std::istringstream >> to double weird behaviour

Question

The following code prints 0 on mac osx with clang. Everywhere else it prints 5 (clang, gcc)

#include <iostream>
#include <sstream>

int main() {
    std::istringstream iss("5C3");

    double n;
    iss >> n;

    std::cout << n << std::endl;

    return 0;
}

---

Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
Target: x86_64-apple-darwin14.5.0
Thread model: posix

When I use int it prints 5 as expected..

How does operator>> of std::istringstream work and why this is happening? Is there a way to make it work in a consistent manner? (that is extract 5)

Answer 1

The relevant part of the standard is [istream.formatted.arithmetic]. The extractor behavior depends on the locale's num_get<> object.

There is a bug in the libc++ num_get::do_get function for double, described in this bug report. On other platforms, you are likely using libstdc++, which gives the behavior you expect.