>> > change this code:
>> > typedef struct NiuRoutingStruct {
>> > sqlite3_vtab vtab;
>> > } NiuRouting;
>> > to this:
>> > struct NiuRouting : public sqlite3_vtab vtab {
>> > ... // your implementation, eventually
>> > };
>> >
>> > This way, the casting is unnecessary and any pointer offsetting will
>> be
>> > done correctly.
>>
>> Just for my curiosity:
>> Is the above really equivalent to the C definition like:
>> typedef struct NiuRoutingStruct {
>> sqlite3_vtab vtab;
>> ... // your implementation, eventually
>> } NiuRouting;
>>
>> ?
>>
>> Reason I'm asking: sqlite uses a pointer to that structure to access
>> "sqlite3_vtab vtab" member - in machine code that would be
>> memory offset 0, all other private members start at
>> +sizeof(sqlite3_vtab)+optional alignment.
>> Now, I'm wondering if a c++ style of inheritance is defined
>> in the same way - Note that sqlite relies on that, or better
>> say "the compiler that produced the sqlite machine code relies
>> on that".
>> Just in case the c++ compiler rearrange the byte offset differently
>> it will crash rights away, I think.
>>
>> Thanks.
>
> Without my copy/paste error, (the 'vtab' should be gone, leaving just the
> struct name), it would be equivalent provided that the alignment of a
> sqlite3_vtab was no less restrictive than the alignment of
> NiuRoutingStruct as it is finally defined. However, static_casting from
> derived to base automatically incorporates the correct offset, (as would
> taking the address of an embedded sqlite3_vtab member, instead of using
> the OP's reinterpret cast), so the crash you worry about (with good
> reason) would be made less likely.
okay, thank you for the explanation. Interesting subject.
Kind regards
Marcus
>
> I think all SQLite relies upon is getting correct pointers to its portion
> of whatever object it is passing around. That virtual table API is
> designed to make it agnostic as to any other, tag-along content.
>
> Best regards,
> --
> Larry Brasfield
>
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