Question

I’m writing porting file-io set of functions from c into a c++ class. “Magic numbers” (unnamed constants) abound.

The functions read a file header which has a number of specific entries whose locations are currently denoted by magic numbers.

I was taught by a veteran programmer a couple years back that using “magic numbers” is inherently evil, and thus, I have since tried to avoid using unnamed constants in my port. So I want to create some sort of list of constants of where the entries are stored.

So far I’ve come up with two solutions that seem relatively safe — use a namespace enclosed set of constants or a namespace enclosed enum.

Can I use either solution safely? Would there be any advantages to one over the other?

e.g.
OPTION 1

namespace hdr_pos {
   const unsigned int item_1_pos=4;
   const unsigned int item_2_pos=8;
   const unsigned int item_3_pos=12;
   const unsigned int item_4_pos=24;
   const unsigned int item_5_pos=32;
};

OPTION 2

namespace hdr_pos {
   enum e {
      item_1_pos=4,
      item_2_pos=8,
      item_3_pos=12,
      item_4_pos=24,
      item_5_pos=32
   };
};

Is there anyway to prevent duplicates, to catch if I change the positions due to a future update to the file header, but forget to change one of them?

Please keep this factual and non-subjective. If there is no advantage you know of, feel free to answer that.

Note: I would use more descriptive names, of course, in my actual implementation; I just called things item_<#>_… for examples sake…

Answer 1

I can see two advantages to using an enum. First, some debuggers can translate constants back into enum variable names (which can make debugging easier in some cases). Also, you can declare a variable of an enumerated type which can only hold a value from that enumeration. This can give you an additional form of type checking that you wouldn’t have simply by using constants.

Checking to see if a value is duplicated might depend on your particular compiler. The easiest way to do so would probably be to write an external script that will parse your enum definition and report whether or not a value is duplicated (you can run this as part of your build process if you like).