3.3.2 Record types

Free Pascal supports fixed records and records with variant parts. The syntax diagram for a record type is

So the following are valid record type declarations:

The variant part must be last in the record. The optional identifier in the case statement serves to access the tag field value, which otherwise would be invisible to the programmer. It can be used to see which variant is active at a certain time¹. In effect, it introduces a new field in the record.

Remark: It is possible to nest variant parts, as in:

By default the size of a record is the sum of the sizes of its fields, each size of a field is rounded up to a power of two. If the record contains a variant part, the size of the variant part is the size of the biggest variant, plus the size of the tag field type if an identifier was declared for it. Here also, the size of each part is first rounded up to two. So in the above example:

• SizeOf would return 24 for Point,
• It would result in 24 for RPoint
• Finally, 26 would be the size of BetterRPoint.
• For MyRec, the value would be 12.

If a typed file with records, produced by a Turbo Pascal program, must be read, then chances are that attempting to read that file correctly will fail. The reason for this is that by default, elements of a record are aligned at 2-byte boundaries, for performance reasons.

This default behaviour can be changed with the {$PACKRECORDS N} switch. Possible values for N are 1, 2, 4, 16 or Default. This switch tells the compiler to align elements of a record or object or class that have size larger than n on n byte boundaries.

Elements that have size smaller or equal than n are aligned on natural boundaries, i.e. to the first power of two that is larger than or equal to the size of the record element.

The keyword Default selects the default value for the platform that the code is compiled for (currently, this is 2 on all platforms) Take a look at the following program:

The output of this program will be :

And this is as expected:

• In Trec1, since B has size 2, it is aligned on a 2 byte boundary, thus leaving an empty byte between A and B, and making the total size 4. In Trec2, B is aligned on a 1-byte boundary, right after A, hence, the total size of the record is 3.
• For Trec3, the sizes of A,B are 1, and hence they are aligned on 1 byte boundaries. The same is true for Trec4.
• For Trec5, since the size of B – 3 – is smaller than 4, B will be on a 4-byte boundary, as this is the first power of two that is larger than its size. The same holds for Trec6.
• For Trec7, B is aligned on a 4 byte boundary, since its size – 7 – is larger than 4. However, in Trec8, it is aligned on a 8-byte boundary, since 8 is the first power of two that is greater than 7, thus making the total size of the record 16.

Free Pascal supports also the ’packed record’, this is a record where all the elements are byte-aligned. Thus the two following declarations are equivalent:

and

Note the {$PackRecords 2} after the first declaration !