interrogating and controlling state of the stream

Question

I am learning C++ Primer 4th edition now and playing with IO stream. When I tried to run the code in the book (Page 289):

#include <iostream>
#include <string>
using namespace std;
int main()
{
    int ival;
    // read cin and test only for EOF; loop is executed even if there are
    // other IO failures
    while (cin >> ival, !cin.eof()){
        if (cin.bad())  // input stream is corrupted; bail out
            throw runtime_error("IO stream corrupted");
        if (cin.fail()){
            cerr << "bad data, try again\n";
            cin.clear(istream::failbit);
            continue;
        }
        //ok to process ival
    }
}

I met an infinite loop after typing a string (say "as"). So what is wrong with this code?

You say "C++ Primer 4th edition". Please disambiguate by providing an author name (& ideally an ISBN) — Lightness Races in Orbit, Jul 12 '17 at 16:14

Lightness Races in Orbit · Answer 1 · 2017-07-12T17:30:17.817

0

When you enter a string, extraction into ival fails and the failbit is set. You then dutifully attempt to clear the bit and continue. But that string is still in the buffer! So the act repeats forever.

Furthermore, you didn't actually clear the bit; you just set the stream state to failbit.

You'll need to call just std::cin.clear(), then pull out the unexpected data from the buffer before you continue.

~~Also, while (cin >> ival, !cin.eof()) is odd; I haven't checked the behaviour or precedence but I guess you'd just write while ((cin >> ival) && !cin.eof()).~~

In conclusion, if this is really the code in the book, you need a much better book because the code is wrong in several serious ways. If you're really programming C++14, you need a more up-to-date book anyway since Lippman's C++ Primer wasn't even updated for C++11 until the 5th edition.

edited Jul 12 '17 at 17:30

answered Jul 12 '17 at 16:11

Lightness Races in Orbit

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I just tried replacing cin.clear(istream::failbit) with cin.clear() but it still didn't work. So how can I clear the buffer? I know this book is really old but it still helps. For the comma thing I think "a, b" is an expression that returns b. – Guojun Zhang Jul 12 '17 at 16:26
@GuojunZhang: Yeah you have to do _everything_ I said, not just part of it – Lightness Races in Orbit Jul 12 '17 at 16:42
@GuojunZhang: Yes, `a, b` evaluates to `b`, but `a << b, c` may not do exactly what you're expecting; depends on the relationship between `<<` and `,`. I don't have time to check right now and my favourite expression checking tool has gone offline :( (`geordi --precedence `) – Lightness Races in Orbit Jul 12 '17 at 16:43
1

I guess while ((cin >> ival) && !cin.eof()) does not help because when you input a string cin >> ival automatically fails, which is not the goal of the program. – Guojun Zhang Jul 12 '17 at 17:27
never mind, I think this piece of code is outdated so I just give up. – Guojun Zhang Jul 12 '17 at 17:28
@GuojunZhang: Yeah that's a really good point actually. I didn't spot that. – Lightness Races in Orbit Jul 12 '17 at 17:29
@GuojunZhang: hehe okay – Lightness Races in Orbit Jul 12 '17 at 17:29

score 0 · Answer 2 · answered Jul 12 '17 at 18:05

Finally made it work by adding cin.ignore(INT_MAX, '\n'). This line is necessary because cin.clear() only removes the error flag while the data remains in the buffer. In order to clear the buffer cin.ignore is needed.

#include <iostream>
#include <string>
using namespace std;
int main()
{
    int ival;
    // read cin and test only for EOF; loop is executed even if there are
    // other IO failures
    while (cin >> ival, (!cin.eof())){
        if (cin.bad())  // input stream is corrupted; bail out
            throw runtime_error("IO stream corrupted");
        if (cin.fail()){
            cerr << "bad data, try again" << endl;
            cin.clear();
            cin.ignore(INT_MAX, '\n');
        }
        //ok to process ival
    }
}

interrogating and controlling state of the stream

2 Answers2