Null vs 0

My application uses dialog boxes that capture and display numbers. However, these numbers are not required. So, my question is - Can I initialize my ints, doubles, and floats to some kind of a NULL value that IS NOT a 0 (number zero)?

Strictly, no, you can't. A weak approach is to pick some value and use it as a "NULL". For example, if a the acceptable range of input is non-negative numbers, you could use -1. For floats, a really large negative number might be appropriate.

If you need a stronger method and need to pass these things around, you could create a structure which includes a initialized flag along with an element to hold the value (unions might make this easier). This is the approach used in the Variant type used in VB and OLE.

If you are dealing with edit boxes, you can map them to strings. An empty string is NULL, and anything else you could convert to a number.