From QB64.org wiki
_DEFINE defines a set of variable names according to their first character as a specified data type.
- Variable start letter range is in the form firstletter-endingletter (like A-C) or just a single letter.
- Data types: INTEGER, SINGLE, DOUBLE, LONG, STRING, _BIT, _BYTE, _INTEGER64, _FLOAT, _OFFSET, _MEM
- Can also use the _UNSIGNED definition for positive whole INTEGER type numerical values.
- When a variable has not been defined or has no type suffix, the value defaults to a SINGLE precision floating point value.
- _DEFINE sets the type of all variable names with the starting letter(s) or letter ranges when encountered in the progression of the program (even in conditional statement blocks not executed and subsequent SUB procedures).
- NOTE: Many Qbasic keyword variable names CAN be used with a STRING suffix ($)! You cannot use them without the suffix, use a numerical suffix or use DIM, REDIM, _DEFINE, BYVAL or TYPE variable AS statements.
- Qbasic's IDE added DEF statements before any SUB or FUNCTION. QB64 (like QB) will change all variable types in subsequent sub-procedures to that default variable type without giving a "Parameter Type Mismatch" warning or adding the proper DEF statement to subsequent procedures. If you do not want that to occur, either remove that DEF statement or add the proper DEF type statements to subsequent procedures.
- May also affect $INCLUDE procedures.
Example: Defining variables that start with the letters A, B, C or F as unsigned integers, including the Add2 FUNCTION.
- Explanation: Unsigned integers can only return positive values while ordinary integers can also return negative values.