Hello, On Mon, Apr 9, 2012 at 7:00 PM, Eric Niebler <e...@boostpro.com> wrote:
> On 4/9/2012 2:21 PM, Fernando Pelliccioni wrote: > > Hello, > > > > I'm wondering if it would be appropriate to treat the fundamental types > > (char, short, int, double, ...) by value, by default. > > > > I wrote this simple piece of code. > > I'm not sure if I'm leaving without considering any other implication, > > but I think it may be an improvement. > > Please, tell me if I am wrong. > > Thanks. I thought long about whether to handle the fundamental types > differently than user-defined types and decided against it. The > capture-everything-by-reference-by-default model is easy to explain and > reason about. Special cases can be handled on a per-domain basis as needed. > > Thanks for your reply. The design philosophy is unobjectionable. > There is a way to change the capture behavior for your domain. The newly > released version of Proto documents how to do this (although the > functionality has been there for a few releases already). > > > http://www.boost.org/doc/libs/1_49_0/doc/html/proto/users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child > > I had read that chapter, but I thought it might be the behavior of default_domain. > In short, you'll need to define an as_child metafunction in your domain > definition: > > class my_domain > : proto::domain< my_generator, my_grammar > > { > // Here is where you define how Proto should handle > // sub-expressions that are about to be glommed into > // a larger expression. > template< typename T > > struct as_child > { > typedef unspecified-Proto-expr-type result_type; > > result_type operator()( T & t ) const > { > return unspecified-Proto-expr-object; > } > }; > }; > > In as_child, you'll have to do this (pseudocode): > > if (is_expr<T>) > return T & > else if(is_fundamental<T>) > return proto::terminal<T>::type > else > return proto::terminal<T &>::type > > Done! I implemented it by defining as_child in my domain. > The metaprogramming is left as an exercise. :-) > > ... Happy metaprogramming :) Thanks for your clarification. Fernando.
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