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/template_util.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 programming 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(void)> func_cb = base::Bind(&Return5);
  49 // LOG(INFO) << func_cb.Run();  // Prints 5.
  50 //
  51 // void PrintHi() { LOG(INFO) << "hi."; }
  52 // base::Closure void_func_cb = base::Bind(&PrintHi);
  53 // 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 // /* Currying parameters. This also works for methods. */
 102 // int Sum(int a, int b, int c) {
 103 //   return a + b + c;
 104 // }
 105 // base::Callback<int(int, int)> sum3_cb = base::Bind(&Sum, 3);
 106 // LOG(INFO) << sum3_cb.Run(4, 5);  // Prints 12.
 107 //
 108 // base::Callback<int(int)> sum7_cb = base::Bind(&Sum, 3, 4);
 109 // LOG(INFO) << sum7_cb.Run(10);  // Prints 17.
 110 //
 111 //
 112 // WHERE IS THIS DESIGN FROM:
 113 //
 114 // The design Callback and Bind is heavily influenced by C++'s
 115 // tr1::function/tr1::bind, and by the "Google Callback" system used inside
 116 // Google.
 117 //
 118 //
 119 // HOW THE IMPLEMENTATION WORKS:
 120 //
 121 // There are three main components to the system:
 122 //   1) The Callback classes.
 123 //   2) The Bind() functions.
 124 //   3) The arguments wrappers (eg., Unretained() and ConstRef()).
 125 //
 126 // The Callback classes represent a generic function pointer. Internally,
 127 // it stores a refcounted piece of state that represents the target function
 128 // and all its bound parameters.  Each Callback specialization has a templated
 129 // constructor that takes an InvokerStorageHolder<> object.  In the context of
 130 // the constructor, the static type of this InvokerStorageHolder<> object
 131 // uniquely identifies the function it is representing, all its bound
 132 // parameters, and a DoInvoke() that is capable of invoking the target.
 133 //
 134 // Callback's constructor is takes the InvokerStorageHolder<> that has the
 135 // full static type and erases the target function type, and the bound
 136 // parameters.  It does this by storing a pointer to the specific DoInvoke()
 137 // function, and upcasting the state of InvokerStorageHolder<> to a
 138 // InvokerStorageBase. This is safe as long as this InvokerStorageBase pointer
 139 // is only used with the stored DoInvoke() pointer.
 140 //
 141 // To create InvokerStorageHolder<> objects, we use the Bind() functions.
 142 // These functions, along with a set of internal templates, are reponsible for
 143 //
 144 //  - Unwrapping the function signature into return type, and parameters
 145 //  - Determining the number of parameters that are bound
 146 //  - Creating the storage for the bound parameters
 147 //  - Performing compile-time asserts to avoid error-prone behavior
 148 //  - Returning an InvokerStorageHolder<> with an DoInvoke() that has an arity
 149 //    matching the number of unbound parameters, and knows the correct
 150 //    refcounting semantics for the target object if we are binding a class
 151 //    method.
 152 //
 153 // The Bind functions do the above using type-inference, and template
 154 // specializations.
 155 //
 156 // By default Bind() will store copies of all bound parameters, and attempt
 157 // to refcount a target object if the function being bound is a class method.
 158 //
 159 // To change this behavior, we introduce a set of argument wrappers
 160 // (eg. Unretained(), and ConstRef()).  These are simple container templates
 161 // that are passed by value, and wrap a pointer to argument.  See the
 162 // file-level comment in base/bind_helpers.h for more info.
 163 //
 164 // These types are passed to the Unwrap() functions, and the MaybeRefcount()
 165 // functions respectively to modify the behavior of Bind().  The Unwrap()
 166 // and MaybeRefcount() functions change behavior by doing partial
 167 // specialization based on whether or not a parameter is a wrapper type.
 168 //
 169 // ConstRef() is similar to tr1::cref.  Unretained() is specific to Chromium.
 170 //
 171 //
 172 // WHY NOT TR1 FUNCTION/BIND?
 173 //
 174 // Direct use of tr1::function and tr1::bind was considered, but ultimately
 175 // rejected because of the number of copy constructors invocations involved
 176 // in the binding of arguments during construction, and the forwarding of
 177 // arguments during invocation.  These copies will no longer be an issue in
 178 // C++0x because C++0x will support rvalue reference allowing for the compiler
 179 // to avoid these copies.  However, waiting for C++0x is not an option.
 180 //
 181 // Measured with valgrind on gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5), the
 182 // tr1::bind call itself will invoke a non-trivial copy constructor three times
 183 // for each bound parameter.  Also, each when passing a tr1::function, each
 184 // bound argument will be copied again.
 185 //
 186 // In addition to the copies taken at binding and invocation, copying a
 187 // tr1::function causes a copy to be made of all the bound parameters and
 188 // state.
 189 //
 190 // Furthermore, in Chromium, it is desirable for the Callback to take a
 191 // reference on a target object when representing a class method call.  This
 192 // is not supported by tr1.
 193 //
 194 // Lastly, tr1::function and tr1::bind has a more general and flexible API.
 195 // This includes things like argument reordering by use of
 196 // tr1::bind::placeholder, support for non-const reference parameters, and some
 197 // limited amount of subtyping of the tr1::function object (eg.,
 198 // tr1::function<int(int)> is convertible to tr1::function<void(int)>).
 199 //
 200 // These are not features that are required in Chromium. Some of them, such as
 201 // allowing for reference parameters, and subtyping of functions, may actually
 202 // become a source of errors. Removing support for these features actually
 203 // allows for a simpler implementation, and a terser Currying API.
 204 //
 205 //
 206 // WHY NOT GOOGLE CALLBACKS?
 207 //
 208 // The Google callback system also does not support refcounting.  Furthermore,
 209 // its implementation has a number of strange edge cases with respect to type
 210 // conversion of its arguments.  In particular, the argument's constness must
 211 // at times match exactly the function signature, or the type-inference might
 212 // break.  Given the above, writing a custom solution was easier.
 213 //
 214 //
 215 // MISSING FUNCTIONALITY
 216 //  - Invoking the return of Bind.  Bind(&foo).Run() does not work;
 217 //  - Binding arrays to functions that take a non-const pointer.
 218 //    Example:
 219 //      void Foo(const char* ptr);
 220 //      void Bar(char* ptr);
 221 //      Bind(&Foo, "test");
 222 //      Bind(&Bar, "test");  // This fails because ptr is not const.
 223
 224 namespace base {
 225
 226 // First, we forward declare the Callback class template. This informs the
 227 // compiler that the template only has 1 type parameter which is the function
 228 // signature that the Callback is representing.
 229 //
 230 // After this, create template specializations for 0-6 parameters. Note that
 231 // even though the template typelist grows, the specialization still
 232 // only has one type: the function signature.
 233 template <typename Sig>
 234 class Callback;
 235
 236 template <typename R>
 237 class Callback<R(void)> : public internal::CallbackBase {
 238  public:
 239   typedef R(*PolymorphicInvoke)(
 240       internal::InvokerStorageBase*);
 241
 242   Callback() : CallbackBase(NULL, NULL) { }
 243
 244   // We pass InvokerStorageHolder by const ref to avoid incurring an
 245   // unnecessary AddRef/Unref pair even though we will modify the object.
 246   // We cannot use a normal reference because the compiler will warn
 247   // since this is often used on a return value, which is a temporary.
 248   //
 249   // Note that this constructor CANNOT be explicit, and that Bind() CANNOT
 250   // return the exact Callback<> type.  See base/bind.h for details.
 251   template <typename T>
 252   Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
 253       : CallbackBase(
 254           reinterpret_cast<InvokeFuncStorage>(&T::Invoker::DoInvoke),
 255           &invoker_holder.invoker_storage_) {
 256     COMPILE_ASSERT((is_same<PolymorphicInvoke,
 257                     typename T::Invoker::DoInvokeType>::value),
 258                    callback_type_does_not_match_bind_result);
 259   }
 260
 261   bool Equals(const Callback& other) const {
 262     return CallbackBase::Equals(other);
 263   }
 264
 265   R Run() const {
 266     PolymorphicInvoke f =
 267         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 268
 269     return f(invoker_storage_.get());
 270   }
 271 };
 272
 273 template <typename R, typename A1>
 274 class Callback<R(A1)> : public internal::CallbackBase {
 275  public:
 276   typedef R(*PolymorphicInvoke)(
 277       internal::InvokerStorageBase*,
 278           typename internal::ParamTraits<A1>::ForwardType);
 279
 280   Callback() : CallbackBase(NULL, NULL) { }
 281
 282   // We pass InvokerStorageHolder by const ref to avoid incurring an
 283   // unnecessary AddRef/Unref pair even though we will modify the object.
 284   // We cannot use a normal reference because the compiler will warn
 285   // since this is often used on a return value, which is a temporary.
 286   //
 287   // Note that this constructor CANNOT be explicit, and that Bind() CANNOT
 288   // return the exact Callback<> type.  See base/bind.h for details.
 289   template <typename T>
 290   Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
 291       : CallbackBase(
 292           reinterpret_cast<InvokeFuncStorage>(&T::Invoker::DoInvoke),
 293           &invoker_holder.invoker_storage_) {
 294     COMPILE_ASSERT((is_same<PolymorphicInvoke,
 295                     typename T::Invoker::DoInvokeType>::value),
 296                    callback_type_does_not_match_bind_result);
 297   }
 298
 299   bool Equals(const Callback& other) const {
 300     return CallbackBase::Equals(other);
 301   }
 302
 303   R Run(typename internal::ParamTraits<A1>::ForwardType a1) const {
 304     PolymorphicInvoke f =
 305         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 306
 307     return f(invoker_storage_.get(), a1);
 308   }
 309 };
 310
 311 template <typename R, typename A1, typename A2>
 312 class Callback<R(A1, A2)> : public internal::CallbackBase {
 313  public:
 314   typedef R(*PolymorphicInvoke)(
 315       internal::InvokerStorageBase*,
 316           typename internal::ParamTraits<A1>::ForwardType,
 317           typename internal::ParamTraits<A2>::ForwardType);
 318
 319   Callback() : CallbackBase(NULL, NULL) { }
 320
 321   // We pass InvokerStorageHolder by const ref to avoid incurring an
 322   // unnecessary AddRef/Unref pair even though we will modify the object.
 323   // We cannot use a normal reference because the compiler will warn
 324   // since this is often used on a return value, which is a temporary.
 325   //
 326   // Note that this constructor CANNOT be explicit, and that Bind() CANNOT
 327   // return the exact Callback<> type.  See base/bind.h for details.
 328   template <typename T>
 329   Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
 330       : CallbackBase(
 331           reinterpret_cast<InvokeFuncStorage>(&T::Invoker::DoInvoke),
 332           &invoker_holder.invoker_storage_) {
 333     COMPILE_ASSERT((is_same<PolymorphicInvoke,
 334                     typename T::Invoker::DoInvokeType>::value),
 335                    callback_type_does_not_match_bind_result);
 336   }
 337
 338   bool Equals(const Callback& other) const {
 339     return CallbackBase::Equals(other);
 340   }
 341
 342   R Run(typename internal::ParamTraits<A1>::ForwardType a1,
 343         typename internal::ParamTraits<A2>::ForwardType a2) const {
 344     PolymorphicInvoke f =
 345         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 346
 347     return f(invoker_storage_.get(), a1,
 348              a2);
 349   }
 350 };
 351
 352 template <typename R, typename A1, typename A2, typename A3>
 353 class Callback<R(A1, A2, A3)> : public internal::CallbackBase {
 354  public:
 355   typedef R(*PolymorphicInvoke)(
 356       internal::InvokerStorageBase*,
 357           typename internal::ParamTraits<A1>::ForwardType,
 358           typename internal::ParamTraits<A2>::ForwardType,
 359           typename internal::ParamTraits<A3>::ForwardType);
 360
 361   Callback() : CallbackBase(NULL, NULL) { }
 362
 363   // We pass InvokerStorageHolder by const ref to avoid incurring an
 364   // unnecessary AddRef/Unref pair even though we will modify the object.
 365   // We cannot use a normal reference because the compiler will warn
 366   // since this is often used on a return value, which is a temporary.
 367   //
 368   // Note that this constructor CANNOT be explicit, and that Bind() CANNOT
 369   // return the exact Callback<> type.  See base/bind.h for details.
 370   template <typename T>
 371   Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
 372       : CallbackBase(
 373           reinterpret_cast<InvokeFuncStorage>(&T::Invoker::DoInvoke),
 374           &invoker_holder.invoker_storage_) {
 375     COMPILE_ASSERT((is_same<PolymorphicInvoke,
 376                     typename T::Invoker::DoInvokeType>::value),
 377                    callback_type_does_not_match_bind_result);
 378   }
 379
 380   bool Equals(const Callback& other) const {
 381     return CallbackBase::Equals(other);
 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) const {
 387     PolymorphicInvoke f =
 388         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 389
 390     return f(invoker_storage_.get(), a1,
 391              a2,
 392              a3);
 393   }
 394 };
 395
 396 template <typename R, typename A1, typename A2, typename A3, typename A4>
 397 class Callback<R(A1, A2, A3, A4)> : public internal::CallbackBase {
 398  public:
 399   typedef R(*PolymorphicInvoke)(
 400       internal::InvokerStorageBase*,
 401           typename internal::ParamTraits<A1>::ForwardType,
 402           typename internal::ParamTraits<A2>::ForwardType,
 403           typename internal::ParamTraits<A3>::ForwardType,
 404           typename internal::ParamTraits<A4>::ForwardType);
 405
 406   Callback() : CallbackBase(NULL, NULL) { }
 407
 408   // We pass InvokerStorageHolder by const ref to avoid incurring an
 409   // unnecessary AddRef/Unref pair even though we will modify the object.
 410   // We cannot use a normal reference because the compiler will warn
 411   // since this is often used on a return value, which is a temporary.
 412   //
 413   // Note that this constructor CANNOT be explicit, and that Bind() CANNOT
 414   // return the exact Callback<> type.  See base/bind.h for details.
 415   template <typename T>
 416   Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
 417       : CallbackBase(
 418           reinterpret_cast<InvokeFuncStorage>(&T::Invoker::DoInvoke),
 419           &invoker_holder.invoker_storage_) {
 420     COMPILE_ASSERT((is_same<PolymorphicInvoke,
 421                     typename T::Invoker::DoInvokeType>::value),
 422                    callback_type_does_not_match_bind_result);
 423   }
 424
 425   bool Equals(const Callback& other) const {
 426     return CallbackBase::Equals(other);
 427   }
 428
 429   R Run(typename internal::ParamTraits<A1>::ForwardType a1,
 430         typename internal::ParamTraits<A2>::ForwardType a2,
 431         typename internal::ParamTraits<A3>::ForwardType a3,
 432         typename internal::ParamTraits<A4>::ForwardType a4) const {
 433     PolymorphicInvoke f =
 434         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 435
 436     return f(invoker_storage_.get(), a1,
 437              a2,
 438              a3,
 439              a4);
 440   }
 441 };
 442
 443 template <typename R, typename A1, typename A2, typename A3, typename A4,
 444     typename A5>
 445 class Callback<R(A1, A2, A3, A4, A5)> : public internal::CallbackBase {
 446  public:
 447   typedef R(*PolymorphicInvoke)(
 448       internal::InvokerStorageBase*,
 449           typename internal::ParamTraits<A1>::ForwardType,
 450           typename internal::ParamTraits<A2>::ForwardType,
 451           typename internal::ParamTraits<A3>::ForwardType,
 452           typename internal::ParamTraits<A4>::ForwardType,
 453           typename internal::ParamTraits<A5>::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     COMPILE_ASSERT((is_same<PolymorphicInvoke,
 470                     typename T::Invoker::DoInvokeType>::value),
 471                    callback_type_does_not_match_bind_result);
 472   }
 473
 474   bool Equals(const Callback& other) const {
 475     return CallbackBase::Equals(other);
 476   }
 477
 478   R Run(typename internal::ParamTraits<A1>::ForwardType a1,
 479         typename internal::ParamTraits<A2>::ForwardType a2,
 480         typename internal::ParamTraits<A3>::ForwardType a3,
 481         typename internal::ParamTraits<A4>::ForwardType a4,
 482         typename internal::ParamTraits<A5>::ForwardType a5) const {
 483     PolymorphicInvoke f =
 484         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 485
 486     return f(invoker_storage_.get(), a1,
 487              a2,
 488              a3,
 489              a4,
 490              a5);
 491   }
 492 };
 493
 494 template <typename R, typename A1, typename A2, typename A3, typename A4,
 495     typename A5, typename A6>
 496 class Callback<R(A1, A2, A3, A4, A5, A6)> : public internal::CallbackBase {
 497  public:
 498   typedef R(*PolymorphicInvoke)(
 499       internal::InvokerStorageBase*,
 500           typename internal::ParamTraits<A1>::ForwardType,
 501           typename internal::ParamTraits<A2>::ForwardType,
 502           typename internal::ParamTraits<A3>::ForwardType,
 503           typename internal::ParamTraits<A4>::ForwardType,
 504           typename internal::ParamTraits<A5>::ForwardType,
 505           typename internal::ParamTraits<A6>::ForwardType);
 506
 507   Callback() : CallbackBase(NULL, NULL) { }
 508
 509   // We pass InvokerStorageHolder by const ref to avoid incurring an
 510   // unnecessary AddRef/Unref pair even though we will modify the object.
 511   // We cannot use a normal reference because the compiler will warn
 512   // since this is often used on a return value, which is a temporary.
 513   //
 514   // Note that this constructor CANNOT be explicit, and that Bind() CANNOT
 515   // return the exact Callback<> type.  See base/bind.h for details.
 516   template <typename T>
 517   Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
 518       : CallbackBase(
 519           reinterpret_cast<InvokeFuncStorage>(&T::Invoker::DoInvoke),
 520           &invoker_holder.invoker_storage_) {
 521     COMPILE_ASSERT((is_same<PolymorphicInvoke,
 522                     typename T::Invoker::DoInvokeType>::value),
 523                    callback_type_does_not_match_bind_result);
 524   }
 525
 526   bool Equals(const Callback& other) const {
 527     return CallbackBase::Equals(other);
 528   }
 529
 530   R Run(typename internal::ParamTraits<A1>::ForwardType a1,
 531         typename internal::ParamTraits<A2>::ForwardType a2,
 532         typename internal::ParamTraits<A3>::ForwardType a3,
 533         typename internal::ParamTraits<A4>::ForwardType a4,
 534         typename internal::ParamTraits<A5>::ForwardType a5,
 535         typename internal::ParamTraits<A6>::ForwardType a6) const {
 536     PolymorphicInvoke f =
 537         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 538
 539     return f(invoker_storage_.get(), a1,
 540              a2,
 541              a3,
 542              a4,
 543              a5,
 544              a6);
 545   }
 546 };
 547
 548
 549 // Syntactic sugar to make Callbacks<void(void)> easier to declare since it
 550 // will be used in a lot of APIs with delayed execution.
 551 typedef Callback<void(void)> Closure;
 552
 553 }  // namespace base
 554
 555 #endif  // BASE_CALLBACK_H