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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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