> I was very much right!
You are, but in my VB6 code the sqlite3* object is always represented as a
pointer (Long in VB6), so that pointer will then have to be passed ByVal.
> It will if you correct that signature.
Ah, OK, thanks.
> I strongly recommend doing one of those online courses about C.
Agree and got the books, just need the time.
RBS
On Sun, Jan 24, 2016 at 10:53 PM, Roger Binns <rogerb at rogerbinns.com> wrote:
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> On 24/01/16 13:12, Bart Smissaert wrote:
> >>> I think that unless the argument is going to be altered I have
> >>> to pass
> > them always ByVal. That wasn't quite right, eg: int
> > sqlite3_close(sqlite3*); sqlite3* is not going to be altered, but I
> > can see that this should be passed ByRef
>
> I was very much right! Both cases apply here - modification and item
> size versus pointer (mailbox number) size.
>
> The type sqlite3 is a structure in C. I don't know the exact
> corresponding terminology in VB6 but it looks like "Type". A C
> structure is very much like this VB:
>
> Type Customer
> Dim FirstName As String
> Dim LastName As String
> Dim Id As Long
> End Type
>
> ie it is a name ("Customer") and has various items of data as members,
> each with their own type (String, Long etc).
>
> The sqlite3 structure has 79 members (more with some optional
> functionality) and occupies about 300 bytes. Even if none of them
> were going to modified, it would be insane to require 79
> parameters/300 bytes be passed, versus one pointer to the structure.
>
> It is possible to mark things as "not going to be modified" in C. The
> tag used is "const". The usage of it varies by code base, as it
> wasn't in the original C and it is easy to bypass if you want, plus
> various other issues. SQLite does use it. In this case it would be:
>
> int sqlite3_close(const sqlite3*);
>
> And logically, how can closing the database not modify at least one of
> the 79 members making up the sqlite3 structure? const was
> deliberately left out for a reason.
>
> > It doesn't work though with complex arguments like this: void
> > (*xFunc)(sqlite3_context*,int,sqlite3_value**)
>
> It will if you correct that signature. Elsewhere in the SQLite code,
> sqlite3_context and sqlite3_value were both defined (as structs). The
> one liner in isolation is not valid because it has no idea what they
> are, although the site could be bit more forgiving. Try that again
> but put "struct" before them, like so:
>
> void (*xFunc)(struct sqlite3_context*,int,struct sqlite3_value**);
>
> This particular case is a callback function. Those can get tricky
> even for regular C programmers.
>
> I strongly recommend doing one of those online courses about C. You
> don't need to become an expert, but at least understanding the
> concepts will make it *far* easier to translate between the VB world
> and C. C is not a big language, and has a lot of simplicity, brevity
> and elegance. Understanding pointers is a good test of a programmer.
>
> Roger
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