g ++和clang ++与流输入和无符号整数的不同行为

Question

I came across a difference in behavior, between gcc (4.9.2) and clang (3.5.0), which surprised me.

When I try to feed an unsigned int from an std::istringstream initialized with a negative value ("-15", in the example) I get

• an error (with fail() bit raised) with clang++
• the initialization with signed(-15) with gcc++

I prepared the trivial following example program.

#include <sstream>
#include <iostream>

int main ()
 {
   std::istringstream iss("-15");

   unsigned int  ui;

   iss >> ui;

   std::cout << "ui[" << ui << "] signed(ui)[" << signed(ui)
      << "] flags[" << iss.fail() << iss.good() << iss.bad()
      << iss.eof() << "]\n";

   return 0;
 }

With clang++, I obtain the following output

ui[0] signed(ui)[0] flags[1001]

With g++, I obtain the following output

ui[4294967281] signed(ui)[-15] flags[0001]

I have two questions.

The first is obvious: who's right? clang++, g++ or is an undefined behaviour?

The second is: how can I force the gcc++ to behave like the clang++, giving an error when extracting an unsigned value from a string beginning with a minus?

Thanks and sorry for my bad english.

EDIT 2016.04.03

I realized that this isn't a difference between g++ and clang++, but a difference between libstd++ and libc++.

Compiling and linking with clang++ and libstd++, I obtain the same output I get with g++.

Sorry.

Answer 1

This has been discussed before here: Negative numeric string (eg "-10") to unsigned short

The answer is that according to the C++ standard 22.4.2.1.2p3, the conversion is required to fail and the value stores should be:

the most negative representable value or zero for an unsigned integer type , if the field represents a value too large negative to be represented in val. ios_base::failbit is assigned to err.

Therefore, clang++ is the correct behavior.