Delphi: All constants are constant, but some are more constant than others?

Question

Consider:

const 
   clHotlight: TColor = $00FF9933;
   clLink = clHotLight; //alias of clHotlight

[Error] file.pas: Constant expression expected


        

Answer 1

The problem arises because a typed constant is not, truly, a constant, as has been explained with varying degrees of clarity and success by others.

What hasn't as yet been shown is how to work around the problem (in a large number of cases), though a couple came tantalisingly close to giving up that secret... :)

In your specific case you can get around the problem by reversing the "aliasing" of the value and the typed constant declaration as follows:

const
  clLink = $00FF9933;
  clHotlight: TColor = clLink;

clLink now provides your true constant and clHotlight is the typed constant that has the same value as clLink.

For the GUID's the same technique can be used, but you have to bear in mind the normal constant expression used to initialise a typed GUID constant - it doesn't use a record but a simple literal string, so:

const
  ID_CONSTANT = '{AA1C8AF2-C290-40AB-9CF5-2888A46E1660}';
  GUID_CONSTANT: TGUID = ID_CONSTANT;

NOTE: Such GUID constants are perfectly usable in all places where TGUID's are required, e.g. IsEqualGUID( tguid, GUID_CONSTANT ) etc.