Chapter 7: User-Defined Types

Custom data types are used very often in programming because they are more descriptive than standard data types and because their usage results in cleaner code. This is especially true for records. But before we learn how to create and use records, we have to understand how to create and use enumerated types, subrange types, and sets.

Enumerated Types

An enumerated type is an ordered list of identifiers and is a good replacement for a collection of constants. For instance, the following constants can be replaced with an enumeration:

const   PAWN = 1;   ROOK = 2;   KNIGHT = 3;   BISHOP = 4;   QUEEN = 5;   KING = 6;

The syntax for declaring an enumerated type is:

type   TEnumeratedType = (value_1, value_2, value_n);   TChessFigure = (cfPawn, cfRook, cfKnight,     cfBishop, cfQueen, cfKing);

By using a TChessFigure variable instead of an integer, the code is easier to read and it enables Delphi to perform type checking. Each identifier in the enumeration gets an ordinal value. The ordinal values in the enumeration start at zero. So, Ord(cfPawn) = 0.

All ordinal procedures and functions can work with enumerated types. You can also explicitly define the ordinal value of an enumeration element:

type   TOddNumber = (oddFirst = 1, oddSecond = 3,     oddThird = 5, oddFourth = 7);   TEvenNumber = (evFirst = 0, evSecond = 2,     evThird = 4, evFourth = 6); var   OddNum: TOddNumber; begin   OddNum := oddThird;   WriteLn(Ord(OddNum)); { 5 }   ReadLn; end.