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Clarion 6: Threading issues with files in methods - 2
2003-01-21 -- Alexey Solovjev

Hello,

> does this means we can have no files declared anymore in class methods ?

No. Class methods are normal procedures just with slightly another syntax,
one predefined parameter. But up to 2 methods per class are special:
constructor and destructor. They are invoking automatically when an
instance they called for is created or to be destroyed.

The common rule is that code can't assume any specific state of data that
has not been initialized yet. This is true for any code and any kind of
data, not for FILEs within constructor. For example, one can write following
code

=====

A         CLASS,TYPE
Construct   PROCEDURE
....
          END

B         CLASS,TYPE
Construct   PROCEDURE
....
          END

InstA     A
InstB     B

....

A.Construct  PROCEDURE
  CODE
  

....

B.Construct  PROCEDURE
  CODE
  

=====

From the point of syntax, all is correct but either InstA or InstB will
use another one before it has been initialized and therefore this code
is wrong. Absolutely the same thing happens with FILEs declared locally
in the constructors of not-threaded instances of some classes: code
tries to use data that has been initialized.

I have an idea how to change the compiler and the RTL to allow programs
like yours run without faults because of initialization but this would be
an allowance to violate rules in very particular situations. Program
should not violate rules.

To summarize:

- you can have FILEs declared locally in any method but constructor
- you can have FILEs declared locally in constructors of classes if
  their instances are threaded
- if you need in a FILE within a constructor of some class that has
  not-threaded static instances, that FILE should be declared out
  of constructor method.

Alexey Solovjev

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