is not open for writing the output sequence cannot be written.
  • A joint file position is maintained for both the input sequence and the output sequence.

Is GCC in the wrong here? Did an implementer misinterpret the "otherwise" in [C++11: 27.7.2.3/4]? Or did I misinterpret the standard?

Upvotes: 3

Views: 249

Answers (2)

Dave S
Dave S

Reputation: 21123

The problem is that the order that the operations on f is called is not specified by the standard. The compiler is free to call f.good(), f.bad(), and f.get() in any order it chooses.

Try changing to print it on separate lines.

Upvotes: 2

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385274

I've just realised I'm being silly:

#include <iostream>
#include <fstream>

int main() {
   std::ifstream f;
   std::cout << f.get() << ", ";
   std::cout << f.good() << ", " << f.bad();
}

// Output: -1, 0, 0

Unspecified evaluation order. Duh.

Upvotes: 1

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