Unified callback system.

base/prebind.h

Index: base/prebind.h
diff --git a/base/prebind.h b/base/prebind.h
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+++ b/base/prebind.h
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+// 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_PREBIND_H_
+#define BASE_PREBIND_H_
+
+#include "base/callback.h"
+#include "base/tuple.h"
+
+namespace base {
+
+// TODO(ajwong): Can we somehow reuse Tuple here?
+
+// Invoker is the class where most of the magic happens.  It stores the functor,
+// the bound arguments, and free arguments.  This class is specialized based on
+// the types of all three of those components.
+//
+// Currently, the specializations are done by encoding the type information via
+// nested function signature types.  For example, a 2 argument function with 2
+// bound parameters is encoded using a type that is a "function of 2 arguments,
+// that returns a pointer to a function of 2 arguments."  The outer function's
+// signature holds the types of the bound parameters, and the returned function
+// pointer holds the type of the acutal function.
+//
+// For the same 2 argument function with 1 bound parameter, the type of Invoker
+// is "a function of 1 argument that returns a pointer to a function of 2
+// arguments."
+//
+// This method of encoding the types allows us to keep the single class name
+// "Invoker".
+//
+// TODO(ajwong): Change this to use the tuple type to avoid having such a crazy
+// signature.
+template <typename BoundFuncType>
+class Invoker;
+
+// 1 -> 0
+template <typename R, typename X0, typename P0>
+class Invoker<R(*(X0))(P0)> : public InvokerBase {
+ public:
+  typedef InvokerBase<R(X0)(P0)> SelfType;

[willchan no longer on Chromium, 2011/01/19 16:57:43]

I'm frankly a bit confused why this is InvokerBase<...> rather than
Invoker<...>.

[awong, 2011/01/20 20:27:35]

Errr...yes, that's just a mistake.  I uploaded without fully getting it to
compile since I ran out of time to work on it. :(

+
+  Invoker(R(*f)(X0), const P0& p0) : f_(f), p0_(p0) {}
+
+  static R PolymorphicInvoke(InvokerBase* invoker) {
+    SelfType* unwrapped = static_cast<SelfType*>(invoker);
+    return unwrapped->f_(unwrapped->p0_);
+  }
+
+ private:
+  Func f_;
+  P0 p0_;
+};
+
+// 2 -> 0
+template <typename R, typename X0, typename P0, typename X1, typename P1>
+class Invoker<R(*(X0,X1))(P0,P1)> : public InvokerBase {
+ public:
+  typedef InvokerBase<R(X0,X1)(P0,P1)> SelfType;
+
+  Invoker(R(*f)(X0,X1), const P0& p0, const P1& p1) : f_(f), p0_(p0), p1_(p1) {
+  }
+
+  static R PolymorphicInvoke(InvokerBase* invoker) {
+    SelfType* unwrapped = static_cast<SelfType*>(invoker);
+    return unwrapped->f_(unwrapped->p0_, unwrapped->p1_);
+  }
+
+ private:
+  Func f_;
+  P0 p0_;
+  P1 p1_;
+};
+
+// 3 -> 0
+template <typename R, typename X0, typename P0, typename X1, typename P1,
+          typename X2, typename P2>
+class Invoker<R(*(X0,X1,X2))(P0,P1,P2)> : public InvokerBase {
+ public:
+  typedef InvokerBase<R(X0,X1,X2)(P0,P1,P2)> SelfType;
+
+  Invoker(R(*f)(X0,X1), const P0& p0, const P1& p1)
+      : f_(f), p0_(p0), p1_(p1), p2_(p2) {
+  }
+
+  static R PolymorphicInvoke(InvokerBase* invoker) {
+    SelfType* unwrapped = static_cast<SelfType*>(invoker);
+    return unwrapped->f_(unwrapped->p0_, unwrapped->p1_);
+  }
+
+ private:
+  Func f_;
+  P0 p0_;
+  P1 p1_;
+  P2 p2_;
+};
+
+// 2 -> 1
+template <typename R, typename X0, typename P0, typename X1, typename A1>
+class Invoker<R(*(*(X0,X1))(P0))(A1)> : public InvokerBase {
+ public:
+  typedef InvokerBase<R(X0,X1)(P0)> SelfType;
+
+  Invoker(R(*f)(X0,X1), const P0& p0, const P1& p1) : f_(f), p0_(p0) {
+  }
+
+  static R PolymorphicInvoke(InvokerBase* invoker, const A1& a1) {
+    SelfType* unwrapped = static_cast<SelfType*>(invoker);
+    return unwrapped->f_(unwrapped->p0_, const_cast<A1&>(a1));

[willchan no longer on Chromium, 2011/01/19 16:57:43]

I'm trying to grok everything, but this const_cast<> looks like it's probably
caused by an implementation error. I don't believe this should be necessary. Is
it? I need to think about it more.

[awong, 2011/01/20 20:27:35]

Actually, I do think it's necessary.  If A1 == "int*", then the variable a1
actually has the type "const int*&" which I think means that if you pass to
"unwrapped->f_", you will get a const conversion violation.

The const_cast<> is actually returning a1 back to the original constness of the
function.

You need "const A1&" because if it's just "A1&", and you pass a literal (eg.,
int 1), you will get a warning.

I need to unittest the edge cases to be sure, but I think this is the correct
solution....sounds like it needs a comment though.

[willchan no longer on Chromium, 2011/01/21 02:30:12]

Hmmmm, I'm not sure this works. How does it handle the case with:

const int kFooArray[] = { 1, 2, 3 };

void Foo(int* array) { array[0] = 999; }

AFAICT, calling Run(kFooArray) will cause A1 == int*, and when Foo() is invoked,
it'll try to modify kFooArray[0], which may be in the readonly data segment, and
SIGSEGV.

It sounds like you're trying to have Run() always take reference arguments in
order to optimize their calls. But this will also create inefficiencies for most
POD types (except large structs), which should just be copied by value.

I have two proposals here.

(1) If the caller wants Run() to be efficient, then have him/her use
std::tr1::cref() to pass it by reference. Use PolymorphicInvoke(InvokerBase*,
A1);
(2) Add a Invoker specialization for POD types (yes, this require multiply the
templates required). Specialize for is_pod<A1>::value. For true, have
PolymorphicInvoke(InvokerBase*, A1). For false, have
PolymorphicInvoke(InvokerBase*, A1&).

I'm open to other options, those are just the ones that immediately come to
mind.

[awong, 2011/01/21 12:00:36]

Ah, I see what you're saying now.  Because I used a const A&, the compiler will
allows a const object to be passed...and then I const_cast it into the wrong
type.  Good catch.

Suggestion #2 sounds the nicest.

For now, I'll revert to a copy and add a TODO.

On 2011/01/21 02:30:12, willchan wrote:

+  }
+
+ private:
+  Func f_;
+  P0 p0_;
+};
+
+// Function 1 -> 0
+template <typename R, typename X0, typename P0>
+Callback<R(void)>
+Prebind(R(*f)(X0), const P0& p0) {
+  typedef Invoker<R(*(X0))(P0)> InvokerType;
+  return Callback<R(void)>(new InvokerType(f, p0),
+                           &InvokerType::PolymorphicInvoke);
+}
+
+// Function 2 -> 0
+template <typename R, typename X0, typename P0, typename X1, typename P1>
+Callback<R(void)>
+Prebind(R(*f)(X0,X1), const P0& p0, const P1& p1) {
+  typedef Invoker<R(*(X0,X1))(P0,P1)> InvokerType;
+  return Callback<R(void)>(new InvokerType(f, p0, p1),
+                           &InvokerType::PolymorphicInvoke);
+}
+
+// Function 3 -> 0
+template <typename R, typename X0, typename P0, typename X1, typename P1
+          typename X2, typename P2>
+Callback<R(void)>
+Prebind(R(*f)(X0,X1,X2), const P0& p0, const P1& p1, const P2& p2) {
+  typedef Invoker<R(*(X0,X1,X2))(P0,P1,P2)> InvokerType;
+  return Callback<R(void)>(new InvokerType(f, p0, p1, p2),
+                           &InvokerType::PolymorphicInvoke);
+}
+
+// Function 2 -> 1
+template <typename R, typename X0, typename P0, typename A0>
+Callback<R(A0)>
+Prebind(R(*f)(X0,X1), const P0& p0) {
+  typedef Invoker<R(*(*(X0,X1,X2))(P0))(A0)> InvokerType;
+  return Callback<R(A0)>(new InvokerType(f, p0),
+                         &InvokerType::PolymorphicInvoke);
+}
+
+}  // namespace base
+
+#endif  // BASE_PREBIND_H_