1// Boost Lambda Library ret.hpp -----------------------------------------
2
3// Copyright (C) 1999, 2000 Jaakko Jarvi (jaakko.jarvi@cs.utu.fi)
4//
5// Distributed under the Boost Software License, Version 1.0. (See
6// accompanying file LICENSE_1_0.txt or copy at
7// http://www.boost.org/LICENSE_1_0.txt)
8//
9// For more information, see www.boost.org
10
11
12#ifndef BOOST_LAMBDA_RET_HPP
13#define BOOST_LAMBDA_RET_HPP
14
15namespace boost {
16namespace lambda {
17
18 // TODO:
19
20// Add specializations for function references for ret, protect and unlambda
21// e.g void foo(); unlambda(foo); fails, as it would add a const qualifier
22 // for a function type.
23 // on the other hand unlambda(*foo) does work
24
25
26// -- ret -------------------------
27// the explicit return type template
28
29 // TODO: It'd be nice to make ret a nop for other than lambda functors
30 // but causes an ambiguiyty with gcc (not with KCC), check what is the
31 // right interpretation.
32
33 // // ret for others than lambda functors has no effect
34 // template <class U, class T>
35 // inline const T& ret(const T& t) { return t; }
36
37
38template<class RET, class Arg>
39inline const
40lambda_functor<
41 lambda_functor_base<
42 explicit_return_type_action<RET>,
43 tuple<lambda_functor<Arg> >
44 >
45>
46ret(const lambda_functor<Arg>& a1)
47{
48 return
49 lambda_functor_base<
50 explicit_return_type_action<RET>,
51 tuple<lambda_functor<Arg> >
52 >
53 (tuple<lambda_functor<Arg> >(a1));
54}
55
56// protect ------------------
57
58 // protecting others than lambda functors has no effect
59template <class T>
60inline const T& protect(const T& t) { return t; }
61
62template<class Arg>
63inline const
64lambda_functor<
65 lambda_functor_base<
66 protect_action,
67 tuple<lambda_functor<Arg> >
68 >
69>
70protect(const lambda_functor<Arg>& a1)
71{
72 return
73 lambda_functor_base<
74 protect_action,
75 tuple<lambda_functor<Arg> >
76 >
77 (tuple<lambda_functor<Arg> >(a1));
78}
79
80// -------------------------------------------------------------------
81
82// Hides the lambda functorness of a lambda functor.
83// After this, the functor is immune to argument substitution, etc.
84// This can be used, e.g. to make it safe to pass lambda functors as
85// arguments to functions, which might use them as target functions
86
87// note, unlambda and protect are different things. Protect hides the lambda
88// functor for one application, unlambda for good.
89
90template <class LambdaFunctor>
91class non_lambda_functor
92{
93 LambdaFunctor lf;
94public:
95
96 // This functor defines the result_type typedef.
97 // The result type must be deducible without knowing the arguments
98
99 template <class SigArgs> struct sig {
100 typedef typename
101 LambdaFunctor::inherited::
102 template sig<typename SigArgs::tail_type>::type type;
103 };
104
105 explicit non_lambda_functor(const LambdaFunctor& a) : lf(a) {}
106
107 typename LambdaFunctor::nullary_return_type
108 operator()() const {
109 return lf.template
110 call<typename LambdaFunctor::nullary_return_type>
111 (cnull_type(), cnull_type(), cnull_type(), cnull_type());
112 }
113
114 template<class A>
115 typename sig<tuple<const non_lambda_functor, A&> >::type
116 operator()(A& a) const {
117 return lf.template call<typename sig<tuple<const non_lambda_functor, A&> >::type >(a, cnull_type(), cnull_type(), cnull_type());
118 }
119
120 template<class A, class B>
121 typename sig<tuple<const non_lambda_functor, A&, B&> >::type
122 operator()(A& a, B& b) const {
123 return lf.template call<typename sig<tuple<const non_lambda_functor, A&, B&> >::type >(a, b, cnull_type(), cnull_type());
124 }
125
126 template<class A, class B, class C>
127 typename sig<tuple<const non_lambda_functor, A&, B&, C&> >::type
128 operator()(A& a, B& b, C& c) const {
129 return lf.template call<typename sig<tuple<const non_lambda_functor, A&, B&, C&> >::type>(a, b, c, cnull_type());
130 }
131};
132
133template <class Arg>
134inline const Arg& unlambda(const Arg& a) { return a; }
135
136template <class Arg>
137inline const non_lambda_functor<lambda_functor<Arg> >
138unlambda(const lambda_functor<Arg>& a)
139{
140 return non_lambda_functor<lambda_functor<Arg> >(a);
141}
142
143 // Due to a language restriction, lambda functors cannot be made to
144 // accept non-const rvalue arguments. Usually iterators do not return
145 // temporaries, but sometimes they do. That's why a workaround is provided.
146 // Note, that this potentially breaks const correctness, so be careful!
147
148// any lambda functor can be turned into a const_incorrect_lambda_functor
149// The operator() takes arguments as consts and then casts constness
150// away. So this breaks const correctness!!! but is a necessary workaround
151// in some cases due to language limitations.
152// Note, that this is not a lambda_functor anymore, so it can not be used
153// as a sub lambda expression.
154
155template <class LambdaFunctor>
156struct const_incorrect_lambda_functor {
157 LambdaFunctor lf;
158public:
159
160 explicit const_incorrect_lambda_functor(const LambdaFunctor& a) : lf(a) {}
161
162 template <class SigArgs> struct sig {
163 typedef typename
164 LambdaFunctor::inherited::template
165 sig<typename SigArgs::tail_type>::type type;
166 };
167
168 // The nullary case is not needed (no arguments, no parameter type problems)
169
170 template<class A>
171 typename sig<tuple<const const_incorrect_lambda_functor, A&> >::type
172 operator()(const A& a) const {
173 return lf.template call<typename sig<tuple<const const_incorrect_lambda_functor, A&> >::type >(const_cast<A&>(a), cnull_type(), cnull_type(), cnull_type());
174 }
175
176 template<class A, class B>
177 typename sig<tuple<const const_incorrect_lambda_functor, A&, B&> >::type
178 operator()(const A& a, const B& b) const {
179 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());
180 }
181
182 template<class A, class B, class C>
183 typename sig<tuple<const const_incorrect_lambda_functor, A&, B&, C&> >::type
184 operator()(const A& a, const B& b, const C& c) const {
185 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());
186 }
187};
188
189// ------------------------------------------------------------------------
190// any lambda functor can be turned into a const_parameter_lambda_functor
191// The operator() takes arguments as const.
192// This is useful if lambda functors are called with non-const rvalues.
193// Note, that this is not a lambda_functor anymore, so it can not be used
194// as a sub lambda expression.
195
196template <class LambdaFunctor>
197struct const_parameter_lambda_functor {
198 LambdaFunctor lf;
199public:
200
201 explicit const_parameter_lambda_functor(const LambdaFunctor& a) : lf(a) {}
202
203 template <class SigArgs> struct sig {
204 typedef typename
205 LambdaFunctor::inherited::template
206 sig<typename SigArgs::tail_type>::type type;
207 };
208
209 // The nullary case is not needed: no arguments, no constness problems.
210
211 template<class A>
212 typename sig<tuple<const const_parameter_lambda_functor, const A&> >::type
213 operator()(const A& a) const {
214 return lf.template call<typename sig<tuple<const const_parameter_lambda_functor, const A&> >::type >(a, cnull_type(), cnull_type(), cnull_type());
215 }
216
217 template<class A, class B>
218 typename sig<tuple<const const_parameter_lambda_functor, const A&, const B&> >::type
219 operator()(const A& a, const B& b) const {
220 return lf.template call<typename sig<tuple<const const_parameter_lambda_functor, const A&, const B&> >::type >(a, b, cnull_type(), cnull_type());
221 }
222
223 template<class A, class B, class C>
224 typename sig<tuple<const const_parameter_lambda_functor, const A&, const B&, const C&>
225>::type
226 operator()(const A& a, const B& b, const C& c) const {
227 return lf.template call<typename sig<tuple<const const_parameter_lambda_functor, const A&, const B&, const C&> >::type>(a, b, c, cnull_type());
228 }
229};
230
231template <class Arg>
232inline const const_incorrect_lambda_functor<lambda_functor<Arg> >
233break_const(const lambda_functor<Arg>& lf)
234{
235 return const_incorrect_lambda_functor<lambda_functor<Arg> >(lf);
236}
237
238
239template <class Arg>
240inline const const_parameter_lambda_functor<lambda_functor<Arg> >
241const_parameters(const lambda_functor<Arg>& lf)
242{
243 return const_parameter_lambda_functor<lambda_functor<Arg> >(lf);
244}
245
246// make void ------------------------------------------------
247// make_void( x ) turns a lambda functor x with some return type y into
248// another lambda functor, which has a void return type
249// when called, the original return type is discarded
250
251// we use this action. The action class will be called, which means that
252// the wrapped lambda functor is evaluated, but we just don't do anything
253// with the result.
254struct voidifier_action {
255 template<class Ret, class A> static void apply(A&) {}
256};
257
258template<class Args> struct return_type_N<voidifier_action, Args> {
259 typedef void type;
260};
261
262template<class Arg1>
263inline const
264lambda_functor<
265 lambda_functor_base<
266 action<1, voidifier_action>,
267 tuple<lambda_functor<Arg1> >
268 >
269>
270make_void(const lambda_functor<Arg1>& a1) {
271return
272 lambda_functor_base<
273 action<1, voidifier_action>,
274 tuple<lambda_functor<Arg1> >
275 >
276 (tuple<lambda_functor<Arg1> > (a1));
277}
278
279// for non-lambda functors, make_void does nothing
280// (the argument gets evaluated immediately)
281
282template<class Arg1>
283inline const
284lambda_functor<
285 lambda_functor_base<do_nothing_action, null_type>
286>
287make_void(const Arg1&) {
288return
289 lambda_functor_base<do_nothing_action, null_type>();
290}
291
292// std_functor -----------------------------------------------------
293
294// The STL uses the result_type typedef as the convention to let binders know
295// the return type of a function object.
296// LL uses the sig template.
297// To let LL know that the function object has the result_type typedef
298// defined, it can be wrapped with the std_functor function.
299
300
301// Just inherit form the template parameter (the standard functor),
302// and provide a sig template. So we have a class which is still the
303// same functor + the sig template.
304
305template<class T>
306struct result_type_to_sig : public T {
307 template<class Args> struct sig { typedef typename T::result_type type; };
308 result_type_to_sig(const T& t) : T(t) {}
309};
310
311template<class F>
312inline result_type_to_sig<F> std_functor(const F& f) { return f; }
313
314
315} // namespace lambda
316} // namespace boost
317
318#endif
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