Re: Handling type, new and old

2014/1/20 Frediano Ziglio <[email protected]>:
> Hi,
>   as discussed recently adding a new type to libTDS is quite a nightmare!
>
> I was thinking a different way to handle our types. Mainly something
> similar to functions pointers we have but extended so support
> everything. I would like to extend it in a way that every upper
> library could have additional "methods" specific to this library.
> However I want to not have to code of a specific library in libTDS.
> libTDS should be able to get the table of a specific type with the
> extended information and call method it needs. Only upper layer will
> see additional methods. I'd like that when a new type is added to
> libTDS compilation of upper layer will fail until type is correctly
> supported. Also I'd like something that does not require many
> allocation but I would prefer statically allocated structures. My
> implementation idea is this:
>
> libTDS has an array of pointer to structures indexed by type to the
> function pointers, something like:
>
> struct tds_type_pointers type_funcs[] = {
>    ...
>    funcs_chars,
>    ...
>    funcs_int,
> };
>
> Now, libTDS will provide implementations for functions but NOT for the
> structures. That way to avoid link failures upper layers has to
> provide the structures, better if they extend them. So for instance
> ODBC could define an extended structure
>
> struct odbc_type_pointers {
>     tds_type_pointers common;
>     unsigned (*get_sql_type)(TDSCOLUM *col);
> };
>
> ...
>
> struct odbc_type_pointers funcs_chars = {
>     { tds_char_get, ...},
>     odbc_get_char_sql_type;
> }
>
> Does we agree on doing that. Probably it will require some changes (a
> lot) but adding a new type will be very easy! Probably will require
> some additional macro and a lot of definitions but beside that we
> avoid to miss some pieces.
>

Some example why. Let's assume libTDS wants to add a new type. For
instance libTDS does not handle tables (yes, mssql 2012 can return a
table as a type!). We add a declaration (but NO a definition) for a
funcs_table. Now we try to compile and links fails as linker does not
fund funcs_table definition (actually it fails in dblib, ctlib and
odbc). We have to define funcs_table in every upper library.

Frediano