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梪p to the reserved word case梚s the same as
that of a standard record type. The remainder of the declaration梖rom case to
the optional final semicolon梚s 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 comma-delimited list of declarations resembling the fieldList: type constructions in the main part of the record type. That is, a variant has the form
fieldList1: type1;
...
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: (Side1, 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 above) 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).
