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

Subject: Re: [geometry] Boost.Geometry models UDLs
From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 2014-01-30 18:14:35

Hi,

2014-01-30 Barend Gehrels <barend_at_[hidden]>:

>
> Adam Wulkiewicz wrote On 30-1-2014 15:32:
>
>  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)
>
>
Ok, thanks. This expression is quite simple, I think it could be easily
implemented without Proto. Wouldn't including Proto significantly increase
the compilation time? I fear that nobody would use it. But now with C++11
something similar could be easily achieved with unified initialization and
initializer lists:

ring_type ring{{16,1},{15,2},{14,3},{13,4},{12,3.14},{1,6}};

// not entirely sure but maybe something like this:
polygon_type
polygon{{{0,0},{0,10},{10,10},{10,0},{0,0}},{{1,1},{2,1},{2,2},{1,2},{1,1}}};

and btw we should probably support it anyway.

Regards,
Adam