Hi Eric, On Jan 11, 9:33 pm, Eric Niebler <e...@boostpro.com> wrote:
See attached.
Thank you very much for the example, it definitely works and does even more that I was expecting to achieve in this example. However, just in order to make it more understandable (to me) at the beginning I want to go even simpler that what you did. I understand that proto is ideal for recursive expression (grammars?) but I just want to define a fixed/rigid grammar that only accepts a rigid expression of the type of 4. + 5.*i. Besides that I don't want proto to evaluate things for me, I want to be able to "see" the expression structure and contained values from a single place. In other words I want a glorified (simulated) constructor. std::complex<double> z = 5. + 6.*i; I guess the "grammar" is simply (correct me if I am wrong): struct i_tag{}; struct complex_cartesian_grammar : proto::plus< proto::terminal<double>, proto::multiplies<proto::terminal<double>, proto::terminal<i_tag> > > {}; yes, it is a very stupid rigid expression. (It just should accept a certain expression.). For this, I striped down your version, to the point it didn't compile but I think the intention of the code is there. The complete code is attached below. In my opinion... what it has: 1) defines abstract symbol "i" 2) defines the only allowed expression in the syntax. 3) defines the domain (I don't know what this does) what it lacks: 4) be able to compile 5) proper conversion of the expression tree to extract the relevant data. 6) expression matches verification?? for the future: 7) allow a few more allowable expressions compatible with the constructor of std::complex<double>, just of the types 4.+5.*i, 4., 5.*i and that's it. Sorry to using your powerful library for such an unpowerful application, I am just trying to learn. The current code gives naturally this error: error: conversion from ‘const boost::proto::exprns_::expr<boost::proto::tag::plus, ... >’ to non- scalar type ‘std::complex<double>’ requested The code follows // Thank you // Alfredo #include <complex> #include <boost/proto/proto.hpp> namespace proto=boost::proto; namespace pretty{ struct i_tag{}; struct complex_cartesian_grammar : proto::plus< proto::terminal<double>, proto::multiplies<proto::terminal<double>, proto::terminal<i_tag> > > {}; template< typename Expr > struct complex_cartesian_expr; struct complex_cartesian_domain : proto::domain< proto::pod_generator< complex_cartesian_expr >, complex_cartesian_grammar > {}; template<typename Expr> struct complex_cartesian_expr { BOOST_PROTO_EXTENDS( Expr, complex_cartesian_expr, complex_cartesian_domain ) template<typename T> operator std::complex<T>() const{ return std::complex<double>(proto::child_c<0>(*this), proto::child_c<0>(*this)); } }; proto::terminal<i_tag>::type const i = {{}}; } int main(){ using namespace pretty; std::complex<double> z = 5. + 6.*i; std::cout << z << std::endl; return 0; }