On Tue, 01 Jun 2010 15:00:47 +0100 Martin <f...@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) > Example 2: > > Procedure Bar; > var a, b: string; > begin > a:= SomeString; > b:=a; > if SomeCondition then > b:=b+"..."; > if SomeOtherCondition then > b:=b+"???"; > end; > > So b may never be changed at all. Sure, I did not mean this cannot happen. I rather spoke of the programmer's intent. My hypothesis is that it does not make sense to copy a string into another var if none will change. Your example is perfect :-) There are cases were this cannot be avoided. Still, is this worth the complication of copy-on-write. I also favor a super-simple and explicit coding style, which produces some more assignments, intermediate and not necessary, than clever coding with complex expressions. From this point of view, copy-on-write is an interesting scheme, in that it avoids useless copies (except for references), thus allowing such a coding style with minor penalty (ref copy only). But if the compiler is smart enough, such intermediate vars can probably be eliminated, no? Denis (*) And to some more constructs in other languages, like foreach (*the* feature I miss in freepascal): foreach name in names do ... end; ________________________________ vit esse estrany ☣ spir.wikidot.com _______________________________________________ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
