boost/lambda/detail/return_type_traits.hpp
// return_type_traits.hpp -- Boost Lambda Library ---------------------------
// Copyright (C) 1999, 2000 Jaakko Jarvi (jaakko.jarvi@cs.utu.fi)
//
// 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)
//
// For more information, see www.boost.org
#ifndef BOOST_LAMBDA_RETURN_TYPE_TRAITS_HPP
#define BOOST_LAMBDA_RETURN_TYPE_TRAITS_HPP
#include "boost/mpl/has_xxx.hpp"
#include <cstddef> // needed for the ptrdiff_t
namespace boost {
namespace lambda {
// Much of the type deduction code for standard arithmetic types
// from Gary Powell
// different arities:
template <class Act, class A1> struct return_type_1; // 1-ary actions
template <class Act, class A1, class A2> struct return_type_2; // 2-ary
template <class Act, class Args> struct return_type_N; // >3- ary
template <class Act, class A1> struct return_type_1_prot;
template <class Act, class A1, class A2> struct return_type_2_prot; // 2-ary
template <class Act, class A1> struct return_type_N_prot; // >3-ary
namespace detail {
template<class> class return_type_deduction_failure {};
// In some cases return type deduction should fail (an invalid lambda
// expression). Sometimes the lambda expression can be ok, the return type
// just is not deducible (user defined operators). Then return type deduction
// should never be entered at all, and the use of ret<> does this.
// However, for nullary lambda functors, return type deduction is always
// entered, and there seems to be no way around this.
// (the return type is part of the prototype of the non-template
// operator()(). The prototype is instantiated, even though the body
// is not.)
// So, in the case the return type deduction should fail, it should not
// fail directly, but rather result in a valid but wrong return type,
// causing a compile time error only if the function is really called.
} // end detail
// return_type_X_prot classes --------------------------------------------
// These classes are the first layer that gets instantiated from the
// lambda_functor_base sig templates. It will check whether
// the action is protectable and one of arguments is "protected" or its
// evaluation will otherwise result in another lambda functor.
// If this is a case, the result type will be another lambda functor.
// The arguments are always non-reference types, except for comma action
// where the right argument can be a reference too. This is because it
// matters (in the builtin case) whether the argument is an lvalue or
// rvalue: int i; i, 1 -> rvalue; 1, i -> lvalue
template <class Act, class A> struct return_type_1_prot {
public:
typedef typename
detail::IF<
is_protectable<Act>::value && is_lambda_functor<A>::value,
lambda_functor<
lambda_functor_base<
Act,
tuple<typename detail::remove_reference_and_cv<A>::type>
>
>,
typename return_type_1<Act, A>::type
>::RET type;
};
// take care of the unavoidable instantiation for nullary case
template<class Act> struct return_type_1_prot<Act, null_type> {
typedef null_type type;
};
// Unary actions (result from unary operators)
// do not have a default return type.
template<class Act, class A> struct return_type_1 {
typedef typename
detail::return_type_deduction_failure<return_type_1> type;
};
namespace detail {
template <class T>
class protect_conversion {
typedef typename boost::remove_reference<T>::type non_ref_T;
public:
// add const to rvalues, so that all rvalues are stored as const in
// the args tuple
typedef typename detail::IF_type<
boost::is_reference<T>::value && !boost::is_const<non_ref_T>::value,
detail::identity_mapping<T>,
const_copy_argument<non_ref_T> // handles funtion and array
>::type type; // types correctly
};
} // end detail
template <class Act, class A, class B> struct return_type_2_prot {
// experimental feature
// We may have a lambda functor as a result type of a subexpression
// (if protect) has been used.
// Thus, if one of the parameter types is a lambda functor, the result
// is a lambda functor as well.
// We need to make a conservative choise here.
// The resulting lambda functor stores all const reference arguments as
// const copies. References to non-const are stored as such.
// So if the source of the argument is a const open argument, a bound
// argument stored as a const reference, or a function returning a
// const reference, that information is lost. There is no way of
// telling apart 'real const references' from just 'LL internal
// const references' (or it would be really hard)
// The return type is a subclass of lambda_functor, which has a converting
// copy constructor. It can copy any lambda functor, that has the same
// action type and code, and a copy compatible argument tuple.
typedef typename boost::remove_reference<A>::type non_ref_A;
typedef typename boost::remove_reference<B>::type non_ref_B;
typedef typename
detail::IF<
is_protectable<Act>::value &&
(is_lambda_functor<A>::value || is_lambda_functor<B>::value),
lambda_functor<
lambda_functor_base<
Act,
tuple<typename detail::protect_conversion<A>::type,
typename detail::protect_conversion<B>::type>
>
>,
typename return_type_2<Act, non_ref_A, non_ref_B>::type
>::RET type;
};
// take care of the unavoidable instantiation for nullary case
template<class Act> struct return_type_2_prot<Act, null_type, null_type> {
typedef null_type type;
};
// take care of the unavoidable instantiation for nullary case
template<class Act, class Other> struct return_type_2_prot<Act, Other, null_type> {
typedef null_type type;
};
// take care of the unavoidable instantiation for nullary case
template<class Act, class Other> struct return_type_2_prot<Act, null_type, Other> {
typedef null_type type;
};
// comma is a special case, as the user defined operator can return
// an lvalue (reference) too, hence it must be handled at this level.
template<class A, class B>
struct return_type_2_comma
{
typedef typename boost::remove_reference<A>::type non_ref_A;
typedef typename boost::remove_reference<B>::type non_ref_B;
typedef typename
detail::IF<
is_protectable<other_action<comma_action> >::value && // it is protectable
(is_lambda_functor<A>::value || is_lambda_functor<B>::value),
lambda_functor<
lambda_functor_base<
other_action<comma_action>,
tuple<typename detail::protect_conversion<A>::type,
typename detail::protect_conversion<B>::type>
>
>,
typename
return_type_2<other_action<comma_action>, non_ref_A, non_ref_B>::type
>::RET type1;
// if no user defined return_type_2 (or plain_return_type_2) specialization
// matches, then return the righthand argument
typedef typename
detail::IF<
boost::is_same<type1, detail::unspecified>::value,
B,
type1
>::RET type;
};
// currently there are no protectable actions with > 2 args
template<class Act, class Args> struct return_type_N_prot {
typedef typename return_type_N<Act, Args>::type type;
};
// take care of the unavoidable instantiation for nullary case
template<class Act> struct return_type_N_prot<Act, null_type> {
typedef null_type type;
};
// handle different kind of actions ------------------------
// use the return type given in the bind invocation as bind<Ret>(...)
template<int I, class Args, class Ret>
struct return_type_N<function_action<I, Ret>, Args> {
typedef Ret type;
};
// ::result_type support
namespace detail
{
BOOST_MPL_HAS_XXX_TRAIT_DEF(result_type)
template<class F> struct get_result_type
{
typedef typename F::result_type type;
};
template<class F, class A> struct get_sig
{
typedef typename function_adaptor<F>::template sig<A>::type type;
};
} // namespace detail
// Ret is detail::unspecified, so try to deduce return type
template<int I, class Args>
struct return_type_N<function_action<I, detail::unspecified>, Args > {
// in the case of function action, the first element in Args is
// some type of function
typedef typename Args::head_type Func;
typedef typename detail::remove_reference_and_cv<Func>::type plain_Func;
public:
// pass the function to function_adaptor, and get the return type from
// that
typedef typename detail::IF<
detail::has_result_type<plain_Func>::value,
detail::get_result_type<plain_Func>,
detail::get_sig<plain_Func, Args>
>::RET::type type;
};
} // namespace lambda
} // namespace boost
#endif