[fpc-pascal] Case in Record

[Frank Peelo]

On 07/10/2009 16:39, Jürgen Hestermann wrote:
>>> IIRC, there is a difference. With the second declaration the compiler 
>>> can add a run-time check that the correct memebrs are accessed based 
>>> on the value of X.
>>> Unfortunately, I cannot find the compiler switch.
>>
>> FPC has no support for this.
> 
> 
> And I don't know any other Pascal Compiler who does any checks in this 
> direction. Do some?

Given that the OP was asking about learning, maybe it would be worth 
mentioning that the way we normally have "data structures which hold 
different data" these days is by polymorphism in objects or classes.

Instead of having an array of variant records, each of which might hold 
a description of a circle, square or pentagon depending on the value of 
a tag variable, we would now have an array of Shape, where Shape is a 
class instance or pointer to an object. The actual elements of the array 
would be Circle, Square or Pentagon, where these are types descended 
from Shape.

This has several benefits, not least the fact that new shapes can be 
added more easily than in the variant record case.

The stuff that has been said about variant records is all good, and 
variant records still have uses in some cases, but we don't want to 
mislead the OP into thinking that this is necessarily the way to go with 
new programs.

FP