Converting 4 character hex string to a signed short with string streams in c++

Question

A similar question has been asked before, and I am using the solution from that question, but it isn't working 100%. Here is my code:

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

using namespace std;

int main () {
    string hexStr1 = "ffff";
    short s1;
    stringstream ss1;
    ss1 << hex << hexStr1;
    ss1 >> s1;

    string hexStr2 = "f";
    short s2;
    stringstream ss2;
    ss2 << hex << hexStr2;
    ss2 >> s2;

    s2 = s2 | -16;



    cout << "s1: " << s1 << "\n";  //Outputs 32767 (max short)
    cout << "s2: " << s2 << "\n";  //Outputs -1 
}

Shouldn't the value stored in s1 be -1 because if a (2-byte) short variable has the value 0xffff it would be negative one?

I don't see what is different about how I'm setting s2 vs. how I'm setting s1, but the results are different...

I would like the program above to print out:

s1: -1
s2: -1

Can anyone explain why the value in s1 is not -1 after my program runs?

Answer 1

The problem is that there is no signed short overload for std::num_get::get() . The stream extraction operator>>(short&) is actually reading 0xffff from the string as an long int, failing to convert the value to a short because of potential overlfow, assigning a defined value of std::numeric_limits<short>::max() and setting std::ios_base::failbit on the stream.

See ISO/IEC 14882 [istream.formatted.arithmetic]/2 for a detailed description of this well-defined and expected behaviour.

Answer 2

The problem most likely is that

ss1 >> hex >> s1;

fails. It does for me.

Update the code to add a check.

if ( !(ss1 >> hex >> s1) )
{
   cout << "Unable to read s1\n";
}

If reading into s1 fails, it does not make sense to rely on its value until it has been set to something valid.