Re: [boost] quince: queries in C++ expressions

Subject: Re: [boost] quince: queries in C++ expressions
From: Michael Shepanski (mps_at_[hidden])
Date: 2014-07-17 21:05:08

On 18/07/2014 2:26 AM, Roland Bock wrote:
> On 2014-07-17 15:34, Michael Shepanski wrote:
>> On 17/07/2014 10:48 PM, Roland Bock wrote:
>>> Shouldn't collections be mentioned on that page, too?
>> I'm not sure I understand.  Do you mean STL containers?  Then no,
>> these are not mapped types (except for the special cases std::string
>> and std::vector<uint8_t>).  You can't have a table<std::list<float>>.
> No, sorry for the confusion, I meant the usage of collector classes.

Oh but they are mentioned!  They are in the category of "user-defined 
class".

Just as a std::tuple can be used to represent data in a table, and can 
also be put together _ad hoc_ by quince to deliver results of a join() 
or select(), the same can be said of user-defined classes (after they 
have been processed with QUINCE_MAP_CLASS).

> Say I have two conditions, userWantsBlob and userWantsOther. I would
> want to have everything in one loop.
>
> In sqlpp11 I could say something like:
>
> auto query = dynamic_select(db, t.meta)...;
> if (userWantsBlob)
>     query.add_column(t.blob);
> if (userWantsOther)
>     query.add_column(t.other);
>
> for (const auto& row : db(query))
> {
>     std::cout << t.meta << '\n';
>     if (userWantsBlob)
>       std::cout << t.at("blob") << '\n';
>     if (userWantsOther)
>       std::cout << t.at("other") << '\n';
> };
>
> How would you do that in quince?

Ah, no, quince's static typing forbids anything like that.  If I wanted 
to provide it, that would be a matter of exposing type-unsafe code that 
I currently keep hidden.

Cheers,
--- Michael