[fpc-pascal] PChar & AnsiString

[Martin]

On 01/06/2010 16:13, spir ☣ wrote:
> On Tue, 01 Jun 2010 15:00:47 +0100
> Martin<fpc at mfriebe.de>  wrote:
>
>    
>> On 01/06/2010 11:23, spir ☣ wrote:
>>      
>>> What is the actual benefit of copy-on-write? I ask because of the following reasoning:
>>> * If a string is just used at several places, for example in output or into bigger strings, then there is no reason reason to copy it into a new variable.
>>> * If a programmer explicitely assigns an existing string to a new variable, the intent is precisely copy-semantics, to make them independent for further changes. If there is no change, there is also no reason for such an assignment.
>>>        
> [...]
>
>    
>> No, an assignment, or increase in ref count is not necessarily followed
>> by a change:
>>
>> Procedure Foo(a:String);
>> begin ... end;
>>
>> Procedure Bar;
>> var s: string;
>> begin
>>     s:= SomeString;
>>     Foo(s);
>> end;
>>
>> Now a copy of s has to be given to Foo, because Foo *may* changes the
>> string. But Foo may also *not* change the string =>  so leave it till later.
>>      
> For me, parameter passing is just an implicit assignment "param:=s" (with the variant of var params). The same applies to return values when assigned back in calling code (*). I would have thus expected s the pointer to be passed. Just checked it is not the case. More secure, esp since legacy shortstrings do not behave that way.
> This example rather backs my pov, I guess, since here copy-on-write does not apply. But it certainly could, since its application should prevent bad surprises, no? (unexpected change of original variable)
>    
I don't know all the internals of FPC, but yes to my understanding, your 
quote:
   "parameter passing is just an implicit assignment"
is absolutely true.

So why do you then say "copy on write" would not apply?
The assignment creates a 2nd reference, and a copy only happens if 
inside Foo a change is made to the string.
=> So according to your own words, copy-on-write is exactly what happens.

Shortstrings have nothing to do with it, they are not reference counted. 
They are a completely different data type.

Martin