boost/spirit/core/composite/optional.hpp
/*=============================================================================
Copyright (c) 1998-2003 Joel de Guzman
Copyright (c) 2001 Daniel Nuffer
Copyright (c) 2002 Hartmut Kaiser
http://spirit.sourceforge.net/
Use, modification and distribution is subject to 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)
=============================================================================*/
#if !defined(BOOST_SPIRIT_OPTIONAL_HPP)
#define BOOST_SPIRIT_OPTIONAL_HPP
#include <boost/spirit/core/parser.hpp>
#include <boost/spirit/core/primitives/primitives.hpp>
#include <boost/spirit/core/composite/composite.hpp>
#include <boost/spirit/meta/as_parser.hpp>
namespace boost { namespace spirit {
///////////////////////////////////////////////////////////////////////////
//
// optional class
//
// Handles expressions of the form:
//
// !a
//
// where a is a parser. The expression returns a composite
// parser that matches its subject zero (0) or one (1) time.
//
///////////////////////////////////////////////////////////////////////////
struct optional_parser_gen;
template <typename S>
struct optional
: public unary<S, parser<optional<S> > >
{
typedef optional<S> self_t;
typedef unary_parser_category parser_category_t;
typedef optional_parser_gen parser_generator_t;
typedef unary<S, parser<self_t> > base_t;
optional(S const& a)
: base_t(a) {}
template <typename ScannerT>
typename parser_result<self_t, ScannerT>::type
parse(ScannerT const& scan) const
{
typedef typename parser_result<self_t, ScannerT>::type result_t;
typedef typename ScannerT::iterator_t iterator_t;
iterator_t save = scan.first;
if (result_t r = this->subject().parse(scan))
{
return r;
}
else
{
scan.first = save;
return scan.empty_match();
}
}
};
struct optional_parser_gen
{
template <typename S>
struct result
{
typedef optional<S> type;
};
template <typename S>
static optional<S>
generate(parser<S> const& a)
{
return optional<S>(a.derived());
}
};
template <typename S>
optional<S>
operator!(parser<S> const& a);
}} // namespace boost::spirit
#endif
#include <boost/spirit/core/composite/impl/optional.ipp>
