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1 $$ This is a pump file for generating file templates. Pump is a python | |
2 $$ script that is part of the Google Test suite of utilities. Description | |
3 $$ can be found here: | |
4 $$ | |
5 $$ http://code.google.com/p/googletest/wiki/PumpManual | |
6 $$ | |
7 | |
8 $$ See comment for MAX_ARITY in base/bind.h.pump. | |
9 $var MAX_ARITY = 7 | |
10 | |
11 // Copyright (c) 2012 The Chromium Authors. All rights reserved. | |
12 // Use of this source code is governed by a BSD-style license that can be | |
13 // found in the LICENSE file. | |
14 | |
15 #ifndef BASE_CALLBACK_H_ | |
16 #define BASE_CALLBACK_H_ | |
17 | |
18 #include "base/callback_forward.h" | |
19 #include "base/callback_internal.h" | |
20 #include "base/template_util.h" | |
21 | |
22 // NOTE: Header files that do not require the full definition of Callback or | |
23 // Closure should #include "base/callback_forward.h" instead of this file. | |
24 | |
25 // ----------------------------------------------------------------------------- | |
26 // Introduction | |
27 // ----------------------------------------------------------------------------- | |
28 // | |
29 // The templated Callback class is a generalized function object. Together | |
30 // with the Bind() function in bind.h, they provide a type-safe method for | |
31 // performing partial application of functions. | |
32 // | |
33 // Partial application (or "currying") is the process of binding a subset of | |
34 // a function's arguments to produce another function that takes fewer | |
35 // arguments. This can be used to pass around a unit of delayed execution, | |
36 // much like lexical closures are used in other languages. For example, it | |
37 // is used in Chromium code to schedule tasks on different MessageLoops. | |
38 // | |
39 // A callback with no unbound input parameters (base::Callback<void(void)>) | |
40 // is called a base::Closure. Note that this is NOT the same as what other | |
41 // languages refer to as a closure -- it does not retain a reference to its | |
42 // enclosing environment. | |
43 // | |
44 // MEMORY MANAGEMENT AND PASSING | |
45 // | |
46 // The Callback objects themselves should be passed by const-reference, and | |
47 // stored by copy. They internally store their state via a refcounted class | |
48 // and thus do not need to be deleted. | |
49 // | |
50 // The reason to pass via a const-reference is to avoid unnecessary | |
51 // AddRef/Release pairs to the internal state. | |
52 // | |
53 // | |
54 // ----------------------------------------------------------------------------- | |
55 // Quick reference for basic stuff | |
56 // ----------------------------------------------------------------------------- | |
57 // | |
58 // BINDING A BARE FUNCTION | |
59 // | |
60 // int Return5() { return 5; } | |
61 // base::Callback<int(void)> func_cb = base::Bind(&Return5); | |
62 // LOG(INFO) << func_cb.Run(); // Prints 5. | |
63 // | |
64 // BINDING A CLASS METHOD | |
65 // | |
66 // The first argument to bind is the member function to call, the second is | |
67 // the object on which to call it. | |
68 // | |
69 // class Ref : public base::RefCountedThreadSafe<Ref> { | |
70 // public: | |
71 // int Foo() { return 3; } | |
72 // void PrintBye() { LOG(INFO) << "bye."; } | |
73 // }; | |
74 // scoped_refptr<Ref> ref = new Ref(); | |
75 // base::Callback<void(void)> ref_cb = base::Bind(&Ref::Foo, ref); | |
76 // LOG(INFO) << ref_cb.Run(); // Prints out 3. | |
77 // | |
78 // By default the object must support RefCounted or you will get a compiler | |
79 // error. If you're passing between threads, be sure it's | |
80 // RefCountedThreadSafe! See "Advanced binding of member functions" below if | |
81 // you don't want to use reference counting. | |
82 // | |
83 // RUNNING A CALLBACK | |
84 // | |
85 // Callbacks can be run with their "Run" method, which has the same | |
86 // signature as the template argument to the callback. | |
87 // | |
88 // void DoSomething(const base::Callback<void(int, std::string)>& callback) { | |
89 // callback.Run(5, "hello"); | |
90 // } | |
91 // | |
92 // Callbacks can be run more than once (they don't get deleted or marked when | |
93 // run). However, this precludes using base::Passed (see below). | |
94 // | |
95 // void DoSomething(const base::Callback<double(double)>& callback) { | |
96 // double myresult = callback.Run(3.14159); | |
97 // myresult += callback.Run(2.71828); | |
98 // } | |
99 // | |
100 // PASSING UNBOUND INPUT PARAMETERS | |
101 // | |
102 // Unbound parameters are specified at the time a callback is Run(). They are | |
103 // specified in the Callback template type: | |
104 // | |
105 // void MyFunc(int i, const std::string& str) {} | |
106 // base::Callback<void(int, const std::string&)> cb = base::Bind(&MyFunc); | |
107 // cb.Run(23, "hello, world"); | |
108 // | |
109 // PASSING BOUND INPUT PARAMETERS | |
110 // | |
111 // Bound parameters are specified when you create thee callback as arguments | |
112 // to Bind(). They will be passed to the function and the Run()ner of the | |
113 // callback doesn't see those values or even know that the function it's | |
114 // calling. | |
115 // | |
116 // void MyFunc(int i, const std::string& str) {} | |
117 // base::Callback<void(void)> cb = base::Bind(&MyFunc, 23, "hello world"); | |
118 // cb.Run(); | |
119 // | |
120 // A callback with no unbound input parameters (base::Callback<void(void)>) | |
121 // is called a base::Closure. So we could have also written: | |
122 // | |
123 // base::Closure cb = base::Bind(&MyFunc, 23, "hello world"); | |
124 // | |
125 // When calling member functions, bound parameters just go after the object | |
126 // pointer. | |
127 // | |
128 // base::Closure cb = base::Bind(&MyClass::MyFunc, this, 23, "hello world"); | |
129 // | |
130 // PARTIAL BINDING OF PARAMETERS | |
131 // | |
132 // You can specify some parameters when you create the callback, and specify | |
133 // the rest when you execute the callback. | |
134 // | |
135 // void MyFunc(int i, const std::string& str) {} | |
136 // base::Callback<void(const std::string&)> cb = base::Bind(&MyFunc, 23); | |
137 // cb.Run("hello world"); | |
138 // | |
139 // When calling a function bound parameters are first, followed by unbound | |
140 // parameters. | |
141 // | |
142 // | |
143 // ----------------------------------------------------------------------------- | |
144 // Quick reference for advanced binding | |
145 // ----------------------------------------------------------------------------- | |
146 // | |
147 // BINDING A CLASS METHOD WITH WEAK POINTERS | |
148 // | |
149 // base::Bind(&MyClass::Foo, GetWeakPtr()); | |
150 // | |
151 // The callback will not be run if the object has already been destroyed. | |
152 // DANGER: weak pointers are not threadsafe, so don't use this | |
153 // when passing between threads! | |
154 // | |
155 // BINDING A CLASS METHOD WITH MANUAL LIFETIME MANAGEMENT | |
156 // | |
157 // base::Bind(&MyClass::Foo, base::Unretained(this)); | |
158 // | |
159 // This disables all lifetime management on the object. You're responsible | |
160 // for making sure the object is alive at the time of the call. You break it, | |
161 // you own it! | |
162 // | |
163 // BINDING A CLASS METHOD AND HAVING THE CALLBACK OWN THE CLASS | |
164 // | |
165 // MyClass* myclass = new MyClass; | |
166 // base::Bind(&MyClass::Foo, base::Owned(myclass)); | |
167 // | |
168 // The object will be deleted when the callback is destroyed, even if it's | |
169 // not run (like if you post a task during shutdown). Potentially useful for | |
170 // "fire and forget" cases. | |
171 // | |
172 // IGNORING RETURN VALUES | |
173 // | |
174 // Sometimes you want to call a function that returns a value in a callback | |
175 // that doesn't expect a return value. | |
176 // | |
177 // int DoSomething(int arg) { cout << arg << endl; } | |
178 // base::Callback<void<int>) cb = | |
179 // base::Bind(base::IgnoreResult(&DoSomething)); | |
180 // | |
181 // | |
182 // ----------------------------------------------------------------------------- | |
183 // Quick reference for binding parameters to Bind() | |
184 // ----------------------------------------------------------------------------- | |
185 // | |
186 // Bound parameters are specified as arguments to Bind() and are passed to the | |
187 // function. A callback with no parameters or no unbound parameters is called a | |
188 // Closure (base::Callback<void(void)> and base::Closure are the same thing). | |
189 // | |
190 // PASSING PARAMETERS OWNED BY THE CALLBACK | |
191 // | |
192 // void Foo(int* arg) { cout << *arg << endl; } | |
193 // int* pn = new int(1); | |
194 // base::Closure foo_callback = base::Bind(&foo, base::Owned(pn)); | |
195 // | |
196 // The parameter will be deleted when the callback is destroyed, even if it's | |
197 // not run (like if you post a task during shutdown). | |
198 // | |
199 // PASSING PARAMETERS AS A scoped_ptr | |
200 // | |
201 // void TakesOwnership(scoped_ptr<Foo> arg) {} | |
202 // scoped_ptr<Foo> f(new Foo); | |
203 // // f becomes null during the following call. | |
204 // base::Closure cb = base::Bind(&TakesOwnership, base::Passed(&f)); | |
205 // | |
206 // Ownership of the parameter will be with the callback until the it is run, | |
207 // when ownership is passed to the callback function. This means the callback | |
208 // can only be run once. If the callback is never run, it will delete the | |
209 // object when it's destroyed. | |
210 // | |
211 // PASSING PARAMETERS AS A scoped_refptr | |
212 // | |
213 // void TakesOneRef(scoped_refptr<Foo> arg) {} | |
214 // scoped_refptr<Foo> f(new Foo) | |
215 // base::Closure cb = base::Bind(&TakesOneRef, f); | |
216 // | |
217 // This should "just work." The closure will take a reference as long as it | |
218 // is alive, and another reference will be taken for the called function. | |
219 // | |
220 // PASSING PARAMETERS BY REFERENCE | |
221 // | |
222 // void foo(int arg) { cout << arg << endl } | |
223 // int n = 1; | |
224 // base::Closure has_ref = base::Bind(&foo, base::ConstRef(n)); | |
225 // n = 2; | |
226 // has_ref.Run(); // Prints "2" | |
227 // | |
228 // Normally parameters are copied in the closure. DANGER: ConstRef stores a | |
229 // const reference instead, referencing the original parameter. This means | |
230 // that you must ensure the object outlives the callback! | |
231 // | |
232 // | |
233 // ----------------------------------------------------------------------------- | |
234 // Implementation notes | |
235 // ----------------------------------------------------------------------------- | |
236 // | |
237 // WHERE IS THIS DESIGN FROM: | |
238 // | |
239 // The design Callback and Bind is heavily influenced by C++'s | |
240 // tr1::function/tr1::bind, and by the "Google Callback" system used inside | |
241 // Google. | |
242 // | |
243 // | |
244 // HOW THE IMPLEMENTATION WORKS: | |
245 // | |
246 // There are three main components to the system: | |
247 // 1) The Callback classes. | |
248 // 2) The Bind() functions. | |
249 // 3) The arguments wrappers (e.g., Unretained() and ConstRef()). | |
250 // | |
251 // The Callback classes represent a generic function pointer. Internally, | |
252 // it stores a refcounted piece of state that represents the target function | |
253 // and all its bound parameters. Each Callback specialization has a templated | |
254 // constructor that takes an BindState<>*. In the context of the constructor, | |
255 // the static type of this BindState<> pointer uniquely identifies the | |
256 // function it is representing, all its bound parameters, and a Run() method | |
257 // that is capable of invoking the target. | |
258 // | |
259 // Callback's constructor takes the BindState<>* that has the full static type | |
260 // and erases the target function type as well as the types of the bound | |
261 // parameters. It does this by storing a pointer to the specific Run() | |
262 // function, and upcasting the state of BindState<>* to a | |
263 // BindStateBase*. This is safe as long as this BindStateBase pointer | |
264 // is only used with the stored Run() pointer. | |
265 // | |
266 // To BindState<> objects are created inside the Bind() functions. | |
267 // These functions, along with a set of internal templates, are responsible for | |
268 // | |
269 // - Unwrapping the function signature into return type, and parameters | |
270 // - Determining the number of parameters that are bound | |
271 // - Creating the BindState storing the bound parameters | |
272 // - Performing compile-time asserts to avoid error-prone behavior | |
273 // - Returning an Callback<> with an arity matching the number of unbound | |
274 // parameters and that knows the correct refcounting semantics for the | |
275 // target object if we are binding a method. | |
276 // | |
277 // The Bind functions do the above using type-inference, and template | |
278 // specializations. | |
279 // | |
280 // By default Bind() will store copies of all bound parameters, and attempt | |
281 // to refcount a target object if the function being bound is a class method. | |
282 // These copies are created even if the function takes parameters as const | |
283 // references. (Binding to non-const references is forbidden, see bind.h.) | |
284 // | |
285 // To change this behavior, we introduce a set of argument wrappers | |
286 // (e.g., Unretained(), and ConstRef()). These are simple container templates | |
287 // that are passed by value, and wrap a pointer to argument. See the | |
288 // file-level comment in base/bind_helpers.h for more info. | |
289 // | |
290 // These types are passed to the Unwrap() functions, and the MaybeRefcount() | |
291 // functions respectively to modify the behavior of Bind(). The Unwrap() | |
292 // and MaybeRefcount() functions change behavior by doing partial | |
293 // specialization based on whether or not a parameter is a wrapper type. | |
294 // | |
295 // ConstRef() is similar to tr1::cref. Unretained() is specific to Chromium. | |
296 // | |
297 // | |
298 // WHY NOT TR1 FUNCTION/BIND? | |
299 // | |
300 // Direct use of tr1::function and tr1::bind was considered, but ultimately | |
301 // rejected because of the number of copy constructors invocations involved | |
302 // in the binding of arguments during construction, and the forwarding of | |
303 // arguments during invocation. These copies will no longer be an issue in | |
304 // C++0x because C++0x will support rvalue reference allowing for the compiler | |
305 // to avoid these copies. However, waiting for C++0x is not an option. | |
306 // | |
307 // Measured with valgrind on gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5), the | |
308 // tr1::bind call itself will invoke a non-trivial copy constructor three times | |
309 // for each bound parameter. Also, each when passing a tr1::function, each | |
310 // bound argument will be copied again. | |
311 // | |
312 // In addition to the copies taken at binding and invocation, copying a | |
313 // tr1::function causes a copy to be made of all the bound parameters and | |
314 // state. | |
315 // | |
316 // Furthermore, in Chromium, it is desirable for the Callback to take a | |
317 // reference on a target object when representing a class method call. This | |
318 // is not supported by tr1. | |
319 // | |
320 // Lastly, tr1::function and tr1::bind has a more general and flexible API. | |
321 // This includes things like argument reordering by use of | |
322 // tr1::bind::placeholder, support for non-const reference parameters, and some | |
323 // limited amount of subtyping of the tr1::function object (e.g., | |
324 // tr1::function<int(int)> is convertible to tr1::function<void(int)>). | |
325 // | |
326 // These are not features that are required in Chromium. Some of them, such as | |
327 // allowing for reference parameters, and subtyping of functions, may actually | |
328 // become a source of errors. Removing support for these features actually | |
329 // allows for a simpler implementation, and a terser Currying API. | |
330 // | |
331 // | |
332 // WHY NOT GOOGLE CALLBACKS? | |
333 // | |
334 // The Google callback system also does not support refcounting. Furthermore, | |
335 // its implementation has a number of strange edge cases with respect to type | |
336 // conversion of its arguments. In particular, the argument's constness must | |
337 // at times match exactly the function signature, or the type-inference might | |
338 // break. Given the above, writing a custom solution was easier. | |
339 // | |
340 // | |
341 // MISSING FUNCTIONALITY | |
342 // - Invoking the return of Bind. Bind(&foo).Run() does not work; | |
343 // - Binding arrays to functions that take a non-const pointer. | |
344 // Example: | |
345 // void Foo(const char* ptr); | |
346 // void Bar(char* ptr); | |
347 // Bind(&Foo, "test"); | |
348 // Bind(&Bar, "test"); // This fails because ptr is not const. | |
349 | |
350 namespace base { | |
351 | |
352 // First, we forward declare the Callback class template. This informs the | |
353 // compiler that the template only has 1 type parameter which is the function | |
354 // signature that the Callback is representing. | |
355 // | |
356 // After this, create template specializations for 0-$(MAX_ARITY) parameters. No
te that | |
357 // even though the template typelist grows, the specialization still | |
358 // only has one type: the function signature. | |
359 // | |
360 // If you are thinking of forward declaring Callback in your own header file, | |
361 // please include "base/callback_forward.h" instead. | |
362 template <typename Sig> | |
363 class Callback; | |
364 | |
365 namespace internal { | |
366 template <typename Runnable, typename RunType, typename BoundArgsType> | |
367 struct BindState; | |
368 } // namespace internal | |
369 | |
370 | |
371 $range ARITY 0..MAX_ARITY | |
372 $for ARITY [[ | |
373 $range ARG 1..ARITY | |
374 | |
375 $if ARITY == 0 [[ | |
376 template <typename R> | |
377 class Callback<R(void)> : public internal::CallbackBase { | |
378 ]] $else [[ | |
379 template <typename R, $for ARG , [[typename A$(ARG)]]> | |
380 class Callback<R($for ARG , [[A$(ARG)]])> : public internal::CallbackBase { | |
381 ]] | |
382 | |
383 public: | |
384 typedef R(RunType)($for ARG , [[A$(ARG)]]); | |
385 | |
386 Callback() : CallbackBase(NULL) { } | |
387 | |
388 // Note that this constructor CANNOT be explicit, and that Bind() CANNOT | |
389 // return the exact Callback<> type. See base/bind.h for details. | |
390 template <typename Runnable, typename BindRunType, typename BoundArgsType> | |
391 Callback(internal::BindState<Runnable, BindRunType, | |
392 BoundArgsType>* bind_state) | |
393 : CallbackBase(bind_state) { | |
394 | |
395 // Force the assignment to a local variable of PolymorphicInvoke | |
396 // so the compiler will typecheck that the passed in Run() method has | |
397 // the correct type. | |
398 PolymorphicInvoke invoke_func = | |
399 &internal::BindState<Runnable, BindRunType, BoundArgsType> | |
400 ::InvokerType::Run; | |
401 polymorphic_invoke_ = reinterpret_cast<InvokeFuncStorage>(invoke_func); | |
402 } | |
403 | |
404 bool Equals(const Callback& other) const { | |
405 return CallbackBase::Equals(other); | |
406 } | |
407 | |
408 R Run($for ARG , | |
409 [[typename internal::CallbackParamTraits<A$(ARG)>::ForwardType a$(ARG)]]
) const { | |
410 PolymorphicInvoke f = | |
411 reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_); | |
412 | |
413 return f(bind_state_.get()[[]] | |
414 $if ARITY != 0 [[, ]] | |
415 $for ARG , | |
416 [[internal::CallbackForward(a$(ARG))]]); | |
417 } | |
418 | |
419 private: | |
420 typedef R(*PolymorphicInvoke)( | |
421 internal::BindStateBase*[[]] | |
422 $if ARITY != 0 [[, ]] | |
423 $for ARG , [[typename internal::CallbackParamTraits<A$(ARG)>::ForwardType]]); | |
424 | |
425 }; | |
426 | |
427 | |
428 ]] $$ for ARITY | |
429 | |
430 // Syntactic sugar to make Callback<void(void)> easier to declare since it | |
431 // will be used in a lot of APIs with delayed execution. | |
432 typedef Callback<void(void)> Closure; | |
433 | |
434 } // namespace base | |
435 | |
436 #endif // BASE_CALLBACK_H | |
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