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

clHotlight: TColor = $00FF9933; is not a constant but a typed constant (=static variable), i.e. the compiler reserves a slot in memory for a TColor which will hold the value $00FF9933 initially at run time.
Because that value can be changed later (with the Assignable Const option ON), it is not a real constant and cannot be accepted by the compiler in clLink = clHotLight;

clHotlight = TColor($00FF9933); is strictly the same as clHotlight = $00FF9933;
It is a true constant and the compiler will replace clHotlight by its value $00FF9933 wherever it appears in the code. And for clLink as well.

Read on this SO question (In Delphi 7, why can I assign a value to a const?) and all the good answers there...

EDIT: about TGUID...
The problem is that writing AdministratorGUID: TGUID = '{DE44EEA0-6712-11D4-ADD4-0006295717DA}'; is not proper.
It is using some compiler magic to call StringToGUID behind the scene, allowing the convenience to express the GUID as a string which they are not by nature. They are records.

So, trying AdministratorGUID = '{DE44EEA0-6712-11D4-ADD4-0006295717DA}'; will not work. That is not a GUID...

A workaround is to have a typed constant and variables pointing to the same memory area using the absolute directive:

const
   AdministratorGUID: TGUID = '{DE44EEA0-6712-11D4-ADD4-0006295717DA}';
var
   SuperuserGUID: TGUID absolute AdministratorGUID; //alias of AdministratorGUID
   RootGUID: TGUID absolute AdministratorGUID;      //alias of AdministratorGUID