[fpc-devel] Managed Types, Undefined Bhaviour

Michael Van Canneyt michael at freepascal.org
Sat Jun 30 14:50:52 CEST 2018

On Sat, 30 Jun 2018, Willibald Krenn wrote:

>>> Managed fields of records are "setup" ;)
>> I will add a section about this in the documentation, seeing that people
>> often confuse the 2 concepts.
> In an ideal world, either the language would not let you write code that has random behavior or the compiler would enforce this.

There is no random behaviour.

> procedure global(memo1: TMemo);
> begin
>  A:=X(3);
>  A[0] := 5;
>  A[1] := 4;
>  B:=X(3);
>  // the following will print 5 twice
>  memo1.lines.add('GlobalA0: %d',[A[0]]);
>  memo1.lines.add('GlobalB0: %d',[B[0]]);

B[0] is not initialized. Printing the contents can print anything.

A and B are different arrays, with length 3, The compiler ensures this. 
But their content is not initialized. Printing the content may therefor
result in any possible vallue.

Your code in essence does the same as

Procedure something;

   B : Array[0..2] of integer;


And - Lo -  it behaves the same...


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