[fpc-devel] ref count issue with out param

Martin Frb lazarus at mfriebe.de
Sat Jun 13 16:27:47 CEST 2015

On 13/06/2015 15:24, Michael Van Canneyt wrote:
> 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.

Yes you are right. But that makes you example different. the value of 
the "out" param passed in is nil (because of the initialization). So the 
refcount of 0 is no surprise at all.

In my example the out param is fine too, but the other param (that gets 
passed the same param on the callee side) is dangling.

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