base/callback.h

   1 // This file was GENERATED by command:
   2 //     pump.py callback.h.pump
   3 // DO NOT EDIT BY HAND!!!
   4
   5
   6
   7 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
   8 // Use of this source code is governed by a BSD-style license that can be
   9 // found in the LICENSE file.
  10
  11 #ifndef BASE_CALLBACK_H_
  12 #define BASE_CALLBACK_H_
  13 #pragma once
  14
  15 #include "base/callback_internal.h"
  16 #include "base/callback_old.h"
  17
  18 // New, super-duper, unified Callback system.  This will eventually replace
  19 // NewRunnableMethod, NewRunnableFunction, CreateFunctor, and CreateCallback
  20 // systems currently in the Chromium code base.
  21 //
  22 // WHAT IS THIS:
  23 //
  24 // The templated Callback class is a generalized function object. Together
  25 // with the Bind() function in bind.h, they provide a type-safe method for
  26 // performing currying of arguments, and creating a "closure."
  27 //
  28 // In programing languages, a closure is a first-class function where all its
  29 // parameters have been bound (usually via currying).  Closures are well
  30 // suited for representing, and passing around a unit of delayed execution.
  31 // They are used in Chromium code to schedule tasks on different MessageLoops.
  32 //
  33 //
  34 // MEMORY MANAGEMENT AND PASSING
  35 //
  36 // The Callback objects themselves should be passed by const-reference, and
  37 // stored by copy. They internally store their state via a refcounted class
  38 // and thus do not need to be deleted.
  39 //
  40 // The reason to pass via a const-reference is to avoid unnecessary
  41 // AddRef/Release pairs to the internal state.
  42 //
  43 //
  44 // EXAMPLE USAGE:
  45 //
  46 // /* Binding a normal function. */
  47 // int Return5() { return 5; }
  48 // base::Callback<int(int)> func_cb = base::Bind(&Return5);
  49 // LOG(INFO) << func_cb.Run(5);  // Prints 5.
  50 //
  51 // void PrintHi() { LOG(INFO) << "hi."; }
  52 // base::Closure void_func_cb = base::Bind(&PrintHi);
  53 // LOG(INFO) << void_func_cb.Run();  // Prints: hi.
  54 //
  55 // /* Binding a class method. */
  56 // class Ref : public RefCountedThreadSafe<Ref> {
  57 //  public:
  58 //   int Foo() { return 3; }
  59 //   void PrintBye() { LOG(INFO) << "bye."; }
  60 // };
  61 // scoped_refptr<Ref> ref = new Ref();
  62 // base::Callback<int(void)> ref_cb = base::Bind(&Ref::Foo, ref.get());
  63 // LOG(INFO) << ref_cb.Run();  // Prints out 3.
  64 //
  65 // base::Closure void_ref_cb = base::Bind(&Ref::PrintBye, ref.get());
  66 // void_ref_cb.Run();  // Prints: bye.
  67 //
  68 // /* Binding a class method in a non-refcounted class.
  69 //  *
  70 //  * WARNING: You must be sure the referee outlives the callback!
  71 //  *          This is particularly important if you post a closure to a
  72 //  *          MessageLoop because then it becomes hard to know what the
  73 //  *          lifetime of the referee needs to be.
  74 //  */
  75 // class NoRef {
  76 //  public:
  77 //   int Foo() { return 4; }
  78 //   void PrintWhy() { LOG(INFO) << "why???"; }
  79 // };
  80 // NoRef no_ref;
  81 // base::Callback<int(void)> base::no_ref_cb =
  82 //     base::Bind(&NoRef::Foo, base::Unretained(&no_ref));
  83 // LOG(INFO) << ref_cb.Run();  // Prints out 4.
  84 //
  85 // base::Closure void_no_ref_cb =
  86 //     base::Bind(&NoRef::PrintWhy, base::Unretained(no_ref));
  87 // void_no_ref_cb.Run();  // Prints: why???
  88 //
  89 // /* Binding a reference. */
  90 // int Identity(int n) { return n; }
  91 // int value = 1;
  92 // base::Callback<int(void)> bound_copy_cb = base::Bind(&Identity, value);
  93 // base::Callback<int(void)> bound_ref_cb =
  94 //     base::Bind(&Identity, base::ConstRef(value));
  95 // LOG(INFO) << bound_copy_cb.Run();  // Prints 1.
  96 // LOG(INFO) << bound_ref_cb.Run();  // Prints 1.
  97 // value = 2;
  98 // LOG(INFO) << bound_copy_cb.Run();  // Prints 1.
  99 // LOG(INFO) << bound_ref_cb.Run();  // Prints 2.
 100 //
 101 //
 102 // WHERE IS THIS DESIGN FROM:
 103 //
 104 // The design Callback and Bind is heavily influenced by C++'s
 105 // tr1::function/tr1::bind, and by the "Google Callback" system used inside
 106 // Google.
 107 //
 108 //
 109 // HOW THE IMPLEMENTATION WORKS:
 110 //
 111 // There are three main components to the system:
 112 //   1) The Callback classes.
 113 //   2) The Bind() functions.
 114 //   3) The arguments wrappers (eg., Unretained() and ConstRef()).
 115 //
 116 // The Callback classes represent a generic function pointer. Internally,
 117 // it stores a refcounted piece of state that represents the target function
 118 // and all its bound parameters.  Each Callback specialization has a templated
 119 // constructor that takes an InvokerStorageHolder<> object.  In the context of
 120 // the constructor, the static type of this InvokerStorageHolder<> object
 121 // uniquely identifies the function it is representing, all its bound
 122 // parameters, and a DoInvoke() that is capable of invoking the target.
 123 //
 124 // Callback's constructor is takes the InvokerStorageHolder<> that has the
 125 // full static type and erases the target function type, and the bound
 126 // parameters.  It does this by storing a pointer to the specific DoInvoke()
 127 // function, and upcasting the state of InvokerStorageHolder<> to a
 128 // InvokerStorageBase. This is safe as long as this InvokerStorageBase pointer
 129 // is only used with the stored DoInvoke() pointer.
 130 //
 131 // To create InvokerStorageHolder<> objects, we use the Bind() functions.
 132 // These functions, along with a set of internal templates, are reponsible for
 133 //
 134 //  - Unwrapping the function signature into return type, and parameters
 135 //  - Determining the number of parameters that are bound
 136 //  - Creating the storage for the bound parameters
 137 //  - Performing compile-time asserts to avoid error-prone behavior
 138 //  - Returning an InvokerStorageHolder<> with an DoInvoke() that has an arity
 139 //    matching the number of unbound parameters, and knows the correct
 140 //    refcounting semantics for the target object if we are binding a class
 141 //    method.
 142 //
 143 // The Bind functions do the above using type-inference, and template
 144 // specializations.
 145 //
 146 // By default Bind() will store copies of all bound parameters, and attempt
 147 // to refcount a target object if the function being bound is a class method.
 148 //
 149 // To change this behavior, we introduce a set of argument wrappers
 150 // (eg. Unretained(), and ConstRef()).  These are simple container templates
 151 // that are passed by value, and wrap a pointer to argument.  See the
 152 // file-level comment in base/bind_helpers.h for more info.
 153 //
 154 // These types are passed to the Unwrap() functions, and the MaybeRefcount()
 155 // functions respectively to modify the behavior of Bind().  The Unwrap()
 156 // and MaybeRefcount() functions change behavior by doing partial
 157 // specialization based on whether or not a parameter is a wrapper type.
 158 //
 159 // ConstRef() is similar to tr1::cref.  Unretained() is specific to Chromium.
 160 //
 161 //
 162 // WHY NOT TR1 FUNCTION/BIND?
 163 //
 164 // Direct use of tr1::function and tr1::bind was considered, but ultimately
 165 // rejected because of the number of copy constructors invocations involved
 166 // in the binding of arguments during construction, and the forwarding of
 167 // arguments during invocation.  These copies will no longer be an issue in
 168 // C++0x because C++0x will support rvalue reference allowing for the compiler
 169 // to avoid these copies.  However, waiting for C++0x is not an option.
 170 //
 171 // Measured with valgrind on gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5), the
 172 // tr1::bind call itself will invoke a non-trivial copy constructor three times
 173 // for each bound parameter.  Also, each when passing a tr1::function, each
 174 // bound argument will be copied again.
 175 //
 176 // In addition to the copies taken at binding and invocation, copying a
 177 // tr1::function causes a copy to be made of all the bound parameters and
 178 // state.
 179 //
 180 // Furthermore, in Chromium, it is desirable for the Callback to take a
 181 // reference on a target object when representing a class method call.  This
 182 // is not supported by tr1.
 183 //
 184 // Lastly, tr1::function and tr1::bind has a more general and flexible API.
 185 // This includes things like argument reordering by use of
 186 // tr1::bind::placeholder, support for non-const reference parameters, and some
 187 // limited amount of subtyping of the tr1::function object (eg.,
 188 // tr1::function<int(int)> is convertible to tr1::function<void(int)>).
 189 //
 190 // These are not features that are required in Chromium. Some of them, such as
 191 // allowing for reference parameters, and subtyping of functions, may actually
 192 // because a source of errors. Removing support for these features actually
 193 // allows for a simpler implementation, and a terser Currying API.
 194 //
 195 //
 196 // WHY NOT GOOGLE CALLBACKS?
 197 //
 198 // The Google callback system also does not support refcounting.  Furthermore,
 199 // its implementation has a number of strange edge cases with respect to type
 200 // conversion of its arguments.  In particular, the argument's constness must
 201 // at times match exactly the function signature, or the type-inference might
 202 // break.  Given the above, writing a custom solution was easier.
 203 //
 204 //
 205 // MISSING FUNCTIONALITY
 206 //  - Invoking the return of Bind.  Bind(&foo).Run() does not work;
 207 //  - Binding arrays to functions that take a non-const pointer.
 208 //    Example:
 209 //      void Foo(const char* ptr);
 210 //      void Bar(char* ptr);
 211 //      Bind(&Foo, "test");
 212 //      Bind(&Bar, "test");  // This fails because ptr is not const.
 213
 214 namespace base {
 215
 216 // First, we forward declare the Callback class template. This informs the
 217 // compiler that the template only has 1 type parameter which is the function
 218 // signature that the Callback is representing.
 219 //
 220 // After this, create template specializations for 0-6 parameters. Note that
 221 // even though the template typelist grows, the specialization still
 222 // only has one type: the function signature.
 223 template <typename Sig>
 224 class Callback;
 225
 226 template <typename R>
 227 class Callback<R(void)> : public internal::CallbackBase {
 228  public:
 229   typedef R(*PolymorphicInvoke)(
 230       internal::InvokerStorageBase*);
 231
 232   Callback() : CallbackBase(NULL, NULL) { }
 233
 234   // We pass InvokerStorageHolder by const ref to avoid incurring an
 235   // unnecessary AddRef/Unref pair even though we will modify the object.
 236   // We cannot use a normal reference because the compiler will warn
 237   // since this is often used on a return value, which is a temporary.
 238   //
 239   // Note that this constructor CANNOT be explicit, and that Bind() CANNOT
 240   // return the exact Callback<> type.  See base/bind.h for details.
 241   template <typename T>
 242   Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
 243       : CallbackBase(
 244           reinterpret_cast<InvokeFuncStorage>(&T::Invoker::DoInvoke),
 245           &invoker_holder.invoker_storage_) {
 246   }
 247
 248   R Run() const {
 249     PolymorphicInvoke f =
 250         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 251
 252     return f(invoker_storage_.get());
 253   }
 254 };
 255
 256 template <typename R, typename A1>
 257 class Callback<R(A1)> : public internal::CallbackBase {
 258  public:
 259   typedef R(*PolymorphicInvoke)(
 260       internal::InvokerStorageBase*,
 261           typename internal::ParamTraits<A1>::ForwardType);
 262
 263   Callback() : CallbackBase(NULL, NULL) { }
 264
 265   // We pass InvokerStorageHolder by const ref to avoid incurring an
 266   // unnecessary AddRef/Unref pair even though we will modify the object.
 267   // We cannot use a normal reference because the compiler will warn
 268   // since this is often used on a return value, which is a temporary.
 269   //
 270   // Note that this constructor CANNOT be explicit, and that Bind() CANNOT
 271   // return the exact Callback<> type.  See base/bind.h for details.
 272   template <typename T>
 273   Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
 274       : CallbackBase(
 275           reinterpret_cast<InvokeFuncStorage>(&T::Invoker::DoInvoke),
 276           &invoker_holder.invoker_storage_) {
 277   }
 278
 279   R Run(typename internal::ParamTraits<A1>::ForwardType a1) const {
 280     PolymorphicInvoke f =
 281         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 282
 283     return f(invoker_storage_.get(), a1);
 284   }
 285 };
 286
 287 template <typename R, typename A1, typename A2>
 288 class Callback<R(A1, A2)> : public internal::CallbackBase {
 289  public:
 290   typedef R(*PolymorphicInvoke)(
 291       internal::InvokerStorageBase*,
 292           typename internal::ParamTraits<A1>::ForwardType,
 293           typename internal::ParamTraits<A2>::ForwardType);
 294
 295   Callback() : CallbackBase(NULL, NULL) { }
 296
 297   // We pass InvokerStorageHolder by const ref to avoid incurring an
 298   // unnecessary AddRef/Unref pair even though we will modify the object.
 299   // We cannot use a normal reference because the compiler will warn
 300   // since this is often used on a return value, which is a temporary.
 301   //
 302   // Note that this constructor CANNOT be explicit, and that Bind() CANNOT
 303   // return the exact Callback<> type.  See base/bind.h for details.
 304   template <typename T>
 305   Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
 306       : CallbackBase(
 307           reinterpret_cast<InvokeFuncStorage>(&T::Invoker::DoInvoke),
 308           &invoker_holder.invoker_storage_) {
 309   }
 310
 311   R Run(typename internal::ParamTraits<A1>::ForwardType a1,
 312         typename internal::ParamTraits<A2>::ForwardType a2) const {
 313     PolymorphicInvoke f =
 314         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 315
 316     return f(invoker_storage_.get(), a1,
 317              a2);
 318   }
 319 };
 320
 321 template <typename R, typename A1, typename A2, typename A3>
 322 class Callback<R(A1, A2, A3)> : public internal::CallbackBase {
 323  public:
 324   typedef R(*PolymorphicInvoke)(
 325       internal::InvokerStorageBase*,
 326           typename internal::ParamTraits<A1>::ForwardType,
 327           typename internal::ParamTraits<A2>::ForwardType,
 328           typename internal::ParamTraits<A3>::ForwardType);
 329
 330   Callback() : CallbackBase(NULL, NULL) { }
 331
 332   // We pass InvokerStorageHolder by const ref to avoid incurring an
 333   // unnecessary AddRef/Unref pair even though we will modify the object.
 334   // We cannot use a normal reference because the compiler will warn
 335   // since this is often used on a return value, which is a temporary.
 336   //
 337   // Note that this constructor CANNOT be explicit, and that Bind() CANNOT
 338   // return the exact Callback<> type.  See base/bind.h for details.
 339   template <typename T>
 340   Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
 341       : CallbackBase(
 342           reinterpret_cast<InvokeFuncStorage>(&T::Invoker::DoInvoke),
 343           &invoker_holder.invoker_storage_) {
 344   }
 345
 346   R Run(typename internal::ParamTraits<A1>::ForwardType a1,
 347         typename internal::ParamTraits<A2>::ForwardType a2,
 348         typename internal::ParamTraits<A3>::ForwardType a3) const {
 349     PolymorphicInvoke f =
 350         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 351
 352     return f(invoker_storage_.get(), a1,
 353              a2,
 354              a3);
 355   }
 356 };
 357
 358 template <typename R, typename A1, typename A2, typename A3, typename A4>
 359 class Callback<R(A1, A2, A3, A4)> : public internal::CallbackBase {
 360  public:
 361   typedef R(*PolymorphicInvoke)(
 362       internal::InvokerStorageBase*,
 363           typename internal::ParamTraits<A1>::ForwardType,
 364           typename internal::ParamTraits<A2>::ForwardType,
 365           typename internal::ParamTraits<A3>::ForwardType,
 366           typename internal::ParamTraits<A4>::ForwardType);
 367
 368   Callback() : CallbackBase(NULL, NULL) { }
 369
 370   // We pass InvokerStorageHolder by const ref to avoid incurring an
 371   // unnecessary AddRef/Unref pair even though we will modify the object.
 372   // We cannot use a normal reference because the compiler will warn
 373   // since this is often used on a return value, which is a temporary.
 374   //
 375   // Note that this constructor CANNOT be explicit, and that Bind() CANNOT
 376   // return the exact Callback<> type.  See base/bind.h for details.
 377   template <typename T>
 378   Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
 379       : CallbackBase(
 380           reinterpret_cast<InvokeFuncStorage>(&T::Invoker::DoInvoke),
 381           &invoker_holder.invoker_storage_) {
 382   }
 383
 384   R Run(typename internal::ParamTraits<A1>::ForwardType a1,
 385         typename internal::ParamTraits<A2>::ForwardType a2,
 386         typename internal::ParamTraits<A3>::ForwardType a3,
 387         typename internal::ParamTraits<A4>::ForwardType a4) const {
 388     PolymorphicInvoke f =
 389         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 390
 391     return f(invoker_storage_.get(), a1,
 392              a2,
 393              a3,
 394              a4);
 395   }
 396 };
 397
 398 template <typename R, typename A1, typename A2, typename A3, typename A4,
 399     typename A5>
 400 class Callback<R(A1, A2, A3, A4, A5)> : public internal::CallbackBase {
 401  public:
 402   typedef R(*PolymorphicInvoke)(
 403       internal::InvokerStorageBase*,
 404           typename internal::ParamTraits<A1>::ForwardType,
 405           typename internal::ParamTraits<A2>::ForwardType,
 406           typename internal::ParamTraits<A3>::ForwardType,
 407           typename internal::ParamTraits<A4>::ForwardType,
 408           typename internal::ParamTraits<A5>::ForwardType);
 409
 410   Callback() : CallbackBase(NULL, NULL) { }
 411
 412   // We pass InvokerStorageHolder by const ref to avoid incurring an
 413   // unnecessary AddRef/Unref pair even though we will modify the object.
 414   // We cannot use a normal reference because the compiler will warn
 415   // since this is often used on a return value, which is a temporary.
 416   //
 417   // Note that this constructor CANNOT be explicit, and that Bind() CANNOT
 418   // return the exact Callback<> type.  See base/bind.h for details.
 419   template <typename T>
 420   Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
 421       : CallbackBase(
 422           reinterpret_cast<InvokeFuncStorage>(&T::Invoker::DoInvoke),
 423           &invoker_holder.invoker_storage_) {
 424   }
 425
 426   R Run(typename internal::ParamTraits<A1>::ForwardType a1,
 427         typename internal::ParamTraits<A2>::ForwardType a2,
 428         typename internal::ParamTraits<A3>::ForwardType a3,
 429         typename internal::ParamTraits<A4>::ForwardType a4,
 430         typename internal::ParamTraits<A5>::ForwardType a5) const {
 431     PolymorphicInvoke f =
 432         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 433
 434     return f(invoker_storage_.get(), a1,
 435              a2,
 436              a3,
 437              a4,
 438              a5);
 439   }
 440 };
 441
 442 template <typename R, typename A1, typename A2, typename A3, typename A4,
 443     typename A5, typename A6>
 444 class Callback<R(A1, A2, A3, A4, A5, A6)> : public internal::CallbackBase {
 445  public:
 446   typedef R(*PolymorphicInvoke)(
 447       internal::InvokerStorageBase*,
 448           typename internal::ParamTraits<A1>::ForwardType,
 449           typename internal::ParamTraits<A2>::ForwardType,
 450           typename internal::ParamTraits<A3>::ForwardType,
 451           typename internal::ParamTraits<A4>::ForwardType,
 452           typename internal::ParamTraits<A5>::ForwardType,
 453           typename internal::ParamTraits<A6>::ForwardType);
 454
 455   Callback() : CallbackBase(NULL, NULL) { }
 456
 457   // We pass InvokerStorageHolder by const ref to avoid incurring an
 458   // unnecessary AddRef/Unref pair even though we will modify the object.
 459   // We cannot use a normal reference because the compiler will warn
 460   // since this is often used on a return value, which is a temporary.
 461   //
 462   // Note that this constructor CANNOT be explicit, and that Bind() CANNOT
 463   // return the exact Callback<> type.  See base/bind.h for details.
 464   template <typename T>
 465   Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
 466       : CallbackBase(
 467           reinterpret_cast<InvokeFuncStorage>(&T::Invoker::DoInvoke),
 468           &invoker_holder.invoker_storage_) {
 469   }
 470
 471   R Run(typename internal::ParamTraits<A1>::ForwardType a1,
 472         typename internal::ParamTraits<A2>::ForwardType a2,
 473         typename internal::ParamTraits<A3>::ForwardType a3,
 474         typename internal::ParamTraits<A4>::ForwardType a4,
 475         typename internal::ParamTraits<A5>::ForwardType a5,
 476         typename internal::ParamTraits<A6>::ForwardType a6) const {
 477     PolymorphicInvoke f =
 478         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 479
 480     return f(invoker_storage_.get(), a1,
 481              a2,
 482              a3,
 483              a4,
 484              a5,
 485              a6);
 486   }
 487 };
 488
 489
 490 // Syntactic sugar to make Callbacks<void(void)> easier to declare since it
 491 // will be used in a lot of APIs with delayed execution.
 492 typedef Callback<void(void)> Closure;
 493
 494 }  // namespace base
 495
 496 #endif  // BASE_CALLBACK_H