Proper use of {$IF}

[Molochnik]

I have two modules
1)

Code: Pascal  [Select][+][-]

1. unit a;
2. interface
3. const
4. UseFeature = 'Yes';

.....
2)

Code: Pascal  [Select][+][-]

1. unit b;
2. uses a{$IF UseFeature='Yes'}, c{$IFEND};
3.  

...

Version 3.2.2 doesn't recognize UseFeature value throughout all unit b.
3.99 recognizes the value of UseFeature in the implementation part of unit b, but still doesn't recognize it in its uses section.
Delphi handles it perfectly - UseFeature is known in the uses section and all over the whole unit b.
Of course if I change $IF to $IFDEF UseFeature and place it in the project options, it will work, but I dont like it - required full rebuild, often conditional parts are marked as erroneous in Delphi (though they are actually not) and for me it's much cleaner and nicer to have conditions directly in the code than in the project options.
What can be done here? Did I miss something?

[marcov]

May be that the uses line is parsed first, and only then the scope is set up (the units loaded)

The construct seems a bit dodgy to me, ad hoc exploitation of specific compiler implementation.

[Zvoni]

{$IF Declared(UseFeature)}?

[wp]

I never was successful with the {$IF} directive in the uses clause. What's definitely working is the {$IFDEF} directive: Create a conditional define {$DEFINE UseFeature} in the project or the package, or add it to an inc file which you {$include ...} in all files needing it. Then in unit b you do

Code: Pascal  [Select][+][-]

1. unit b;
2. {$INCLUDE defs.inc}  // assuming that "UseFeature" is defined in file "defs.inc"
3. uses
4.   a{$IFDEF UseFeature}, b{$ENDIF};
5. ...

[Thaddy]

I never was successful with the {$IF} directive

Can you provide an example?

[wp]

I never was successful with the {$IF} directive

Can you provide an example?

I wrote: "I never was successful with the {$IF} directive in the uses clause."

[Molochnik]

I never was successful with the {$IF} directive in the uses clause.

Yes I have the same problem, but thought I missed something. As I said before Delphi handles $IF in the uses section properly.

[Thaddy]

Confirm this, but strange. Is there any bug report for that?

[PascalDragon]

What can be done here? Did I miss something?

This is currently simply not possible. You can't use constants declared in another unit inside preprocessor macros inside the uses-clause due to the way how the compiler loads units. It simply doesn't know these constants until the trailing semicolon of the clause has been parsed.

[Molochnik]

This is currently simply not possible. You can't use constants declared in another unit inside preprocessor macros inside the uses-clause due to the way how the compiler loads units. It simply doesn't know these constants until the trailing semicolon of the clause has been parsed.

Ok got it. It's not a big problem, though it actually reduces the $IF value almost to zero. Why use it if $IFDEF works always and everywhere?

[PascalDragon]

This is currently simply not possible. You can't use constants declared in another unit inside preprocessor macros inside the uses-clause due to the way how the compiler loads units. It simply doesn't know these constants until the trailing semicolon of the clause has been parsed.

Ok got it. It's not a big problem, though it actually reduces the $IF value almost to zero. Why use it if $IFDEF works always and everywhere?

$IFDEF has the same restrictions here, there is no difference between $IF and $IFDEF regarding that. And after the uses-clause you can $IF on identifiers from another unit.

Also the advantage of $IF compared to $IFDEF is that you can combine multiple expressions using OR and AND.