Re: [boost] [offtopic] user-defined raw string literals

Subject: Re: [boost] [offtopic] user-defined raw string literals
From: Abel Sinkovics (abel_at_[hidden])
Date: 2014-02-01 03:54:08

Hi Adam,

On 2014-01-30 17:25, Adam Wulkiewicz wrote:
> Ok so now the tricky part. It should be parsed in compile-time, the 
> type of the geometry retrieved from the string literal and proper type 
> returned (bg::polygon<> in this case).
> Unfortunately string literals can't be handled as raw literals, e.g. 
> using the variadic-template version of operator"":
>
> template <char...>
> void operator "" ()
>
> Why? Is there a reason for this?
> If we had the ability to use the compile-time version with string 
> literals it would be possible to implement various compile-time 
> parsers, not only for the mentioned WKT.
>
> So what do you think about it?

I don't know the reason for the string literal limitation, but I have a 
workaround for it. You can use the MPLLIBS_STRING macro from Metaparse - 
not in Boost yet. (see 
http://abel.web.elte.hu/mpllibs/metaparse/manual.html#the-input-of-the-parsers 
and http://abel.web.elte.hu/mpllibs/metaparse/MPLLIBS_STRING.html). 
Using that you can parse string literals at compile-time.

Metaparse itself is a library supporting the creation of parsers for 
DSLs that are parsed at compile time. You might be able to create an 
interface like this using that:

auto polygon = WKT("POLYGON((30 10,40 40,20 40,10 20,30 10))");
auto point = WKT("POINT(30 10)");
// ...

where WKT is a macro expanding to a template metaprogram-based parser 
invocation. If you are interested in trying it out, there is a tutorial 
which helps you getting started: 
https://github.com/sabel83/metaparse_tutorial#metaparse-tutorial.

Regards,
   Ábel