Question

I’m a Java developer who tries to code C++ in the limited Arduino environment. Exceptions and dynamic objects are not available or have to be avoid because of memory restrictions.

My task, create a method which parses a character buffer (hex nibble) and returns a boolean value to indicate the success as well as the actual result. My attempt so far (unrelated stuff avoided):

class Parser {
   unsigned char buffer[SIZE];
   unsigned char index;

   void parse();
   bool parseHexNibble(unsigned char &result);
};

void Parser::parse() {
  unsigned char result = 0;
  if (!parseHexNibble(result)) {
    return;
  }

  // do some work with result
}

bool Parser::parseHexNibble(unsigned char &result) {
  unsigned char chr = buffer[index];
  if (chr >= '0' && chr <= '9') {
    result = chr - '0';
  }
  else if (chr >= 'A' && chr <= 'F') {
    result = chr - 'A' + 10;
  }
  else {
    return false;
  }

  index++;
  return true;
}

Will this work? I’m unsure when to use * and when to use &. Is there a better solution?

Answer 1

There are a few ways:

The way you did it is one of them, although I recommend using a pointer rather than a reference (* instead of &) because with references, the function call looks like you are passing a value (p.parseHexNibble(c)) while the pointer version is more clear that you may want something stored in it (p.parseHexNibble(&c)).

Another option, (this one is my personal favorite) is the same thing that getc does: Return an int that either contains a char value (0 <= i < 256) or is -1 to signal an error. In the case of no error, the int can safely be typecast and/or stored to a char. (This assumes that it is a char in fact that you want to return.)

The third way is to return a structure containing a boolean and a character (either by making your own or by using std::pair or the like).

On an unrelated note, you should be using char rather than unsigned char to store actual character values. unsigned char should only be used for arbitrary raw data.