Show Lecture.IosStateFlags as a slide show.

CS253 Ios State Flags

Overview

• By default, streams do not throw errors when things go bad.
  • Though you can enable that if you wish.
• Instead, they maintain several bits (flags) that indicate the state of the stream.
• Several methods exist which interrogate the stream’s state.

ios State Flags

• An ios (Input/Output Stream) object maintains state.
• Several sorts of things can go wrong, so three different types of errors are maintained:
  • ios::eofbit
  • ios::failbit
  • ios::badbit
  • ios::goodbit is not really a bit—just the absence of the other bits.
• Yes, “fail” and “bad” are quite similar words. Life is difficult.

Summary of flags

• ios::eofbit — End-of-file has been encountered.
• ios::failbit — Open or conversion failed.
• ios::badbit — Something underneath it all went bad.

ios::eofbit

• ios::eofbit is set when end-of-file is encountered.
• That’s when you try to read, but can’t, because there’s no more data. It doesn’t predict.
• Let’s try reading from the special file /dev/null, which is always empty.

ifstream in("/dev/null");
char c;
in.get(c);
if (in.rdstate() & ios::eofbit)  cout << "After, eofbit\n";
if (in.rdstate() & ios::failbit) cout << "After, failbit\n";
if (in.rdstate() & ios::badbit)  cout << "After, badbit\n";

After, eofbit
After, failbit

eofbit ≡ hit eof trying to read; failbit ≡ tried to get a char and couldn’t.

ios::failbit

• ios::failbit is set when an I/O operation fails—you didn’t get what you wanted.
• ios::failbit was set in the previous example, because we tried to read a character, and couldn’t.
• Let’s try to read a float from /etc/hostname, which has contains the name of this computer, “beethoven” (not a number).

ifstream in("/etc/hostname");
float f;
in >> f;
if (in.rdstate() & ios::eofbit)  cout << "After, eofbit\n";
if (in.rdstate() & ios::failbit) cout << "After, failbit\n";
if (in.rdstate() & ios::badbit)  cout << "After, badbit\n";

After, failbit

ios::failbit upon open failure

Also, ios::failbit is set when a file can’t be opened.

ifstream in("/this/file/doesn’t/exist");
if (in.rdstate() & ios::eofbit)  cout << "eofbit\n";
if (in.rdstate() & ios::failbit) cout << "failbit\n";
if (in.rdstate() & ios::badbit)  cout << "badbit\n";

failbit

ios::badbit

• ios::badbit is not set upon a mere conversion error, but, rather, when a low-level I/O error occurs: the system runs out of memory, the disk becomes full, etc.
• Let’s try writing to the special file /dev/full, which simulates a disk that is always full.

ofstream out("/dev/full");
out << "foo" << endl;
if (out.rdstate() & ios::eofbit)  cout << "After, eofbit\n";
if (out.rdstate() & ios::failbit) cout << "After, failbit\n";
if (out.rdstate() & ios::badbit)  cout << "After, badbit\n";

After, badbit

The bits are sticky

ifstream in("/etc/hostname");
double d;
in >> d;
if (in.rdstate() & ios::failbit) cout << "#1: failbit\n";
char c;
in >> c;
if (in.rdstate() & ios::failbit) cout << "#2: failbit\n";

#1: failbit
#2: failbit

• Why doesn’t the second read succeed? Because the bits are sticky.
• Once ios::failbit is set, it stays set.
• The second extraction gives up without trying, because ios::failbit is set.

Clearing bits

ifstream in("/etc/hostname");
double d;
in >> d;
if (in.rdstate() & ios::failbit) cout << "#1: failbit\n";
char c;
in >> c;
if (in.rdstate() & ios::failbit) cout << "#2: failbit\n";
in.clear();
in >> c;
if (in.rdstate() & ios::failbit) cout << "#3: failbit\n";
cout << "c=‘" << c << "’\n";

#1: failbit
#2: failbit
c=‘b’

ios::clear() clears ios::badbit, ios::eofbit, and ios::failbit.

Setting bits

• Despite its name, if you give ios::clear() an argument, it sets the state of a stream to exactly those bits:

// I want to set failbit=1, eofbit=0, badbit=0:
in.clear(ios::failbit);

• Another method, ios::setstate(), sets sets only a particular bit.

// I want to set failbit=1, and leave the other bits unchanged.
in.setstate(ios::failbit);

Interrogating bits

The previous slides have been interrogating the bits in a straightforward, but clumsy fashion. There are several other ways:

Use of fail()

$ cat /proc/uptime
5380626.68 63583945.07

The fail() method is sometimes misused:

ifstream in("/proc/uptime");
for (float f; in >> f; )
    cout << f << '\n';

if (in.fail())
    cerr << "failure!\n";

5.38063e+06
6.35839e+07
failure!

Advice

I generally only use boolean context, and occasionally eof() when I want to be rigorous. I also avoid .is_open():

ifstream in("/etc/hostname");
if (!in) {
    cerr << "Can’t open /etc/hostname for reading\n";
    return 1;
}

for (float f; in >> f; )
    cout << "Hooray, got a number: " << f << '\n';

// All done reading.  Why did we stop?
// Was it end-of-file, or did something go wrong?
// Don’t check .fail(), that will always be true at this point.
if (!in.eof())
    cerr << "Error!\n";

Error!