Geometry Boost List: Re: [geometry] Boost.Geometry models UDLs

Subject: Re: [geometry] Boost.Geometry models UDLs
From: Barend Gehrels (barend_at_[hidden])
Date: 2014-01-30 16:57:10

Hi Adam,

Adam Wulkiewicz wrote On 30-1-2014 15:32:
> Hi,
>
> I was wondering if it would be possible to implement some nice UDL for 
> BG models. I thought it would be great to have a compile-time WKT 
> which whould work e.g. like this:
>
> "LINESTRING(0 0, 1 1, 2 2)"_wkt
>
> if we could parse this in compile-time we'd be able to return proper 
> type. Unfortunately AFAIK it's (currently?) impossible to parse string 
> literals in compile-time. If you could proof me wrong I'd be very glad 
> :). So I've implemented some simplified version which can be found here:
>
> https://github.com/awulkiew/test_boost_geometry_udl
>
> Any ideas are welcome. It works like this:
> |using namespace bg::literals::cart2d;
>
> "(1 1)"_pt; // point<double, 2, cs::cartesian>
> "(0 0, 2 2)"_box; // box<p2d>
> "(0 0, 1 1, 2 1)"_ls; // linestring<p2d>
>
> // polygon<p2d> - for now the default cw closed, should probably be ccw
> "((1 0, 1 3, 3 3, 3 0, 1 0))"_poly
>
> // ring<p2d> - for now the default cw closed, should probably be ccw
> // also non-WKT definition could be used - without doubled parentheses
> "((1 0, 1 3, 3 3, 3 0, 1 0))"_ring|
> Of course you must compile it with some compiler supporting 
> user-defined literals, e.g. GCC >= 4.7 or Clang >= 3.1

Interesting.

Note that I did some experiments in the past to get a similar effect, 
though implemented differently. If we would use Proto we are able to 
compile things like this:

     // Initialize a ring:
     typedef bg::model::ring<bg::model::d2::point_xy<double> > ring_type;
     ring_type ring = geometry_of(16, 1)(15,2)(14, 3)(13,4)(12, 3.14)(1,6);
     std::cout << bg::wkt(ring) << std::endl;

It does obviously not have the WKT syntax, but it should be possible to 
parse any geometry type with this.

See ....\extensions\example\experimental\geometry_of.cpp
(it is unfinished)

Regards, Barend