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- //[ CheckedCalc
- // Copyright 2011 Eric Niebler. Distributed under the Boost
- // Software License, Version 1.0. (See accompanying file
- // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
- //
- // This is an example of how to specify a transform externally so
- // that a single grammar can be used to drive multiple differnt
- // calculations. In particular, it defines a calculator grammar
- // that computes the result of an expression with either checked
- // or non-checked division.
- #include <iostream>
- #include <boost/assert.hpp>
- #include <boost/mpl/int.hpp>
- #include <boost/mpl/next.hpp>
- #include <boost/mpl/min_max.hpp>
- #include <boost/fusion/container/vector.hpp>
- #include <boost/fusion/container/generation/make_vector.hpp>
- #include <boost/proto/proto.hpp>
- namespace mpl = boost::mpl;
- namespace proto = boost::proto;
- namespace fusion = boost::fusion;
- // The argument placeholder type
- template<typename I> struct placeholder : I {};
- // Give each rule in the grammar a "name". This is so that we
- // can easily dispatch on it later.
- struct calc_grammar;
- struct divides_rule : proto::divides<calc_grammar, calc_grammar> {};
- // Use external transforms in calc_gramar
- struct calc_grammar
- : proto::or_<
- proto::when<
- proto::terminal<placeholder<proto::_> >
- , proto::functional::at(proto::_state, proto::_value)
- >
- , proto::when<
- proto::terminal<proto::convertible_to<double> >
- , proto::_value
- >
- , proto::when<
- proto::plus<calc_grammar, calc_grammar>
- , proto::_default<calc_grammar>
- >
- , proto::when<
- proto::minus<calc_grammar, calc_grammar>
- , proto::_default<calc_grammar>
- >
- , proto::when<
- proto::multiplies<calc_grammar, calc_grammar>
- , proto::_default<calc_grammar>
- >
- // Note that we don't specify how division nodes are
- // handled here. Proto::external_transform is a placeholder
- // for an actual transform.
- , proto::when<
- divides_rule
- , proto::external_transform
- >
- >
- {};
- template<typename E> struct calc_expr;
- struct calc_domain : proto::domain<proto::generator<calc_expr> > {};
- template<typename E>
- struct calc_expr
- : proto::extends<E, calc_expr<E>, calc_domain>
- {
- calc_expr(E const &e = E()) : calc_expr::proto_extends(e) {}
- };
- calc_expr<proto::terminal<placeholder<mpl::int_<0> > >::type> _1;
- calc_expr<proto::terminal<placeholder<mpl::int_<1> > >::type> _2;
- // Use proto::external_transforms to map from named grammar rules to
- // transforms.
- struct non_checked_division
- : proto::external_transforms<
- proto::when< divides_rule, proto::_default<calc_grammar> >
- >
- {};
- struct division_by_zero : std::exception {};
- struct do_checked_divide
- : proto::callable
- {
- typedef int result_type;
- int operator()(int left, int right) const
- {
- if (right == 0) throw division_by_zero();
- return left / right;
- }
- };
- // Use proto::external_transforms again, this time to map the divides_rule
- // to a transforms that performs checked division.
- struct checked_division
- : proto::external_transforms<
- proto::when<
- divides_rule
- , do_checked_divide(calc_grammar(proto::_left), calc_grammar(proto::_right))
- >
- >
- {};
- int main()
- {
- non_checked_division non_checked;
- int result2 = calc_grammar()(_1 / _2, fusion::make_vector(6, 2), non_checked);
- BOOST_ASSERT(result2 == 3);
- try
- {
- checked_division checked;
- // This should throw
- int result3 = calc_grammar()(_1 / _2, fusion::make_vector(6, 0), checked);
- BOOST_ASSERT(false); // shouldn't get here!
- }
- catch(division_by_zero)
- {
- std::cout << "caught division by zero!\n";
- }
- }
- //]
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