why does std::cin fail to throw an exception when I explicitly tell it to do so?

Question

I may be misunderstanding how std::cin.exceptions is supposed to work. Here is a simple example:

#include <iostream>

int main(){
    std::cin.exceptions(std::ios_base::failbit);
    int i=0;
    try{ std::cin >> i; }
    catch(std::ios_base::failure &b){ std::cout << "exception caught\n"; }
}

compiling with g++ --std=c++11, then running with an invalid input, such as a non-numeric character like 'f', does not throw any exception, e.g. I don't see "exception caught". This is confusing to me because I'm assuming that when attempting to read an int, cin will set the stream state to failbit, and the first line in my code says to throw an exception whenever this happens. In that case, why do I not see an exception?

here is the output when I type g++ -v

Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 9.0.0 (clang-900.0.39.2)
Target: x86_64-apple-darwin17.3.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

update: it seems to be a problem specific to the compiler described above, (or at least my particular copy of it). As noted by one of the comments, replacing by

std::cin.exceptions(std::ios_base::badbit);

doesn't fix the issue, but strangely,

std::cin.exceptions(std::ios_base::failbit | std::ios_base::badbit);

does.

Also works fine here: http://coliru.stacked-crooked.com/a/17ab6735f5826ca5 — Support Ukraine, Jan 30 '18 at 20:28
that's odd; I updated my question with my compiler info. Any particular reason why it wouldn't work on that compiler? — xdavidliu, Jan 30 '18 at 20:29
In my case, odd enough. Both `failbit` and `badbit` don't work, however, `failbit | badbit` works.... Mac OSX, clang++ 5.0 — llllllllll, Jan 30 '18 at 20:32
@liliscent i am getting the same. 4386427 that would catch all exceptions. the issue here isn't that I'm not catching an exception, it's that the exception isn't being thrown at all — xdavidliu, Jan 30 '18 at 20:34
It seems to be clang's problem, on my Ubuntu g++, `failbit` works. — llllllllll, Jan 30 '18 at 20:34
The iostream exceptions are an in practice **unused feature**. One reason that it's unused is that encountering EOF sets failbit. Splat. — Cheers and hth. - Alf, Jan 30 '18 at 20:37
@4386427 This is clang 4.0, old version. My clang++ version is the latest released. — llllllllll, Jan 30 '18 at 20:41

why does std::cin fail to throw an exception when I explicitly tell it to do so?

0 Answers0