How do you convert a `std::string` hex value to an `unsigned char`

Question

sample input

a8 49 7f ac 24 77 c3 6e 70 ca 99 ca fc e2 c5 7b

This fucntion converts the hex values in the sample to a string to be later converted into an unsigned char

std::vector<unsigned char> cipher_as_chars(std::string cipher) 
{
    std::vector<unsigned char> hex_char;
    int j =0 ;
    for (int i = 0; i < cipher.length();)
    {

        std::string x = "";
        x = x + cipher[i] + cipher[i+1];
        
        unsigned char hexchar[2] ;
        strcpy( (char*) hexchar, x.c_str() );
        hex_char[j] = *hexchar;
        j++;


        
        cout << "Current Index : " << i << " " << x  << " <> " << hexchar << endl;
        i = i+3;
    }


    return hex_char;
}

You can use `std::istringstream` the `std::hex` i/o manipulator and the `>>` operator to do that. — πάντα ῥεῖ, Jan 01 '22 at 16:10
First, copy three characters into a two-character array: fail. Then, retrieve the first character. Even assuming that the undefined behavior gets fixed, how do you expect to `strcpy` the string "a8" into an `unsigned char` buffer, then retrieve the first character, which would be "a", and get 168 as the result? — Sam Varshavchik, Jan 01 '22 at 16:11
@khalid someone already did that here: https://stackoverflow.com/questions/3381614/c-convert-string-to-hexadecimal-and-vice-versa — πάντα ῥεῖ, Jan 01 '22 at 16:13
And what's your question about this? Anything not working with the given code? — Nico Haase, Jan 05 '22 at 13:41

Aykhan Hagverdili · Accepted Answer · 2022-01-01T16:39:44.590

As a very simple solution, you can use a istringstream, which allows parsing hex strings:

#include <cstdio>
#include <iterator>
#include <sstream>
#include <string>
#include <vector>

std::vector<unsigned char> cipher_as_chars(std::string const& cipher) {
    std::istringstream strm{cipher};
    strm >> std::hex;

    return {std::istream_iterator<int>{strm}, {}};
}

int main() {
    auto const cipher = "a8 49 7f ac 24 77 c3 6e 70 ca 99 ca fc e2 c5 7b";
    auto const sep = cipher_as_chars(cipher);
    for (auto elm : sep) {
        std::printf("%hhx ", elm);
    }
    std::putchar('\n');
}

How do you convert a `std::string` hex value to an `unsigned char`

1 Answers1