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

18 Jul 2014


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