326 lines
10 KiB
C++
326 lines
10 KiB
C++
// Boost Lambda Library ret.hpp -----------------------------------------
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// Copyright (C) 1999, 2000 Jaakko Jarvi (jaakko.jarvi@cs.utu.fi)
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//
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// Distributed under the Boost Software License, Version 1.0. (See
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// accompanying file LICENSE_1_0.txt or copy at
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// http://www.boost.org/LICENSE_1_0.txt)
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//
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// For more information, see www.boost.org
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#ifndef BOOST_LAMBDA_RET_HPP
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#define BOOST_LAMBDA_RET_HPP
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namespace boost {
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namespace lambda {
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// TODO:
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// Add specializations for function references for ret, protect and unlambda
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// e.g void foo(); unlambda(foo); fails, as it would add a const qualifier
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// for a function type.
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// on the other hand unlambda(*foo) does work
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// -- ret -------------------------
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// the explicit return type template
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// TODO: It'd be nice to make ret a nop for other than lambda functors
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// but causes an ambiguiyty with gcc (not with KCC), check what is the
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// right interpretation.
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// // ret for others than lambda functors has no effect
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// template <class U, class T>
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// inline const T& ret(const T& t) { return t; }
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template<class RET, class Arg>
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inline const
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lambda_functor<
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lambda_functor_base<
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explicit_return_type_action<RET>,
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tuple<lambda_functor<Arg> >
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>
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>
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ret(const lambda_functor<Arg>& a1)
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{
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return
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lambda_functor_base<
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explicit_return_type_action<RET>,
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tuple<lambda_functor<Arg> >
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>
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(tuple<lambda_functor<Arg> >(a1));
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}
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// protect ------------------
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// protecting others than lambda functors has no effect
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template <class T>
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inline const T& protect(const T& t) { return t; }
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template<class Arg>
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inline const
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lambda_functor<
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lambda_functor_base<
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protect_action,
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tuple<lambda_functor<Arg> >
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>
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>
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protect(const lambda_functor<Arg>& a1)
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{
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return
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lambda_functor_base<
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protect_action,
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tuple<lambda_functor<Arg> >
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>
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(tuple<lambda_functor<Arg> >(a1));
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}
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// -------------------------------------------------------------------
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// Hides the lambda functorness of a lambda functor.
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// After this, the functor is immune to argument substitution, etc.
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// This can be used, e.g. to make it safe to pass lambda functors as
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// arguments to functions, which might use them as target functions
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// note, unlambda and protect are different things. Protect hides the lambda
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// functor for one application, unlambda for good.
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template <class LambdaFunctor>
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class non_lambda_functor
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{
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LambdaFunctor lf;
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public:
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// This functor defines the result_type typedef.
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// The result type must be deducible without knowing the arguments
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template <class SigArgs> struct sig {
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typedef typename
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LambdaFunctor::inherited::
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template sig<typename SigArgs::tail_type>::type type;
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};
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explicit non_lambda_functor(const LambdaFunctor& a) : lf(a) {}
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typename LambdaFunctor::nullary_return_type
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operator()() const {
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return lf.template
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call<typename LambdaFunctor::nullary_return_type>
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(cnull_type(), cnull_type(), cnull_type(), cnull_type());
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}
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template<class A>
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typename sig<tuple<const non_lambda_functor, A&> >::type
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operator()(A& a) const {
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return lf.template call<typename sig<tuple<const non_lambda_functor, A&> >::type >(a, cnull_type(), cnull_type(), cnull_type());
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}
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template<class A, class B>
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typename sig<tuple<const non_lambda_functor, A&, B&> >::type
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operator()(A& a, B& b) const {
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return lf.template call<typename sig<tuple<const non_lambda_functor, A&, B&> >::type >(a, b, cnull_type(), cnull_type());
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}
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template<class A, class B, class C>
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typename sig<tuple<const non_lambda_functor, A&, B&, C&> >::type
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operator()(A& a, B& b, C& c) const {
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return lf.template call<typename sig<tuple<const non_lambda_functor, A&, B&, C&> >::type>(a, b, c, cnull_type());
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}
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};
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template <class Arg>
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inline const Arg& unlambda(const Arg& a) { return a; }
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template <class Arg>
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inline const non_lambda_functor<lambda_functor<Arg> >
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unlambda(const lambda_functor<Arg>& a)
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{
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return non_lambda_functor<lambda_functor<Arg> >(a);
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}
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// Due to a language restriction, lambda functors cannot be made to
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// accept non-const rvalue arguments. Usually iterators do not return
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// temporaries, but sometimes they do. That's why a workaround is provided.
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// Note, that this potentially breaks const correctness, so be careful!
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// any lambda functor can be turned into a const_incorrect_lambda_functor
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// The operator() takes arguments as consts and then casts constness
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// away. So this breaks const correctness!!! but is a necessary workaround
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// in some cases due to language limitations.
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// Note, that this is not a lambda_functor anymore, so it can not be used
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// as a sub lambda expression.
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template <class LambdaFunctor>
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struct const_incorrect_lambda_functor {
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LambdaFunctor lf;
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public:
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explicit const_incorrect_lambda_functor(const LambdaFunctor& a) : lf(a) {}
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template <class SigArgs> struct sig {
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typedef typename
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LambdaFunctor::inherited::template
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sig<typename SigArgs::tail_type>::type type;
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};
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// The nullary case is not needed (no arguments, no parameter type problems)
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template<class A>
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typename sig<tuple<const const_incorrect_lambda_functor, A&> >::type
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operator()(const A& a) const {
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return lf.template call<typename sig<tuple<const const_incorrect_lambda_functor, A&> >::type >(const_cast<A&>(a), cnull_type(), cnull_type(), cnull_type());
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}
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template<class A, class B>
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typename sig<tuple<const const_incorrect_lambda_functor, A&, B&> >::type
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operator()(const A& a, const B& b) const {
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return lf.template call<typename sig<tuple<const const_incorrect_lambda_functor, A&, B&> >::type >(const_cast<A&>(a), const_cast<B&>(b), cnull_type(), cnull_type());
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}
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template<class A, class B, class C>
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typename sig<tuple<const const_incorrect_lambda_functor, A&, B&, C&> >::type
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operator()(const A& a, const B& b, const C& c) const {
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return lf.template call<typename sig<tuple<const const_incorrect_lambda_functor, A&, B&, C&> >::type>(const_cast<A&>(a), const_cast<B&>(b), const_cast<C&>(c), cnull_type());
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}
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};
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// ------------------------------------------------------------------------
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// any lambda functor can be turned into a const_parameter_lambda_functor
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// The operator() takes arguments as const.
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// This is useful if lambda functors are called with non-const rvalues.
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// Note, that this is not a lambda_functor anymore, so it can not be used
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// as a sub lambda expression.
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template <class LambdaFunctor>
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struct const_parameter_lambda_functor {
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LambdaFunctor lf;
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public:
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explicit const_parameter_lambda_functor(const LambdaFunctor& a) : lf(a) {}
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template <class SigArgs> struct sig {
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typedef typename
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LambdaFunctor::inherited::template
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sig<typename SigArgs::tail_type>::type type;
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};
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// The nullary case is not needed: no arguments, no constness problems.
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template<class A>
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typename sig<tuple<const const_parameter_lambda_functor, const A&> >::type
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operator()(const A& a) const {
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return lf.template call<typename sig<tuple<const const_parameter_lambda_functor, const A&> >::type >(a, cnull_type(), cnull_type(), cnull_type());
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}
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template<class A, class B>
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typename sig<tuple<const const_parameter_lambda_functor, const A&, const B&> >::type
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operator()(const A& a, const B& b) const {
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return lf.template call<typename sig<tuple<const const_parameter_lambda_functor, const A&, const B&> >::type >(a, b, cnull_type(), cnull_type());
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}
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template<class A, class B, class C>
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typename sig<tuple<const const_parameter_lambda_functor, const A&, const B&, const C&>
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>::type
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operator()(const A& a, const B& b, const C& c) const {
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return lf.template call<typename sig<tuple<const const_parameter_lambda_functor, const A&, const B&, const C&> >::type>(a, b, c, cnull_type());
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}
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};
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template <class Arg>
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inline const const_incorrect_lambda_functor<lambda_functor<Arg> >
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break_const(const lambda_functor<Arg>& lf)
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{
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return const_incorrect_lambda_functor<lambda_functor<Arg> >(lf);
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}
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template <class Arg>
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inline const const_parameter_lambda_functor<lambda_functor<Arg> >
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const_parameters(const lambda_functor<Arg>& lf)
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{
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return const_parameter_lambda_functor<lambda_functor<Arg> >(lf);
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}
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// make void ------------------------------------------------
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// make_void( x ) turns a lambda functor x with some return type y into
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// another lambda functor, which has a void return type
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// when called, the original return type is discarded
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// we use this action. The action class will be called, which means that
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// the wrapped lambda functor is evaluated, but we just don't do anything
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// with the result.
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struct voidifier_action {
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template<class Ret, class A> static void apply(A&) {}
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};
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template<class Args> struct return_type_N<voidifier_action, Args> {
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typedef void type;
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};
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template<class Arg1>
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inline const
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lambda_functor<
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lambda_functor_base<
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action<1, voidifier_action>,
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tuple<lambda_functor<Arg1> >
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>
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>
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make_void(const lambda_functor<Arg1>& a1) {
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return
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lambda_functor_base<
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action<1, voidifier_action>,
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tuple<lambda_functor<Arg1> >
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>
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(tuple<lambda_functor<Arg1> > (a1));
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}
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// for non-lambda functors, make_void does nothing
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// (the argument gets evaluated immediately)
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template<class Arg1>
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inline const
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lambda_functor<
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lambda_functor_base<do_nothing_action, null_type>
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>
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make_void(const Arg1&) {
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return
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lambda_functor_base<do_nothing_action, null_type>();
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}
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// std_functor -----------------------------------------------------
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// The STL uses the result_type typedef as the convention to let binders know
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// the return type of a function object.
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// LL uses the sig template.
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// To let LL know that the function object has the result_type typedef
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// defined, it can be wrapped with the std_functor function.
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// Just inherit form the template parameter (the standard functor),
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// and provide a sig template. So we have a class which is still the
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// same functor + the sig template.
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template<class T>
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struct result_type_to_sig : public T {
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template<class Args> struct sig { typedef typename T::result_type type; };
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result_type_to_sig(const T& t) : T(t) {}
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};
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template<class F>
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inline result_type_to_sig<F> std_functor(const F& f) { return f; }
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} // namespace lambda
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} // namespace boost
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#endif
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