Variant Records: The equivalent to the C-union structure

Variant Records: The equivalent to the C-union structure

Question:
Is there a way to create a C ‘union’-like structure in Delphi? That is, a structure that uses the same memory area?

Answer:
The Delphi (Pascal/ObjectPascal) equivalent to a C-union structure iscalled a Variant Record (not to be confused with the Variant “type”available in Delphi 2.0+). As with a C-union, the Pascal variant recordallows several structure types to be combined into one, and allwill occupy the same memory space. Look up the syntax declaration under “Records”in the help file. But here’s an example:

type TPerson = record   FirstName, LastName: string[40];   BirthDate: TDate;   case Citizen: Boolean of     True: (BirthPlace: string[40]);     False: (Country: string[20];       EntryPort: string[20];       EntryDate: TDate;       ExitDate: TDate); end;
The record above is actually a single expression of two records that coulddescribe a person:
type TPersonCitizen = record   FirstName, LastName: string[40];   BirthDate: TDate;   BirthPlace: string[40] end;
and
type TPersonAlien = record   FirstName, LastName: string[40];   BirthDate: TDate;   Country: string[20];   EntryPort: string[20];   EntryDate: TDate;   ExitDate: TDate; end;
And as in a union, the combination of the two types of records makesfor much more efficient programming, because a person could be expressed in avariety of ways.

Everything I explained above is pretty hypothetical stuff. In Delphi, theTRect structure that describes a rectangle is actually a variant record:

type TPoint = record  X: Longint;  Y: Longint;end;TRect = record  case Integer of    0: (Left, Top, Right, Bottom: Integer);    1: (TopLeft, BottomRight: TPoint);  end;
where the coordinates of the rectangle can be expressed using either fourinteger values or two TPoints.

I realize this is pretty quick and dirty, so I suggest yourefer to the help file for a more in-depth explanation, or go to yournearest book store or library and look at any Pascal book (not Delphi — mostwon’t explain this fairly esoteric structure). However, if you’re familiarwith the C-union, this stuff should be an absolute breeze.

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