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 

and the alternate wording that works:

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

Explain.

---

Then consider:

const     AdministratorGUID: TGUID = '{DE44EEA0-6712-11D4-ADD4-0006295717DA}';    SuperuserGUID = AdministratorGUID; //alias of AdministratorGUID  [Error] file.pas: Constant expression expected 

And fix.

Edit: Added keyword const before declarations; someone didn't believe they were const.

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 

Answer 2

I tried this code :

  const     CAnswer1 = 42;     CAnswer2 : Integer = 42;    var     LAnswer : Integer;    begin     LAnswer := CAnswer1;     LAnswer := CAnswer2;   end; 

and here is the produced code :

Project9.dpr.18: LAnswer := CAnswer1; 004101AC C7056C6E41002A00 mov [$00416e6c],$0000002a //<- assign a hard-coded "42" value Project9.dpr.19: LAnswer := CAnswer2; 004101B6 A1701C4100       mov eax,[$00411c70] //<- fetch a variable's content 004101BB A36C6E4100       mov [$00416e6c],eax //<- assign this content  

You are right : some constants are more constant than others. The second constant is actually treated by the compiler as a variable.