Inconsistent behavior when parsing numbers from stringstream on different platforms

Question

In a project i'm using a stringstream to read numeric values using the operator>>. I'm now getting reports indicating that the parsing bahaviour is inconsistent across different platforms if additional characters are appended to the number (for instance "2i"). Compiling the sample below with GCC/VCC/LLVM on Linux results in:

val=2; fail=0

Compiling and running it on iOS with either GCC or LLVM reportedly yields:

val=0; fail=1

What does the standard say about the behavior of operator>> in such a case?

--- Sample Code ---------------------------------------------

#include <sstream>
#include <iostream>

int main(int argc, const char **args)
{
  double val;
  std::stringstream ss("2i"); 
  ss >> val;
  std::cout << "val=" << val << "; fail=" << ss.fail() << std::endl;
  return 0;
}

Answer 1

According to this reference :

Thus, in either case:

• if your compiler is pre C++11 and reading fails it would leave the value of val intact and flag failbit with 0.

• if you compiler is post C++11 and reading fails it would set the value of val equal to 0 and flag failbit with 0.

• However, operator>> extracts and parses characters sequentially with function num_get::get [ 27.7.2.2.2 Arithmetic extractors ], from the stream as long as it can interpret them as the representation of a value of the proper type.

• Thus, in your case operator>> will call num_get::get for the first character (ie, 2 ) and the reading will succeed, then it will move on to read the next character (ie, i ). i doesn't fit a numerical value and consequently num_get::get will fail and reading will stop. However, there are already valid characters been read. These valid characters will be processed and assigned to val , the rest of the characters will remain in the stringstream . To illustrate this I'll give an example:

---

#include <sstream>
#include <iostream>
#include <string>

int main(int argc, const char **args)
{
  double val(0.0);
  std::stringstream ss("2i");

  ss >> val;

  std::cout << "val=" << val << "; fail=" << ss.fail() << std::endl;

  std::string str;
  ss >> str;
  std::cout << str << std::endl;

  return 0;
}

Output:

val=2; fail=0

i

• You see that if I use extract operator again to a std::string , the character i is extracted.

• The above however, doesn't explain why you don't get the same behaviour in ios.

Answer 2

This is a known bug with libc++ that was submitted to Bugzilla . The problem as I see it is with std::num_get::do_get() 's double overload somehow continuing to parse the characters a, b, c, d, e, f, i, x, p, n and their captial equivalents despite those being invalid characters for an integral type (other than e where it denotes scientific notation but must be followed by a numeric value otherwise failure). Normally do_get() would stop when it finds an invalid character and not set failbit as long as characters were sucessfully extracted (as explained above).