Re: [fpc-pascal] Declaring an array as var or not in function calls?
Bo Berglund via fpc-pascal schrieb am Do., 13. Apr. 2023, 07:49: > What is the use case for "out" compared to "var" when it seemingly is > doing the > same thing? > Or is it exactly this: > With "out" you get a pristine empty variable which you have to initialize > if > needed (setting its length in this case)? > And you are ensured no stale data will be part of it? > When the compiler finds an "out" parameter in a call it does not need to check whether the variable you pass is has been written to. With a "var" parameter you'll get "variable not initialized" warnings/hints instead. It's the task of the called function then to provide a correct value for the parameter. > > > >And if so, what is the rule for *when* to use var in function call > argument > >> lists for items that will be changed inside the called function and used > >> afterwards > >> > > > >The rule is simple: if you need to modify the parameter value itself (e.g. > >changing a primitive type, changing a field in a record, changing an > >array's length or data, changing the value of a class instance - but not > >its fields!) you need to pass it as "var". > >If it's a write only parameter you can even use "out". > > So "out" it is unless I need to use the existing data or append new data > to it, > in which case I need "var", right? > Correct. Regards, Sven > ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-pascal
Re: [fpc-pascal] Declaring an array as var or not in function calls?
On Thu, 13 Apr 2023 00:06:16 +0200, Sven Barth via fpc-pascal wrote: >Bo Berglund via fpc-pascal schrieb am >Mi., 12. Apr. 2023, 21:17: > >> Now my question: >> >> Since I am modifying the WArgs variable inside the BuildArgumentsArray >> function and using its new content afterwards in the WriteItem function >> I assumed that I had to use var as shown above, but I am not certain this >> is the case... >> > >If you modify the length or the contents of a dynamic array parameter then >you need to use "var" or "out" modifiers (the later only if you don't need >the data passed in). In this case I am assuming that there is no existing data because I change the length of the array depending on the calculation of the number of needed calls to complete the write operation. So it is definitely only an "out" argument (I have never used out before...). > >> It seems like it works also if I do not use the vars specification... >> > >It should not. If it does it will likely be an implementation detail and >you shall not rely on it. Do you have an explicit example for this >behavior? Well, I think I have done this earlier but now I have made a quick test using TBytes and it seems like var *is* needed... Since I have not used out before I also tested that and it worked like var does. What is the use case for "out" compared to "var" when it seemingly is doing the same thing? Or is it exactly this: With "out" you get a pristine empty variable which you have to initialize if needed (setting its length in this case)? And you are ensured no stale data will be part of it? > >And if so, what is the rule for *when* to use var in function call argument >> lists for items that will be changed inside the called function and used >> afterwards >> > >The rule is simple: if you need to modify the parameter value itself (e.g. >changing a primitive type, changing a field in a record, changing an >array's length or data, changing the value of a class instance - but not >its fields!) you need to pass it as "var". >If it's a write only parameter you can even use "out". So "out" it is unless I need to use the existing data or append new data to it, in which case I need "var", right? Thanks! -- Bo Berglund Developer in Sweden ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-pascal
Re: [fpc-pascal] Declaring an array as var or not in function calls?
Bo Berglund via fpc-pascal schrieb am Mi., 12. Apr. 2023, 21:17: > Now my question: > > Since I am modifying the WArgs variable inside the BuildArgumentsArray > function > and using its new content afterwards in the WriteItem function I assumed > that I > had to use var as shown above, but I am not certain this is the case... > If you modify the length or the contents of a dynamic array parameter then you need to use "var" or "out" modifiers (the later only if you don't need the data passed in). > It seems like it works also if I do not use the vars specification... > It should not. If it does it will likely be an implementation detail and you shall not rely on it. Do you have an explicit example for this behavior? And if so, what is the rule for *when* to use var in function call argument > lists for items that will be changed inside the called function and used > afterwards > The rule is simple: if you need to modify the parameter value itself (e.g. changing a primitive type, changing a field in a record, changing an array's length or data, changing the value of a class instance - but not its fields!) you need to pass it as "var". If it's a write only parameter you can even use "out". Regards, Sven > ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-pascal
Re: [fpc-pascal] Declaring an array as var or not in function calls?
Giuliano Colla via fpc-pascal schrieb am Mi., 12. Apr. 2023, 22:54: > Il 12/04/2023 21:17, Bo Berglund via fpc-pascal ha scritto: > > And if so, what is the rule for **when** to use var in function call argument > lists for items that will be changed inside the called function and used > afterwards? > > Your Wargs is defined as an open array, i.e. its length is unknown at > compile time. Therefore the compiler cannot pass it in the subroutine call > other than a pointer to the array. It cannot pass the array content because > it doesn't know its length. > In that case you may omit the var specifier. > But if one day you change your mind, and decide to establish a size for > your array, without var the compiler may decide to pass the full array data > to the procedure, and therefore whatever you do in your procedure is done > on the local copy of the array and not in the original array. > My suggestion is to specify "var" whenever your procedure is expected to > modify the data passed. It makes the program more readable, even if "var" > is redundant. > There is *no* open array in the example given. Also even if there were changing the length of an open array would not work... Regards, Sven ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-pascal
Re: [fpc-pascal] Declaring an array as var or not in function calls?
Il 12/04/2023 21:17, Bo Berglund via fpc-pascal ha scritto: And if so, what is the rule for*when* to use var in function call argument lists for items that will be changed inside the called function and used afterwards? Your Wargs is defined as an open array, i.e. its length is unknown at compile time. Therefore the compiler cannot pass it in the subroutine call other than a pointer to the array. It cannot pass the array content because it doesn't know its length. In that case you may omit the var specifier. But if one day you change your mind, and decide to establish a size for your array, without var the compiler may decide to pass the full array data to the procedure, and therefore whatever you do in your procedure is done on the local copy of the array and not in the original array. My suggestion is to specify "var" whenever your procedure is expected to modify the data passed. It makes the program more readable, even if "var" is redundant. Giuliano -- Do not do to others as you would have them do to you.They might have different tastes. ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-pascal
