Converting a hex string to a byte array

Question

What is the best way to convert a variable length hex string e.g. \"01A1\" to a byte array containing that data.

i.e converting this:

st         


        

Answer 1

I found this question, but the accepted answer didn't look like a C++ way of solving the task to me (this doesn't mean it's a bad answer or anything, just explaining motivation behind adding this one). I recollected this nice answer and decided to implement something similar. Here is complete code of what I ended up with (it also works for std::wstring):

#include 
#include 

#include 
#include 
#include 
#include 
#include 
#include 
#include 

template 
class hex_ostream_iterator :
    public std::iterator
{
    OutputIt out;
    int digitCount;
    int number;

public:
    hex_ostream_iterator(OutputIt out) : out(out), digitCount(0), number(0)
    {
    }

    hex_ostream_iterator &
    operator=(char c)
    {
        number = (number << 4) | char2int(c);
        digitCount++;

        if (digitCount == 2) {
            digitCount = 0;
            *out++ = number;
            number = 0;
        }
        return *this;
    }

    hex_ostream_iterator &
    operator*()
    {
        return *this;
    }

    hex_ostream_iterator &
    operator++()
    {
        return *this;
    }

    hex_ostream_iterator &
    operator++(int)
    {
        return *this;
    }

private:
    int
    char2int(char c)
    {
        static const std::string HEX_CHARS = "0123456789abcdef";

        const char lowerC = std::tolower(c);
        const std::string::size_type pos = HEX_CHARS.find_first_of(lowerC);
        if (pos == std::string::npos) {
            throw std::runtime_error(std::string("Not a hex digit: ") + c);
        }
        return pos;
    }
};

template 
hex_ostream_iterator
hex_iterator(OutputIt out)
{
    return hex_ostream_iterator(out);
}

template 
hex_ostream_iterator
from_hex_string(InputIt first, InputIt last, OutputIt out)
{
    if (std::distance(first, last) % 2 == 1) {
        *out = '0';
        ++out;
    }
    return std::copy(first, last, out);
}

int
main(int argc, char *argv[])
{
    if (argc != 2) {
        std::cout << "Usage: " << argv[0] << " hexstring" << std::endl;
        return EXIT_FAILURE;
    }

    const std::string input = argv[1];
    std::vector bytes;
    from_hex_string(input.begin(), input.end(),
                    hex_iterator(std::back_inserter(bytes)));

    typedef std::ostream_iterator osit;
    std::copy(bytes.begin(), bytes.end(), osit(std::cout));

    return EXIT_SUCCESS;
}

And the output of ./hex2bytes 61a062a063 | hexdump -C:

00000000  61 a0 62 a0 63                                    |a.b.c|
00000005

And of ./hex2bytes 6a062a063 | hexdump -C (note odd number of characters):

00000000  06 a0 62 a0 63                                    |..b.c|
00000005