Question

I’d like to take the next two hex characters from a stream and store them as the associated associated hex->decimal numeric value in a char.

So if an input file contains 2a3123, I’d like to grab 2a, and store the numeric value (decimal 42) in a char.

I’ve tried

char c;
instream >> std::setw(2) >> std::hex >> c;

but this gives me garbage (if I replace c with an int, I get the maximum value for signed int).

Any help would be greatly appreciated! Thanks!

edit: I should note that the characters are guaranteed to be within the proper range for chars and that the file is valid hexadecimal.

Answer 1

OK I think dealing with ASCII decoding is a bad idea at all and does not really answer the question.

I think your code does not work because setw() or istream::width() works only when you read to std::string or char*. I guess it from here

How ever you can use the goodness of standard c++ iostream converters. I came up with idea that uses stringstream class and string as buffer. The thing is to read n chars into buffer and then use stringstream as a converter facility.

I am not sure if this is the most optimal version. Probably not.

Code:

#include <iostream>
#include <sstream>

int main(void){
    int c;
    std::string buff;
    std::stringstream ss_buff;
    std::cin.width(2);
    std::cin >> buff;
    ss_buff << buff;
    ss_buff >> std::hex >> c;
    std::cout << "read val: " << c << '\n';
}

Result:

luk32@genaker:~/projects/tmp$ ./a.out 
0a10
read val: 10
luk32@genaker:~/projects/tmp$ ./a.out 
10a2
read val: 16
luk32@genaker:~/projects/tmp$ ./a.out 
bv00   
read val: 11
luk32@genaker:~/projects/tmp$ ./a.out 
bc01
read val: 188
luk32@genaker:~/projects/tmp$ ./a.out
01bc
read val: 1

And as you can see not very error resistant. Nonetheless, works for the given conditions, can be expanded into a loop and most importantly uses the iostream converting facilities so no ASCII magic from your side. C/ASCII would probably be way faster though.

PS. Improved version. Uses simple char[2] buffer and uses non-formatted write/read to move data thorough the buffer (get/write as opposed to operator<</operator>>). The rationale is pretty simple. We do not need any fanciness to move 2 bytes of data. We ,however, use formatted extractor to make the conversion. I made it a loop version for the convenience. It was not super simple though. It took me good 40 minutes of fooling around to figure out very important lines. With out them the extraction works for 1st 2 characters.

#include <iostream>
#include <sstream>

int main(void){
    int c;
    char* buff = new char[3];
    std::stringstream ss_buff;
    std::cout << "read vals: ";
    std::string tmp;
    while( std::cin.get(buff, 3).gcount() == 2 ){
       std::cout << '(' << buff << ") ";
       ss_buff.seekp(0); //VERY important lines
       ss_buff.seekg(0); //VERY important lines
       ss_buff.write(buff, 2);
       if( ss_buff.fail() ){ std::cout << "error\n"; break;}
       std::cout << ss_buff.str() << ' ';
       ss_buff >> std::hex >> c;
       std::cout << c << '\n';
    }
    std::cout << '\n';
    delete [] buff;
}

Sample output:

luk32@genaker:~/projects/tmp$ ./a.out
read vals: 0aabffc
(0a) 0a 10
(ab) ab 171
(ff) ff 255

Please note, the c was not read as intended.

I found everything needed here http://www.cplusplus.com/reference/iostream/