[fpc-devel] ref count issue with out param

[Michael Van Canneyt]

On Sat, 13 Jun 2015, Martin Frb wrote:

> On 13/06/2015 15:12, Michael Van Canneyt wrote:
>> 
>> 
>> Note the 0 !
>> 
>> Secondly, the section about 'out' parameters is very old; there were not 
>> nearly so much managed types at the time.
>> I will update it, I was just documenting the new string types anyway. The 
>> above will nicely illustrate the point.
>
> What about the (unverified) remark on the forum that delphi does it different 
> (apparently passing nil in that case)?
>
> "const" is an exception. The purpose of const is to skip the ref counting.
>
> "Out" has a different purpose. Should it really create a dangling pointer in 
> a ref counted variable?
>
> In your example you are lucky you got a refcount of 0. The memory is 
> after-all freed. you called the StringRefCount on a dangling pointer. 
> Anything could have happen.

No, the compiler *must* initialize it. 
Without that guarantee, ansistring out parameters could never work.

If the out got a dangling pointer as input, it would never be able to decrease the ref. count when assigning something to it.
Ergo, the compiler must initialize out params.

Michael.