An assignment of a record variable on a non-by-reference type should give a warning. Don't you think?

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An assignment of a record variable on a non-by-reference type should give a warning. Don't you think?

RWithPointers = record
Name: string;
Plugin: TSomeElaborateClass;
class procedure Clean(aInstance: RWithPointers);
//class procedure Clean(var aInstance: RWithPointers); static;
end;

Will not work. But compiles w/o warnings.

Need a 'var' parameter:
class procedure Clean(var aInstance: RWithPointers);

Body:
class procedure RWithPointers.Clean(aInstance: RWithPointers);
begin
// Assigments of members on local variable (going out of scope)... Hey compiler!
aInstance.Name := '';
aInstance.Plugin := nil;
end;

Just asking. Bit me again. Stupid, i know.

Comments

  1. Thomas Mueller (dummzeuch)May 23, 2018 at 10:50 AM

    It probably should, yes. The compiler could easily check that this code has no effect. Actually, that's not strictly true: Assigning an empty string to Name will decrement the reference count of the original string.

    ReplyDelete
  2. Dany MarmurMay 23, 2018 at 11:12 AM

    Thomas Mueller Wow! That sound like ARC... having to prepend all and everything with [weak] or some such.

    But does the statement not still hold true? Even if there is a reference that decreases it does not ACTUALLY change the content of the record after the procedure has finished.

    ReplyDelete
  3. Hallvard VassbotnMay 23, 2018 at 12:01 PM

    The compiler does what you tell it to do. Maybe a hint could be emitted in some cases.

    ReplyDelete
  4. David HeffernanMay 23, 2018 at 1:03 PM

    Certainly should not be an error. A hint perhaps.

    ReplyDelete
  5. Paul TOTHMay 23, 2018 at 11:47 PM

    there's no reason you could not use the parameter to store temporary data. TRect is often used that way.

    ReplyDelete
  6. Dany MarmurMay 24, 2018 at 12:16 AM

    Hallvard Vassbotn, David Heffernan i wrote "warning". Should have written "hint"?
    Paul TOTH of course, that is useful and logical. Should have said "Not the result i wanted" instead of "not work". I would never want to change the behaviour of the resulting code.

    ReplyDelete
  7. Paul TOTHMay 24, 2018 at 12:21 AM

    Dany Marmur I understand, I would also like that Delphi puts a warning on each of my bugs :)

    ReplyDelete
  8. Rudy VelthuisJuly 19, 2018 at 1:21 PM

    Paul TOTH Yes, it is often doen that way, but I personally find re-using parameters for temporary storage bad practice. A variable (and even more a parameter) should be for one single purpose, and re-using them for a different purpose is not good practice.

    But otherwise, I am not even sure if it should be a hint. If you don't want to write to the parameter, make it const. Otherwise you should be able to, and it is not uncommon to give a plain value parameter a new value in the course of a function or procedure, e.g. `S := Trim(S);` or similar. You don't want to get a hint, nor do you want that to have any side effects.

    ReplyDelete

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