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

StackOverflow https://stackoverflow.com/questions/2714365

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.

Was it helpful?

Solution

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

OTHER TIPS

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.

clHotlight: TColor = $00FF9933; 
          ^

Is declaring clHotlight as a 'variable' (well, ok an 'assignable constant' if you've permitted this in compiler options) by means of the :.

As you've found, declaring:

clHotlight = TColor($00FF9933);

Makes no allocation of clHotlight until it is specified later.

The same applies to your GUID.

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.

The right side of a constant declaration must be a "constant expression", which is defined as "a constant expression is an expression that the compiler can evaluate without executing the program in which it occurs". You can find the whole accepted syntax for constant expression in the language guide. Note that the language guide explicitly states "Typed constants cannot occur in constant expressions." - and that's why your declarations fails, both clHotlight: TColor = $00FF9933; and AdministratorGUID: TGUID =...; are typed constants. Also, constant expression cannot include functions calls except those listed in the language guide (i.e. Length(), SizeOf(), and some others) that the compiler is able to compute at compile time. Rewrite this way:

const
  AdminGUID = '{DE44EEA0-6712-11D4-ADD4-0006295717DA}';
  AdministratorGUID: TGUID = AdminGUID;
  SuperuserGUID: TGUID = AdminGUID;

And it will work.

welcome to Delphi evolution. in delphi 1 & 2, you can not assign initial constant value to a global var (ex: var xVar: Integer = 1). The only way you can do that is using const xVar: Integer = 1) and some where in you codes, you can then change it to somethingelse if desired. Until they get rid of this ancient feature, you can not using "const xVar: Integer" construct as a const value.

Cheers A Pham

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