boost/proto/transform/when.hpp
#ifndef BOOST_PP_IS_ITERATING
///////////////////////////////////////////////////////////////////////////////
/// \file when.hpp
/// Definition of when transform.
//
// Copyright 2008 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)
#ifndef BOOST_PROTO_TRANSFORM_WHEN_HPP_EAN_10_29_2007
#define BOOST_PROTO_TRANSFORM_WHEN_HPP_EAN_10_29_2007
#include <boost/preprocessor/cat.hpp>
#include <boost/preprocessor/repetition/enum_params.hpp>
#include <boost/preprocessor/repetition/enum_trailing_params.hpp>
#include <boost/preprocessor/iteration/iterate.hpp>
#include <boost/mpl/if.hpp>
#include <boost/proto/proto_fwd.hpp>
#include <boost/proto/traits.hpp>
#include <boost/proto/transform/call.hpp>
#include <boost/proto/transform/make.hpp>
#include <boost/proto/transform/impl.hpp>
namespace boost { namespace proto
{
/// \brief A grammar element and a PrimitiveTransform that associates
/// a transform with the grammar.
///
/// Use <tt>when\<\></tt> to override a grammar's default transform
/// with a custom transform. It is for used when composing larger
/// transforms by associating smaller transforms with individual
/// rules in your grammar, as in the following transform which
/// counts the number of terminals in an expression.
///
/// \code
/// // Count the terminals in an expression tree.
/// // Must be invoked with initial state == mpl::int_<0>().
/// struct CountLeaves
/// : or_<
/// when<terminal<_>, mpl::next<_state>()>
/// , otherwise<fold<_, _state, CountLeaves> >
/// >
/// {};
/// \endcode
///
/// In <tt>when\<G, T\></tt>, when \c T is a class type it is a
/// PrimitiveTransform and the following equivalencies hold:
///
/// <tt>boost::result_of\<when\<G,T\>(E,S,V)\>::::type</tt> is the same as
/// <tt>boost::result_of\<T(E,S,V)\>::::type</tt>.
///
/// <tt>when\<G,T\>()(e,s,d)</tt> is the same as
/// <tt>T()(e,s,d)</tt>.
template<typename Grammar, typename PrimitiveTransform /*= Grammar*/>
struct when
: PrimitiveTransform
{
typedef typename Grammar::proto_base_expr proto_base_expr;
};
/// \brief A specialization that treats function pointer Transforms as
/// if they were function type Transforms.
///
/// This specialization requires that \c Fun is actually a function type.
///
/// This specialization is required for nested transforms such as
/// <tt>when\<G, T0(T1(_))\></tt>. In C++, functions that are used as
/// parameters to other functions automatically decay to funtion
/// pointer types. In other words, the type <tt>T0(T1(_))</tt> is
/// indistinguishable from <tt>T0(T1(*)(_))</tt>. This specialization
/// is required to handle these nested function pointer type transforms
/// properly.
template<typename Grammar, typename Fun>
struct when<Grammar, Fun *>
: when<Grammar, Fun>
{};
/// \brief Syntactic sugar for <tt>when\<_, Fun\></tt>, for use
/// in grammars to handle all the cases not yet handled.
///
/// Use <tt>otherwise\<T\></tt> in your grammars as a synonym for
/// <tt>when\<_, T\></tt> as in the following transform which
/// counts the number of terminals in an expression.
///
/// \code
/// // Count the terminals in an expression tree.
/// // Must be invoked with initial state == mpl::int_<0>().
/// struct CountLeaves
/// : or_<
/// when<terminal<_>, mpl::next<_state>()>
/// , otherwise<fold<_, _state, CountLeaves> >
/// >
/// {};
/// \endcode
template<typename Fun>
struct otherwise
: when<_, Fun>
{};
#define BOOST_PP_ITERATION_PARAMS_1 (3, (0, BOOST_PROTO_MAX_ARITY, <boost/proto/transform/when.hpp>))
#include BOOST_PP_ITERATE()
}} // namespace boost::proto
#endif
#else
#define N BOOST_PP_ITERATION()
/// \brief A grammar element and a PrimitiveTransform that associates
/// a transform with the grammar.
///
/// Use <tt>when\<\></tt> to override a grammar's default transform
/// with a custom transform. It is for used when composing larger
/// transforms by associating smaller transforms with individual
/// rules in your grammar, as in the following transform which
/// counts the number of terminals in an expression.
///
/// \code
/// // Count the terminals in an expression tree.
/// // Must be invoked with initial state == mpl::int_<0>().
/// struct CountLeaves
/// : or_<
/// when<terminal<_>, mpl::next<_state>()>
/// , otherwise<fold<_, _state, CountLeaves> >
/// >
/// {};
/// \endcode
///
/// The <tt>when\<G, R(A0,A1,...)\></tt> form accepts either a
/// CallableTransform or an ObjectTransform as its second parameter.
/// <tt>when\<\></tt> uses <tt>is_callable\<R\>::::value</tt> to
/// distinguish between the two, and uses <tt>call\<\></tt> to
/// evaluate CallableTransforms and <tt>make\<\></tt> to evaluate
/// ObjectTransforms.
template<typename Grammar, typename R BOOST_PP_ENUM_TRAILING_PARAMS(N, typename A)>
struct when<Grammar, R(BOOST_PP_ENUM_PARAMS(N, A))>
: transform<when<Grammar, R(BOOST_PP_ENUM_PARAMS(N, A))> >
{
typedef typename Grammar::proto_base_expr proto_base_expr;
// Note: do not evaluate is_callable<R> in this scope.
// R may be an incomplete type at this point.
template<typename Expr, typename State, typename Data>
struct impl : transform_impl<Expr, State, Data>
{
// OK to evaluate is_callable<R> here. R should be compete by now.
typedef
typename mpl::if_c<
is_callable<R>::value
, call<R(BOOST_PP_ENUM_PARAMS(N, A))> // "R" is a function to call
, make<R(BOOST_PP_ENUM_PARAMS(N, A))> // "R" is an object to construct
>::type
which;
typedef typename which::template impl<Expr, State, Data>::result_type result_type;
/// Evaluate <tt>R(A0,A1,...)</tt> as a transform either with
/// <tt>call\<\></tt> or with <tt>make\<\></tt> depending on
/// whether <tt>is_callable\<R\>::::value</tt> is \c true or
/// \c false.
///
/// \param e The current expression
/// \param s The current state
/// \param d An arbitrary data
/// \pre <tt>matches\<Expr, Grammar\>::::value</tt> is \c true
/// \return <tt>which()(e, s, d)</tt>
result_type operator ()(
typename impl::expr_param e
, typename impl::state_param s
, typename impl::data_param d
) const
{
return typename which::template impl<Expr, State, Data>()(e, s, d);
}
};
};
#undef N
#endif
