Variant Parts in Records

A record type can have a variant part, which looks like a case statement. The variant part must follow the other fields in the record declaration.

To declare a record type with a variant part, use the following syntax.

type recordTypeName = record fieldList1: type1;

fieldListn: typen; case tag: ordinalType of constantList1: (variant1);

constantListn: (variantn);

end;

The first part of the declaration - up to the reserved word case - is the same as that of a standard record type. The remainder of the declaration - from case to the optional final semicolon - is called the variant part. In the variant part,

  • tag is optional and can be any valid identifier. If you omit tag, omit the colon (:) after it as well.
  • ordinalType denotes an ordinal type.
  • Each constantList is a constant denoting a value of type ordinalType, or a comma-delimited list of such constants. No value can be represented more than once in the combined constantLists.

Each variant is a semicolon-delimited list of declarations resembling the fieldList: type constructions in the main part of the record type. That is, a variant has the form fieldListl: typel; fieldListn: typen;

where each fieldList is a valid identifier or comma-delimited list of identifiers, each type denotes a type, and the final semicolon is optional. The types must not be long strings, dynamic arrays, variants (that is, Variant types), or interfaces, nor can they be structured types that contain long strings, dynamic arrays, variants, or interfaces; but they can be pointers to these types.

Records with variant parts are complicated syntactically but deceptively simple semantically. The variant part of a record contains several variants which share the same space in memory. You can read or write to any field of any variant at any time; but if you write to a field in one variant and then to a field in another variant, you may be overwriting your own data. The tag, if there is one, functions as an extra field (of type ordinalType) in the non-variant part of the record.

Variant parts have two purposes. First, suppose you want to create a record type that has fields for different kinds of data, but you know that you will never need to use all of the fields in a single record instance. For example, type

TEmployee = record

FirstName, LastName: string[40];

BirthDate: TDate;

case Salaried: Boolean of

True: (AnnualSalary: Currency); False: (HourlyWage: Currency);

end;

The idea here is that every employee has either a salary or an hourly wage, but not both. So when you create an instance of TEmployee, there is no reason to allocate enough memory for both fields. In this case, the only difference between the variants is in the field names, but the fields could just as easily have been of different types. Consider some more complicated examples:

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, ExitDate: TDate);

end;

type

TShapeList = (Rectangle, Triangle, Circle, Ellipse, Other); TFigure = record case TShapeList of

Rectangle: (Height, Width: Real); Triangle: (Sidel, Side2, Angle: Real); Circle: (Radius: Real); Ellipse, Other: ();

end;

For each record instance, the compiler allocates enough memory to hold all the fields in the largest variant. The optional tag and the constantLists (like Rectangle, Triangle, and so forth in the last example) play no role in the way the compiler manages the fields; they are there only for the convenience of the programmer.

The second reason for variant parts is that they let you treat the same data as belonging to different types, even in cases where the compiler would not allow a typecast. For example, if you have a 64-bit Real as the first field in one variant and a 32-bit Integer as the first field in another, you can assign a value to the Real field and then read back the first 32 bits of it as the value of the Integer field (passing it, say, to a function that requires integer parameters).

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