Another istream discrepancy between libstdc++ and libc++

Question

This simple code:

#include <iostream>
#include <sstream>
int main()
{
   float x = 0.0;
   std::stringstream ss("NA");
   ss >> x;

    std::cout << ( ss.eof() ? "is" : "is not") << " at eof; x is " << x << std::endl;
}

returns different results, depending on which library I choose (I'm running clang from xcode 5 on OSX 10.9):

clang++ -std=c++11 -stdlib=libstdc++ -> not at eof
clang++ -stdlib=libstdc++ -> not at eof
/usr/bin/g++-4.2 -> not at eof

clang++ -std=c++11 -stdlib=libc++ -> at eof
clang++ -stdlib=libc+ -> at eof

It seems to me that if I try to read an alphabetic character into a float, the operation should fail, but it shouldn't eat the invalid characters - so I should get fail() but not eof(), so this looks like a bug in libc++.

Is there a c++ standard somewhere that describes what the behavior should be?

p.s. I've expanded the original test program like so:

#include <iostream>
#include <sstream>
int main()
{
   float x = 0.0;
   std::stringstream ss("NA");
   ss >> x;
    std::cout << "stream " << ( ss.eof() ? "is" : "is not") << " at eof and " << (ss.fail() ? "is" : "is not") << " in fail; x is " << x << std::endl;
    if (ss.fail())
    {
        std::cout << "Clearing fail flag" << std::endl;
        ss.clear();
    }
   char c = 'X';
    ss >> c;
    std::cout << "Read character: \'" << c << "\'" << std::endl;
}

and here's what I see:

With libc++:

stream is at eof and is in fail; x is 0
Clearing fail flag
Read character: 'X'

with stdlibc++:

stream is not at eof and is in fail; x is 0
Clearing fail flag
Read character: 'N'

p.p.s. As described in the issue that n.m. links to, the problem doesn't occur if the stringstream is set to "MA" instead of "NA". Apparently libc++ starts parsing the string, thinking it's going to get "NAN" and then when it doesn't, it gets all upset.

Answer 1

In this case, you should get fail, but not eof. Basically, if, after failure, you can clear the error and successfully do a get() of a character, you should not get eof. (If you cannot successfully read a character after clear(), whether you get eof or not may depend on the type you were trying to read, and perhaps in some cases, even on the implementation.)