Delphi in a Nutshell

Name

Set Keyword

Syntax

type Name = set of Ordinal type;

Description

A set type is defined in terms of a base type, which must be an ordinal type: enumeration, integer, or character. Delphi implements the set type using a bit mask, where each member of the base type is assigned a corresponding bit in the set type. Set types are limited to 256 elements, and the ordinal values of the base type are likewise limited to the range to 255.

The set’s bit mask is stored as an array of bytes. The ordinal value of a set member determines which bit of which bytes the member occupies: ordinal value is the least significant bit of the first byte, and 255 is the most significant bit of the 32nd byte. Storage for a set does not include any bytes that have no members, but if a byte contains at least one member, the set includes the entire byte. The representation for a set ensures that the same ordinal value always occupies the same bit position, regardless of the set’s actual type.

For example, set of 5..10 takes up 2 bytes. The first byte stores members 5..7 at bits 5 to 7 and the next byte stores members 8..10 at bits to 2.

Tips and Tricks

• Sets are often easier to use than bit masks.

• Programmers coming to Delphi from other languages are unaccustomed to Pascal’s set operators. Familiarize yourself with the operators described in Chapter 7.

• You can type cast a small set to an integer bit mask. The type cast must use an integer type that is the same size as the set. To type cast a font style, for example, ...