Re: [boost] sqlpp11, 3rd iteration

Subject: Re: [boost] sqlpp11, 3rd iteration
From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 2014-08-18 20:09:53

Roland Bock wrote:
> On 2014-08-18 22:43, Adam Wulkiewicz wrote:
>> But AFAIU if the C++ objects were created along with the
>> representation of a row, the results could be stored as optionals. Or
>> am I missing something?
> Yes the values are set when the row is fetched. The objects are re-used
> for each row. And the backend is given pointers to write the value into
> as explained above.
>
> But while writing this, I just realized: I could do is offer a
> conversion operator for std::optional<int64_t>, of course. Thus, given a
> row with a column `a` which can be NULL, you would then have
>
> std::optional<int64_t> a1 = row.a; // OK
> int64_t a2 = row.a; // compile failure

Is it the same way with bigger objects, e.g. std::strings?
Is the pointer to std::string passed to the backend (which means that 
some temporary buffer must be used) or must the pointer to the memory 
owned by std::string be passed, after resizeing a string?

AFAIU the reason of a problem with optionals is a limitation of the 
implementation.
Couldn't a reference to optional be passed to the backend and then the 
optional filled with data? E.g. using some temporary buffer or whatever 
method suitable to get the data. And then the optional re-created using 
this data.

Even temporary buffers could be ommited. Hmm, it probably even wouldn't 
be required to pass an optional to the backend. An optional could be 
created with default-constructed value:

member = optional<int64_t>(int64_t());

or re-constructed in case there were no valid value stored, probably 
like this:

if ( !member )
     member = optional<int64_t>(int64_t());

and then the address of this value already stored within optional could 
be passed deeper:

if ( backend.fill(member.get_ptr()) == false )
     member = optional<int64_t>();// null

Or am I missing something?
Btw, I'm not saying you should change the interface. I was just 
surprised that it couldn't be done.

Regards,
Adam