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 01:08:36

On 17/07/2014 2:38 PM, pfultz2 wrote:
>> When I come back from the tuples detour, how will I get this class
>> defined?
> There is no need to define this class. Boost.Fusion takes care of it
> already.

Again, you say "takes care of it" -- but your "it" is not the "it" that 
I need. :)

Let's look from the point of view of a quince user.  The user writes 
some code like this:

     extern table<point> points;

     for (const point &p: points.where(points->x < 4.0f))
         std::cout << p.y << std::endl;

Look at the expression "points->x".  "points" is the table. Its 
operator->() returns a pointer to that table's value mapper. points->x 
is a mapper for one part of points.

In order to make this syntax work, the mapper type has to have a member 
named x, i.e. a member whose name is the same as the name of the value 
type's member.  This is a requirement, so that my EDSL looks the way I 
want it to look.

The QUINCE_MAP_CLASS macro makes this happen by defining a structure 
with members named in this way.

If you can do *that* with Boost.Fusion then perhaps you can show us the 
code that does it, and we discuss costs and benefits.

>> If I were to take the tuples detour, that would be the only
>> site where it happens.
> What do you mean?

I mean, if I were to take the approach of implementing QUINCE_MAP_CLASS 
and QUINCE_MAP_CLASS_WITH_BASES by (a) using Boost.Fusion to define some 
kind of tuple, and then (b) using some other means to turn that tuple 
into the class I need, then I expect it would all be done in one place, 
_viz._ inside the definition of quince's private macro 
QUINCE_DEFINE_CLASS_MAPPER_WITH_BASES.  So I would be introducing a 
dependency on Boost.Fusion for the sake of changing one point in my code.

--- Michael