Unified callback system.

base/uber_callback.h.pump

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+$$ This is a pump file for generating file templates.  Pump is a python
+$$ script that is part of the Google Test suite of utilities.  Description
+$$ can be found here:
+$$
+$$ http://code.google.com/p/googletest/wiki/PumpManual
+$$
+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_UBER_CALLBACK_H_
+#define BASE_UBER_CALLBACK_H_
+#pragma once
+
+#include "base/uber_callback_helpers.h"
+
+// New, super-duper, unified Callback system.  This will eventually replace

[willchan no longer on Chromium, 2011/02/06 10:26:50]

Change comments. This will look silly in 2-3 years time :) I try to keep
comments referring to to the present.

+// NewRunnableMethod, NewRunnableFunction, CreateFunctor, and CreateCallback
+// systems currently in the Chromium code base.
+//
+// WHAT IS THIS:
+//
+// The templated Callback class is a generalized funciton object. Together
+// with the Prebind() function in prebind.h, they provide a type-safe method
+// for performing currying of arguments, and createing a "closure."
+//
+// In programing languages, a closure is a first-class function where all its
+// parameters have been bound (usually via currying).  Closures are well
+// suited for representing, and passing around a unit of delayed execution.
+// They are used in Chromium code to schedule tasks on different MessageLoops.
+//
+// EXAMPLE USAGE:
+//
+// /* Binding a class member. */
+// class Ref : public RefCountedThreadSafe<Ref> {
+//   int Foo() { return 3; }
+// };
+// scoped_refptr<Ref> ref = new Ref();
+// Callback<int(void)> ref_cb = Prebind(&Ref::Foo, ref.get());
+// LOG(INFO) << ref_cb.Run();  // Prints out 3.
+//
+// /* Binding a class member for a non-refcounted class. */
+// class NoRef {
+//   int Foo() { return 4; }
+// };
+// NoRef no_ref;
+// Callback<int(void)> no_ref_cb = Prebind(&NoRef::Foo, Unretained(&no_ref));
+// LOG(INFO) << ref_cb.Run();  // Prints out 4.
+//
+// /* Binding a normal function. */
+// int Return5() { return 5; }
+// Callback<int(int)> func_cb = Prebind(&Return5);
+// LOG(INFO) << func_cb.Run(5);  // Prints 5.
+//
+// /* Binding a reference. */
+// int Identity(int n) { return n; }
+// int value = 1;
+// Callback<int(void)> bound_copy_cb = Prebind(&Identity, value);
+// Callback<int(void)> bound_ref_cb = Prebind(&Identity, ConstRef(value));
+// LOG(INFO) << bound_copy_cb.Run();  // Prints 1.
+// LOG(INFO) << bound_ref_cb.Run();  // Prints 1.
+// value = 2;
+// LOG(INFO) << bound_copy_cb.Run();  // Prints 1.
+// LOG(INFO) << bound_ref_cb.Run();  // Prints 2.
+//
+//
+// WHERE IS THIS DESIGN FROM:
+//
+// The design Callback and Prebind is heavily influenced by C++'s
+// tr1::function/tr1::bind, and by the "Google Callback" system used inside
+// Google.
+//
+//
+// WHY NOT TR1 FUNCTION/BIND?
+//
+// Direct use of tr1::function and tr1::bind was considered, but ultimately
+// rejected because of the number of copy constructors invocations involved
+// in the binding of arguments during construction, and the forwarding of
+// arguments during invocation.  These copies will no longer be an issue in
+// C++0x because C++0x will support rvalue reference allowing for the compiler
+// to avoid these copies.  However, waiting for C++0x is not an option.
+//
+// Measured with valgrind on gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5), the
+// tr1::bind call itself will invoke a non-trivial copy constructor three times
+// for each bound parameter.  Also, each when passing a tr1::function, each
+// bound argument will be copied again.
+//
+// In addition to the copies taken at binding and invocation, copying a
+// tr1::function causes a copy to be made of all the bound parameters and
+// state.
+//
+// Furthermore, in Chromium, it is desirable for the Callback to take a
+// reference on a target object when representing a class method call.  This
+// is not supported by tr1.
+//
+// Lastly, tr1::function and tr1::bind has a more general and flexible API.
+// This includes things like argument reordering by use of
+// tr1::bind::placeholder, support for non-const reference parameters, and some
+// limited amount of subtyping of the tr1::function object (eg.,
+// tr1::function<int(int)> is convertable to tr1::function<void(int)>).
+//
+// These are not features that are required in Chromium. Some of them, such as
+// allowing for reference parameters, and subtyping of functions, may actually
+// because a source of errors. Removing support for these features actually
+// allows for a simpler implementation, and a terser Currying API.
+
+namespace base {
+
+$var MAX_ARITY = 6
+
+// First, we forward declare the Callback class template. This informs the
+// compiler that ther template only have 1 type parameter: the function
+// signature that the Callback is abstracting.
+//
+// After this, create template specializations for 0-$(MAX_ARITY) parameters. Note that
+// even though the template typelist grows, that the specialization still
+// only has one type: the function signature.
+//
+// Also, note that the templated constructor should *not* be explicit. This is
+// to allow the natural assignment syntax from the result of Prebind(), which
+// is not the same type as Callback().  See the description of Prebind for
+// details.
+template <typename Sig>
+class Callback;
+
+
+$range ARITY 0..MAX_ARITY
+$for ARITY [[
+$range ARG 1..ARITY
+
+$if ARITY == 0 [[
+template <typename R>
+class Callback<R(void)> {
+]] $else [[
+template <typename R, $for ARG , [[typename A$(ARG)]]>
+class Callback<R($for ARG , [[A$(ARG)]])> {
+]]
+
+ public:
+  Callback() : polymorphic_invoke_(NULL) { }
+
+  typedef R(*PolymorphicInvoke)(internal::InvokerStorageBase*[[]]
+$if ARITY != 0 [[, ]] $for ARG , [[const A$(ARG)&]]);
+
+  template <typename T>
+  Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
+      : polymorphic_invoke_(&T::FunctionTraits::DoInvoke) {
+    invoker_storage_.swap(invoker_holder.invoker_storage_);
+  }
+
+
+$if ARITY == 0 [[
+  R Run(void) {
+]] $else [[
+  R Run($for ARG , [[const A$(ARG)& a$(ARG)]]) {
+]]
+
+    return polymorphic_invoke_(invoker_storage_.get()[[]]
+$if ARITY != 0 [[, ]] $for ARG , [[a$(ARG)]]);
+  }
+
+ private:
+  scoped_refptr<internal::InvokerStorageBase> invoker_storage_;
+  PolymorphicInvoke polymorphic_invoke_;
+};
+
+
+]]  $$ for ARITY
+
+// Syntactic sugar to make Callbacks<void(void)> easier to read since it will
+// be used in a lot of APIs with delayed execution.
+typedef Callback<void(void)> Closure;
+
+}  // namespace base
+
+#endif  // BASE_UBER_CALLBACK_H