Return -1

[vi87]

Hi guys.
How can I return 255 in this function when
char& is not const???

char& myString::operator[] (const int index) 
{
    if (index < this->len || index> this->len)
      return 255; 
    return this->s[index]; 
}

[tolga gerekci]

return unsigned char

[Disch]

There are a few problems with this code:

char& myString::operator[] (const int index) 
{
    if (index < this->len || index> this->len)  // this is saying that 'len' is the only possible index
      return 255;   // you cannot return a literal 255 as a non-const reference
    return this->s[index]; 
}

The reason you can't return a literal is simple. What if someone tries to do this:

mystr[i] = 'v';

where 'i' is an out of range index. You'd effectively be trying to do 255 = 'v'; which is nonsense.

If you really want to do this, you'll need to have a dummy value:

char& myString::operator[] (const int index) 
{
    static char dummy;
    if ( /*index is out of bounds*/ )
      return dummy;

However note that you cannot really reliably specify what dummy is. You can initialize it with -1, but it can be changed at any time by the user doing this:

// assuming 'i' is out of bounds
mystr[i] = 'v';  // changes dummy to 'v'
cout << mystr[i];  // will print 'v', not -1 / 255 

That's besides the point that char's range is only [-128..127], so it can't even represent 255 anyway.


Really the best way to handle an out of range error in this situation is to throw an exception.

[Mathhead200]

(Or use pointers, and return NULL on error...) but I think exceptions are the way to go too...

[vi87]

Thanks for your answers guys.

Sure thing that exceptions are the way better than monkeying around with return values of different data types.
I just got a teacher who really waned us to return 255 as a char and then cast it as a int...

    char num = 255;
    cout << int(num) << endl; 

       -1