Re: Usage of auto keyword in X3 tutorials

Baptiste Wicht <[email protected]> Fri, 11 Nov 2016 15:26:15 +0100
Newsgroups	gmane.comp.parsers.spirit.general
Message-ID	<20161111142615.GA25813@frigg>
Hi Stephan,

For the particular example, you can probably get around using auto by
using std::function. The problem here is not related to spirit X3 but to
the use of lambda expressions for which you cannot know the type but you
store them in a std::function of the correct type. Replacing auto
functors by std::function is likely to be detrimental to performance
though.

If you also don't like generic lambda, you can compute the type of
context using typedefs without too much issue to avoid the "auto& ctx",
but I don't really see the point.

But you gonna have other problems if you don't like auto if you have
a big grammar. For instance, if you look at this:
http://ciere.com/cppnow15/x3_docs/spirit/tutorials/rexpr.html With
larger grammars, you are going to use auto for the definitions of the
rules. In that particular case, the type of the definitions should only
be deduced, never written even with typedefs... Unless you can to use
decltype, but I guess you don't like it either. In that case, I don't
know how to do it without auto, but there is probably a solution.

Honestly, since x3 is a C++14 library and makes extensive use of modern
C++, I would not use it without using auto. Using x3 without auto would
feel like a crippled version of x3, in my opinion (just like C++>11
without auto is crippled).

Moreover, auto usage in Spirit is not only related to x3. In Qi, you
could already use auto rules in order to improve performance.

Hope that helps

Baptiste

On Fri, Nov 11, 2016 at 01:55:46PM +0100, Stephan Menzel wrote:
> Hello everyone,
>
> as a long term user of spirit I finally wanted to take a lot at the X3 docs
> to see what changes.
> What I noticed quickly is the abundance of "auto" in the tutorials which
> leads me to believe that the ease of use is probably in parts due to this.
>
> Am I right with this? My problem is a deep distrust of auto and I have
> banned it in all our code.
> Looking at this for example:
> http://ciere.com/cppnow15/x3_docs/spirit/tutorials/complex___our_first_complex_parser.html
>
> Would this still look easy if instead of auto there would be actual types
> or typedefs?
>
> Or, to phrase my question more tongue in cheek: Would auto haters like the
> step towards X3?
>
> Thanks for opinions..
>
> Stephan

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