[fpc-pascal] Wrong constructor returning the right object

[Mark Morgan Lloyd]

Marco van de Voort wrote:
> In our previous episode, Mark Morgan Lloyd said:
> 
> That logical, since the constructors are not an virtual; and override; pair.
>  
>> I find that if I explicitly decorate TB5500BaseUnit.Create() as virtual 
>> and TB5500SPOUnit.Create() as override then TB5500SPOUnit.Create() is 
>> called correctly. 
> 
> That's the normal way.
> 
>> I didn't think this was necessary, 
> 
> _why_ did you think this?

In part, because the documentation on method declarations at 
http://www.freepascal.org/docs-html/ref/refsu25.html#x66-730005.5.1 
explicitly allows a  virtual  method directive, but the documentation on 
constructor declarations at 
http://www.freepascal.org/docs-html/ref/refse28.html#x64-710005.4 
explicitly does not allow it.

>> but on reflection I 
>> assume that it's because u is declared as a TB5500BaseUnit: the 
>> variable's compile-time rather than run-time class is being used for the 
>> constructor unless explicitly overridden.
> 
> In theory this kind of information could propagate in simple cases, but then
> behaviour would differ between cases where this information could be
> propagated (a deeper version called), and not (base version called).

-- 
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk

[Opinions above are the author's, not those of his employers or colleagues]