Converting a hex string to a byte array

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:

std::string = "01A1";

into this

char* hexArray;
int hexLength;

or this

std::vector<char> hexArray;

so that when I write this to a file and hexdump -C it I get the binary data containing 01A1.

This ought to work:

int char2int(char input)
{
  if(input >= '0' && input <= '9')
    return input - '0';
  if(input >= 'A' && input <= 'F')
    return input - 'A' + 10;
  if(input >= 'a' && input <= 'f')
    return input - 'a' + 10;
  throw std::invalid_argument("Invalid input string");
}

// This function assumes src to be a zero terminated sanitized string with
// an even number of [0-9a-f] characters, and target to be sufficiently large
void hex2bin(const char* src, char* target)
{
  while(*src && src[1])
  {
    *(target++) = char2int(*src)*16 + char2int(src[1]);
    src += 2;
  }
}

Depending on your specific platform there's probably also a standard implementation though.