123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269 |
- // 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
|