Convert string to bytes in c++

Question

I am new to c++ and still trying to feel my way in. I have attempted to adapt a function I have found on SO to convert my string into bytes as I need:

void hexconvert(const char *text, unsigned char bytes[])
{
    int i;
    int temp;

    for (i = 0; i < 4; ++i) {
        sscanf(text + 2 * i, "%2x", &temp);
        bytes[i] = temp;
    }

    cout << bytes;
}

hexconvert("SKY 000.001\n", );

Issues I have are:

1) I am not sure how to amend the for loop to handle my string. 2) I am not sure what I should use as the input for the second parameter in the function.

Can anyone assist?

Thanks

Why dont you just do std::cout << std::hex << whatever << std::endl; ? — Jesper Juhl
– Jesper Juhl, Commented Jun 23, 2018 at 14:12
hi, thanks for replying. if i need access to my variable for something other than a console print, how would this work? — gdogg371
– gdogg371, Commented Jun 23, 2018 at 14:23

Steztric · Accepted Answer · 2018-09-24 07:43:51Z

This is my suggested solution. I used it to encode a GUID as a byte array. It should achieve higher performance than having to do printf on all the characters.

typedef unsigned char byte;

std::map<char, byte> char2hex = 
{
  {'0', 0x0},
  {'1', 0x1},
  {'2', 0x2},
  {'3', 0x3},
  {'4', 0x4},
  {'5', 0x5},
  {'6', 0x6},
  {'7', 0x7},
  {'8', 0x8},
  {'9', 0x9},
  {'a', 0xa},
  {'b', 0xb},
  {'c', 0xc},
  {'d', 0xd},
  {'e', 0xe},
  {'f', 0xf}
};

void convertToBytes(const string &chars, byte bytes[])
{
  for (size_t i = 0; i < chars.length() / 2; i++) {
    byte b1 = (byte)(char2hex[chars[2*i]] << 4);
    byte b2 = char2hex[chars[2*i+1]];
    byte f = b1 | b2;
    *(bytes + i) = f;
  }
}

Remember that two ascii characters make up one byte, so for every pair of characters, I have to convert the first character to byte, then shift it up by 4 bits, then or it with the next character to get one byte.

Thomas Matthews · Accepted Answer · 2018-06-23 17:29:06Z

To print a string as bytes:

const size_t length = data.length();
for (size_t i = 0; i < length; ++i)
{
  unsigned int value = data[i];
  std::cout << std::dec << std::fill(' ') << value
            << " (0x" << std::setw(2) << std::setfill('0') << std::hex << value << ')'
            << "\n";
}

Some important rules to remember:
1. Copy the character into an integer type variable, so that cout doesn't print as character.
2. Bytes are unsigned.
3. When filling the width with 0 for hex, remember to reset it to space before printing decimal.
4. Use std::hex for printing in hexadecimal, and remember to reset it with std::dec afterwards (if you are printing in decimal afterwards).

See <iomanip>.

Edit 1: C-Style
To use the C language style:

static const char data[] = "Hello World!";
const size_t length = strlen(data);
for (size_t i = 0; i < length; ++i)
{
  printf("%3d (0x%02X)\n", data[i], data[i]);
}

The above assumes that data is a character array, nul terminated.

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