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