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Variant records: the equivalent to the C-union structure (Views: 100)


Problem/Question/Abstract:

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 is called a Variant Record (not to be confused with the Variant "type" available in Delphi 2.0+). As with a C- union, the Pascal variant record allows several structure types to be combined into one, and all will 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 could describe 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 makes for much more efficient programming, because a person could be expressed in a variety of ways.

Everything I explained above is pretty hypothetical stuff. In Delphi, the TRect 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 four integer values or two TPoints.

I realize this is pretty quick and dirty, so I suggest you refer to the help file for a more in-depth explanation, or go to your nearest book store or library and look at any Pascal book (not Delphi -- most won't explain this fairly esoteric structure). However, if you're familiar with the C-union, this stuff should be an absolute breeze.

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