1 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
5 // This defines a set of argument wrappers and related factory methods that
6 // can be used specify the refcounting and reference semantics of arguments
7 // that are bound by the Bind() function in base/bind.h.
9 // The public functions are base::Unretained(), base::Owned(),
10 // base::ConstRef(), and base::IgnoreReturn().
12 // Unretained() allows Bind() to bind a non-refcounted class, and to disable
13 // refcounting on arguments that are refcounted objects.
14 // Owned() transfers ownership of an object to the Callback resulting from
15 // bind; the object will be deleted when the Callback is deleted.
16 // ConstRef() allows binding a constant reference to an argument rather
18 // IgnoreReturn() is used to adapt a 0-argument Callback with a return type to
19 // a Closure. This is useful if you need to PostTask with a function that has
20 // a return value that you don't care about.
23 // EXAMPLE OF Unretained():
27 // void func() { cout << "Foo:f" << endl; }
30 // // In some function somewhere.
32 // Closure foo_callback =
33 // Bind(&Foo::func, Unretained(&foo));
34 // foo_callback.Run(); // Prints "Foo:f".
36 // Without the Unretained() wrapper on |&foo|, the above call would fail
37 // to compile because Foo does not support the AddRef() and Release() methods.
40 // EXAMPLE OF Owned():
42 // void foo(int* arg) { cout << *arg << endl }
44 // int* pn = new int(1);
45 // Closure foo_callback = Bind(&foo, Owned(pn));
47 // foo_callback.Run(); // Prints "1"
48 // foo_callback.Run(); // Prints "1"
50 // foo_callback.Run(); // Prints "2"
52 // foo_callback.Reset(); // |pn| is deleted. Also will happen when
53 // // |foo_callback| goes out of scope.
55 // Without Owned(), someone would have to know to delete |pn| when the last
56 // reference to the Callback is deleted.
59 // EXAMPLE OF ConstRef():
61 // void foo(int arg) { cout << arg << endl }
64 // Closure no_ref = Bind(&foo, n);
65 // Closure has_ref = Bind(&foo, ConstRef(n));
67 // no_ref.Run(); // Prints "1"
68 // has_ref.Run(); // Prints "1"
71 // no_ref.Run(); // Prints "1"
72 // has_ref.Run(); // Prints "2"
74 // Note that because ConstRef() takes a reference on |n|, |n| must outlive all
75 // its bound callbacks.
78 // EXAMPLE OF IgnoreReturn():
80 // int DoSomething(int arg) { cout << arg << endl; }
81 // Callback<int(void)> cb = Bind(&DoSomething, 1);
82 // Closure c = IgnoreReturn(cb); // Prints "1"
84 // ml->PostTask(FROM_HERE, IgnoreReturn(cb)); // Prints "1" on |ml|
86 #ifndef BASE_BIND_HELPERS_H_
87 #define BASE_BIND_HELPERS_H_
90 #include "base/basictypes.h"
91 #include "base/bind.h"
92 #include "base/callback.h"
93 #include "base/memory/weak_ptr.h"
94 #include "base/template_util.h"
99 // Use the Substitution Failure Is Not An Error (SFINAE) trick to inspect T
100 // for the existence of AddRef() and Release() functions of the correct
103 // http://en.wikipedia.org/wiki/Substitution_failure_is_not_an_error
104 // http://stackoverflow.com/questions/257288/is-it-possible-to-write-a-c-template-to-check-for-a-functions-existence
105 // http://stackoverflow.com/questions/4358584/sfinae-approach-comparison
106 // http://stackoverflow.com/questions/1966362/sfinae-to-check-for-inherited-member-functions
108 // The last link in particular show the method used below.
110 // For SFINAE to work with inherited methods, we need to pull some extra tricks
111 // with multiple inheritance. In the more standard formulation, the overloads
112 // of Check would be:
114 // template <typename C>
115 // Yes NotTheCheckWeWant(Helper<&C::TargetFunc>*);
117 // template <typename C>
118 // No NotTheCheckWeWant(...);
120 // static const bool value = sizeof(NotTheCheckWeWant<T>(0)) == sizeof(Yes);
122 // The problem here is that template resolution will not match
123 // C::TargetFunc if TargetFunc does not exist directly in C. That is, if
124 // TargetFunc in inherited from an ancestor, &C::TargetFunc will not match,
125 // |value| will be false. This formulation only checks for whether or
126 // not TargetFunc exist directly in the class being introspected.
128 // To get around this, we play a dirty trick with multiple inheritance.
129 // First, We create a class BaseMixin that declares each function that we
130 // want to probe for. Then we create a class Base that inherits from both T
131 // (the class we wish to probe) and BaseMixin. Note that the function
132 // signature in BaseMixin does not need to match the signature of the function
133 // we are probing for; thus it's easiest to just use void(void).
135 // Now, if TargetFunc exists somewhere in T, then &Base::TargetFunc has an
136 // ambiguous resolution between BaseMixin and T. This lets us write the
139 // template <typename C>
140 // No GoodCheck(Helper<&C::TargetFunc>*);
142 // template <typename C>
143 // Yes GoodCheck(...);
145 // static const bool value = sizeof(GoodCheck<Base>(0)) == sizeof(Yes);
147 // Notice here that the variadic version of GoodCheck() returns Yes here
148 // instead of No like the previous one. Also notice that we calculate |value|
149 // by specializing GoodCheck() on Base instead of T.
151 // We've reversed the roles of the variadic, and Helper overloads.
152 // GoodCheck(Helper<&C::TargetFunc>*), when C = Base, fails to be a valid
153 // substitution if T::TargetFunc exists. Thus GoodCheck<Base>(0) will resolve
154 // to the variadic version if T has TargetFunc. If T::TargetFunc does not
155 // exist, then &C::TargetFunc is not ambiguous, and the overload resolution
156 // will prefer GoodCheck(Helper<&C::TargetFunc>*).
158 // This method of SFINAE will correctly probe for inherited names, but it cannot
159 // typecheck those names. It's still a good enough sanity check though.
161 // Works on gcc-4.2, gcc-4.4, and Visual Studio 2008.
163 // TODO(ajwong): Move to ref_counted.h or template_util.h when we've vetted
166 // TODO(ajwong): Make this check for Release() as well.
167 // See http://crbug.com/82038.
168 template <typename T
>
169 class SupportsAddRefAndRelease
{
177 // MSVC warns when you try to use Base if T has a private destructor, the
178 // common pattern for refcounted types. It does this even though no attempt to
179 // instantiate Base is made. We disable the warning for this definition.
181 #pragma warning(disable:4624)
183 struct Base
: public T
, public BaseMixin
{
186 #pragma warning(default:4624)
189 template <void(BaseMixin::*)(void)> struct Helper
{};
191 template <typename C
>
192 static No
& Check(Helper
<&C::AddRef
>*);
195 static Yes
& Check(...);
198 static const bool value
= sizeof(Check
<Base
>(0)) == sizeof(Yes
);
202 // Helpers to assert that arguments of a recounted type are bound with a
204 template <bool IsClasstype
, typename T
>
205 struct UnsafeBindtoRefCountedArgHelper
: false_type
{
208 template <typename T
>
209 struct UnsafeBindtoRefCountedArgHelper
<true, T
>
210 : integral_constant
<bool, SupportsAddRefAndRelease
<T
>::value
> {
213 template <typename T
>
214 struct UnsafeBindtoRefCountedArg
: false_type
{
217 template <typename T
>
218 struct UnsafeBindtoRefCountedArg
<T
*>
219 : UnsafeBindtoRefCountedArgHelper
<is_class
<T
>::value
, T
> {
223 template <typename T
>
224 class UnretainedWrapper
{
226 explicit UnretainedWrapper(T
* o
) : ptr_(o
) {}
227 T
* get() const { return ptr_
; }
232 template <typename T
>
233 class ConstRefWrapper
{
235 explicit ConstRefWrapper(const T
& o
) : ptr_(&o
) {}
236 const T
& get() const { return *ptr_
; }
241 // An alternate implementation is to avoid the destructive copy, and instead
242 // specialize ParamTraits<> for OwnedWrapper<> to change the StorageType to
243 // a class that is essentially a scoped_ptr<>.
245 // The current implementation has the benefit though of leaving ParamTraits<>
246 // fully in callback_internal.h as well as avoiding type conversions during
248 template <typename T
>
251 explicit OwnedWrapper(T
* o
) : ptr_(o
) {}
252 ~OwnedWrapper() { delete ptr_
; }
253 T
* get() const { return ptr_
; }
254 OwnedWrapper(const OwnedWrapper
& other
) {
264 // Unwrap the stored parameters for the wrappers above.
265 template <typename T
>
266 T
Unwrap(T o
) { return o
; }
268 template <typename T
>
269 T
* Unwrap(UnretainedWrapper
<T
> unretained
) { return unretained
.get(); }
271 template <typename T
>
272 const T
& Unwrap(ConstRefWrapper
<T
> const_ref
) {
273 return const_ref
.get();
276 template <typename T
>
277 T
* Unwrap(const scoped_refptr
<T
>& o
) { return o
.get(); }
279 template <typename T
>
280 const WeakPtr
<T
>& Unwrap(const WeakPtr
<T
>& o
) { return o
; }
282 template <typename T
>
283 T
* Unwrap(const OwnedWrapper
<T
>& o
) {
287 // Utility for handling different refcounting semantics in the Bind()
289 template <typename IsMethod
, typename T
>
290 struct MaybeRefcount
;
292 template <typename T
>
293 struct MaybeRefcount
<base::false_type
, T
> {
294 static void AddRef(const T
&) {}
295 static void Release(const T
&) {}
298 template <typename T
, size_t n
>
299 struct MaybeRefcount
<base::false_type
, T
[n
]> {
300 static void AddRef(const T
*) {}
301 static void Release(const T
*) {}
304 template <typename T
>
305 struct MaybeRefcount
<base::true_type
, T
*> {
306 static void AddRef(T
* o
) { o
->AddRef(); }
307 static void Release(T
* o
) { o
->Release(); }
310 template <typename T
>
311 struct MaybeRefcount
<base::true_type
, UnretainedWrapper
<T
> > {
312 static void AddRef(const UnretainedWrapper
<T
>&) {}
313 static void Release(const UnretainedWrapper
<T
>&) {}
316 template <typename T
>
317 struct MaybeRefcount
<base::true_type
, OwnedWrapper
<T
> > {
318 static void AddRef(const OwnedWrapper
<T
>&) {}
319 static void Release(const OwnedWrapper
<T
>&) {}
322 // No need to additionally AddRef() and Release() since we are storing a
323 // scoped_refptr<> inside the storage object already.
324 template <typename T
>
325 struct MaybeRefcount
<base::true_type
, scoped_refptr
<T
> > {
326 static void AddRef(const scoped_refptr
<T
>& o
) {}
327 static void Release(const scoped_refptr
<T
>& o
) {}
330 template <typename T
>
331 struct MaybeRefcount
<base::true_type
, const T
*> {
332 static void AddRef(const T
* o
) { o
->AddRef(); }
333 static void Release(const T
* o
) { o
->Release(); }
336 template <typename T
>
337 struct MaybeRefcount
<base::true_type
, WeakPtr
<T
> > {
338 static void AddRef(const WeakPtr
<T
>&) {}
339 static void Release(const WeakPtr
<T
>&) {}
342 template <typename R
>
343 void VoidReturnAdapter(Callback
<R(void)> callback
) {
347 } // namespace internal
349 template <typename T
>
350 inline internal::UnretainedWrapper
<T
> Unretained(T
* o
) {
351 return internal::UnretainedWrapper
<T
>(o
);
354 template <typename T
>
355 inline internal::ConstRefWrapper
<T
> ConstRef(const T
& o
) {
356 return internal::ConstRefWrapper
<T
>(o
);
359 template <typename T
>
360 inline internal::OwnedWrapper
<T
> Owned(T
* o
) {
361 return internal::OwnedWrapper
<T
>(o
);
364 template <typename R
>
365 Closure
IgnoreReturn(Callback
<R(void)> callback
) {
366 return Bind(&internal::VoidReturnAdapter
<R
>, callback
);
371 #endif // BASE_BIND_HELPERS_H_