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

Subject: Re: [boost] quince: queries in C++ expressions
From: Michael Shepanski (mps_at_[hidden])
Date: 2014-07-16 21:32:05

On 17/07/2014 2:13 AM, pfultz2 wrote:
>> Could you clarify what you mean by "do this"?  QUINCE_MAP_CLASS builds a
>> class with exactly the data members, function members, and bases that
>> quince requires.
> Boost.Fusion already provides the ability to map the classes to fusion
> sequences.

But that is not what QUINCE_MAP_CLASS and QUINCE_MAP_CLASS_WITH_BASES 
do. Those macros define mapper classes with specific data members, 
function members, and bases, which are required by the rest of quince. 
Also the members and bases are exposed to user code, so they have to be 
defined in exactly the way that those macros define them.

>   Here is a very simple example how to build an ORM using
> Boost.Fusion in C++14:
>
> https://gist.github.com/pfultz2/abbea635bdc8a4971424
>
> Of course as a simple example, it doesn't map the types to the database
> types(it just uses `typeid` for illustration purposes only).
>
> Now this example just demonstrates mapping to the database with fusion. The
> other half is building queries in C++. This can easily be done by taking
> advantage that the structs are adapted as associative sequences. So we can
> use
> the keys to refer to the fields. So we could build queries something like
> this:
>
>      table<person> p("person");
>      auto query = from(p)
>                   .where(at_key<fields::age>(p) > 60)
>                   .select(at_key<fields::name>(p));
>
> Of course, some additional macros would be useful to reduce the boilerplate,
> but an ORM in C++ should really start with Boost.Fusion.

It sounds like you are proposing a new product, rather than a change to 
quince. I wish you well.

>
>> I'm not actually doing tuple manipulation here. That's not to say the
>> task couldn't be solved by a detour through tuples, but it /would/ be a
>> detour.
> But it would be a 'detour' that would save you from writing mapping macros.

I still need to define my mapper *class*, with its appropriate members 
and bases, as described here 
http://quince-lib.com/mapped_data_types/user_defined_class_types.html .  
When I come back from the tuples detour, how will I get this class 
defined? I think I will be back to using Boost.Preprocessor magic.

By the way, you say "mapping macros" but it's really just one. The 
implementations of QUINCE_MAP_CLASS and QUINCE_MAP_CLASS_WITH_BASES both 
invoke one macro (QUINCE_DEFINE_CLASS_MAPPER_WITH_BASES) to do all the 
hard work. If I were to take the tuples detour, that would be the only 
site where it happens.

Regards,
--- Michael