Re: [Boost-users] [proto] Question on grammar and operator overloading

Hal Finkel wrote:

Hello,

I'm writing a complex number class using boost::proto by extending proto::terminal< boost::array<_, 2> >. I've attached a copy of my current code, which compiles, runs, and (at least with both g++ and icc on my machine) demonstrates a significant speed improvement compared to std::complex.

However, there is (at least) one thing which don't work as it should:

In the grammar I have to specify lazy_complex< double > instead of lazy_complex< _ >, otherwise the compiler cannot find proto's operator overloads, why?

In your grammar, lazy_complex<_> is equivalent to proto::terminal< boost::array< _, 2 > >; that is, they describe the same set of expression types. Unfortunately, Proto cannot handle a grammar like that. The problem is the "2". Proto can only peer inside template instantiations as long as none of the template parameters are non-type template parameters. There are a number of hackish solutions. You could define your own type: template > struct my_array : boost::array< T, Size::value > {}; and use my_array throughout. Then your grammar becomes proto::terminal< boost::array< _, mpl::int_<2> > >, or just proto::terminal< boost::array< _, _ > >. The other alternative is to define a predicate that matches boost::array types: template<typename T> struct is_boost_array : mpl::false_ {}; template struct is_boost_array > : mpl::true_ {}; and change your grammar to: proto::and_< proto::terminal<_>, proto::if_proto::_value()>

Either way. You could even just use some other terminal type besides boost::array. std::complex or std::pair would do.

Note: This code will not compile under 1.37, but will compile under 1.40, because of some issue in 1.37 which manifests itself as a problem with the overloading of the '<<' operator. I've not tested the releases in between 1.37 and 1.40.

Yes, I've fixed some bugs in this area. -- Eric Niebler BoostPro Computing http://www.boostpro.com