Re: How to read/write continuous hex dump using STL

From:

"Mike Wahler" <mkwahler@mkwahler.net>

Newsgroups:

comp.lang.c++

Date:

Sat, 30 Jun 2007 17:32:46 GMT

Message-ID:

<2twhi.2291$rR.1902@newsread2.news.pas.earthlink.net>

<capnwhit@yahoo.com> wrote in message
news:1183222924.315645.130770@k29g2000hsd.googlegroups.com...

Hello all,

The program below has a bug... The program is supposed to convert a
string to a continuous hex dump and back to a string. The output
should be as follows:

abcd
61626364
abcd

But the output I see is as follows:

abcd
61626364
dddd

Can someone please show me what I'm doing wrong and what is the proper
way of reading a continuous hex dump using the STL?

// BEGIN SAMPLE PROGRAM
#include <iostream> // For std::cout
#include <string> // For std::string
#include <sstream> // For std::ostringstream
#include <iomanip> // For std::setfill

#include <cassert> // for assert

int
main()
{
// Set "input"
//
std::string input = "abcd";
std::cout << input << std::endl;

// Set "hexDump"
//
std::ostringstream oss;
oss << std::hex << std::setfill('0');
for(int i = 0; i < input.length(); i++)
{
   int temp = input[i];
   oss << std::setw(2) << temp;
}
std::string hexDump = oss.str();
assert(hexDump.length() == 2 * input.length());
std::cout << hexDump << std::endl;

// Set "output"
//
std::string output;
int numChars = hexDump.length() / 2;
output.resize(numChars);
std::istringstream iss(hexDump);
iss >> std::hex;
for(int i = 0; i < numChars; i++)
{
   int temp;
   iss >> std::setw(2) >> temp; // BUG: Using setw DOES NOT WORK!!!

// Not true. 'setw' sets *output* width
*/

Had you checked the stream state of 'iss' (which should always be
done with any stream), you'd have seen the conversion failed.
The '>>' operator stops reading upon encountering whitespace.
So it tried to read hex value '6162636465', which is
418262508645 in decimal, most likely outside the guaranteed range
of type 'int' for your platform (32 bits?).

You'll need to either insert whitespace between your hex values,
or only read two characters at a time (you could use 'iss.get()'
or 'iss.read()'.

output[i] = temp;
}
std::cout << output << std::endl;
}
// END SAMPLE PROGRAM

-Mike