What Effect Would LWG2349 Have?

Question

While libstdc++ does not, libc++ does follow the standard which states that passing ios_base::failbit to basic_istream::exceptions has no effect on formatted input. For example this code:

istringstream is{"ASD"};    
double foo;

is.exceptions(istream::failbit);

try {
    is >> foo;
    cout << foo << endl;
} catch(ios_base::failure& fail) {
    cout << "ouch\n";
}

Would result in:

My reading of LWG2349 is that it would cause basic_istream to not throw on any formatted input.

For example LWG2349 proposes a change to the standard's 27.7.2.3 [istream]/1 which was cited with reference to the invalidation of a bug that would have made libc++ behave like libstdc++. The change is in bold and strike through below:

If an exception , other than the ones thrown from clear(), if any, is thrown during input then ios::badbit is turned on in *this’s error state. ~~(Exceptions thrown from basic_ios<>::clear() are not caught or rethrown.)~~ If (exceptions()&badbit) != 0 then the exception is rethrown.

I understand that basic_istream::clear is what throws in reaction to bad formatted input so am I misreading LWG2349 or is it in fact going to stop basic_istream from throwing any errors?

ecatmur · Accepted Answer · 2016-01-29 16:54:39Z

9

The point of the language excluding exceptions "thrown from clear()" is to ensure that if clear() throws, because an input function has called clear(failbit) and (exceptions() & failbit) != 0, then badbit is not set as a result. clear() will continue to throw in that case, it just will not set badbit.

As described in the commentary to LWG2349, the intention is that badbit is set when an exception is thrown from user code:

PJ and Matt both agree that the intention (of badbit + rethrow) is "to signify an exception arising in user code, not the iostreams package".

Now, when can an exception be thrown by "user code" but within the iostreams machinery? One example would be by the locale getters:

struct my_get : std::num_get<char> {
    using iter_type = std::istreambuf_iterator<char>;
    iter_type do_get(iter_type, iter_type, std::ios_base&, std::ios_base::iostate&, bool&) const override {
        throw std::logic_error{"my_get::do_get"};
    }
};
int main() {
    std::istringstream iss;
    iss.imbue({std::locale{}, new my_get});
    iss.exceptions(std::ios_base::failbit | std::ios_base::badbit);
    try {
        bool b;
        iss >> b;
    } catch (std::exception& ex) {
        std::cout << ex.what() << '\n';
    }
    std::cout
        << ((iss.rdstate() & std::ios_base::eofbit) ? "eof " : "")
        << ((iss.rdstate() & std::ios_base::failbit) ? "fail " : "")
        << ((iss.rdstate() & std::ios_base::badbit) ? "bad " : "")
        << '\n';
}

At present, gcc outputs:

eof fail

clang outputs:

eof fail

After LWG2349, the correct behavior is to set badbit and rethrow the exception:

my_get::do_get
eof bad

answered Jan 29, 2016 at 16:54

ecatmur

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15 Comments

Jonathan Mee Over a year ago

Your 2nd sentence says: "clear() will continue to throw in that case, it just will not set badbit." And that throw would still not be rethrown, if the exceptions() & ios_base::badbit == 0, right? So this won't do anything to align the question's example behaviors of libc++ and libstdc++?

ecatmur Over a year ago

@JonathanMee the intention is that the throw from clear() is not caught (or, if it is caught, it is invisibly rethrown without setting badbit); the catch and rethrow only applies to user-thrown exceptions. So in the question's example, the correct output would be "ouch".

ecatmur Over a year ago

@JonathanMee it helps to be aware of e.g. 27.7.2.1 [istream]/4: "If one of these called functions throws an exception, then unless explicitly noted otherwise, the input function sets badbit in error state. If badbit is on in exceptions(), the input function rethrows the exception without completing its actions, otherwise it does not throw anything and proceeds as if the called function had returned a failure indication." The purpose of LWG2349 is to ensure that user-thrown exceptions from other user code (not just sbumpc and sgetc) behave similarly.

T.C. Over a year ago

@JonathanMee That particular change was apparently intended to be stylistic - the discussion section says that the previous version with the parenthetical "lost some context (the word "rethrown" is not seen before this sentence within this section)". Again, at this point, one should focus on the intent, not the exact proposed wording.

ecatmur Over a year ago

@JonathanMee yes, definitely. I think a small part of the problem is that throwing the clear exception "all the way" is not actually possible - a conformant implementation would have to wrap user operations in a try block, so would have to specifically catch ios_base::failure and rethrow it with throw;. But that's an implementation detail - it should indeed appear to the user as if the ios_base::failure thrown by clear is thrown all the way.

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