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// Copyright (c) 2012 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.
// This file contains utility functions and classes that help the
// implementation, and management of the Callback objects.
#include <stddef.h>
#include <map>
#include <memory>
#include <type_traits>
#include <vector>
#include "base/base_export.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/memory/scoped_ptr.h"
#include "base/template_util.h"
namespace base {
namespace internal {
class CallbackBase;
// BindStateBase is used to provide an opaque handle that the Callback
// class can use to represent a function object with bound arguments. It
// behaves as an existential type that is used by a corresponding
// DoInvoke function to perform the function execution. This allows
// us to shield the Callback class from the types of the bound argument via
// "type erasure."
// At the base level, the only task is to add reference counting data. Don't use
// RefCountedThreadSafe since it requires the destructor to be a virtual method.
// Creating a vtable for every BindState template instantiation results in a lot
// of bloat. Its only task is to call the destructor which can be done with a
// function pointer.
class BindStateBase {
explicit BindStateBase(void (*destructor)(BindStateBase*))
: ref_count_(0), destructor_(destructor) {}
~BindStateBase() = default;
friend class scoped_refptr<BindStateBase>;
friend class CallbackBase;
void AddRef();
void Release();
std::atomic<int32_t> ref_count_;
// Pointer to a function that will properly destroy |this|.
void (*destructor_)(BindStateBase*);
// Holds the Callback methods that don't require specialization to reduce
// template bloat.
class BASE_EXPORT CallbackBase {
CallbackBase(const CallbackBase& c);
CallbackBase& operator=(const CallbackBase& c);
// Returns true if Callback is null (doesn't refer to anything).
bool is_null() const { return bind_state_.get() == NULL; }
// Returns the Callback into an uninitialized state.
void Reset();
// In C++, it is safe to cast function pointers to function pointers of
// another type. It is not okay to use void*. We create a InvokeFuncStorage
// that that can store our function pointer, and then cast it back to
// the original type on usage.
using InvokeFuncStorage = void(*)();
// Returns true if this callback equals |other|. |other| may be null.
bool Equals(const CallbackBase& other) const;
// Allow initializing of |bind_state_| via the constructor to avoid default
// initialization of the scoped_refptr. We do not also initialize
// |polymorphic_invoke_| here because doing a normal assignment in the
// derived Callback templates makes for much nicer compiler errors.
explicit CallbackBase(BindStateBase* bind_state);
// Force the destructor to be instantiated inside this translation unit so
// that our subclasses will not get inlined versions. Avoids more template
// bloat.
scoped_refptr<BindStateBase> bind_state_;
InvokeFuncStorage polymorphic_invoke_;
// A helper template to determine if given type is non-const move-only-type,
// i.e. if a value of the given type should be passed via std::move() in a
// destructive way. Types are considered to be move-only if they have a
// sentinel MoveOnlyTypeForCPP03 member: a class typically gets this from using
// It would be easy to generalize this trait to all move-only types... but this
// confuses template deduction in VS2013 with certain types such as
// std::unique_ptr.
// TODO(dcheng): Revisit this when Windows switches to VS2015 by default.
template <typename T> struct IsMoveOnlyType {
template <typename U>
static YesType Test(const typename U::MoveOnlyTypeForCPP03*);
template <typename U>
static NoType Test(...);
static const bool value = sizeof((Test<T>(0))) == sizeof(YesType) &&
// Specialization of IsMoveOnlyType so that std::unique_ptr is still considered
// move-only, even without the sentinel member.
template <typename T>
struct IsMoveOnlyType<std::unique_ptr<T>> : std::true_type {};
template <typename>
struct CallbackParamTraitsForMoveOnlyType;
template <typename>
struct CallbackParamTraitsForNonMoveOnlyType;
// TODO(tzik): Use a default parameter once MSVS supports variadic templates
// with default values.
// This is a typetraits object that's used to take an argument type, and
// extract a suitable type for storing and forwarding arguments.
// In particular, it strips off references, and converts arrays to
// pointers for storage; and it avoids accidentally trying to create a
// "reference of a reference" if the argument is a reference type.
// This array type becomes an issue for storage because we are passing bound
// parameters by const reference. In this case, we end up passing an actual
// array type in the initializer list which C++ does not allow. This will
// break passing of C-string literals.
template <typename T>
struct CallbackParamTraits
: std::conditional<IsMoveOnlyType<T>::value,
CallbackParamTraitsForNonMoveOnlyType<T>>::type {
template <typename T>
struct CallbackParamTraitsForNonMoveOnlyType {
using ForwardType = const T&;
using StorageType = T;
// The Storage should almost be impossible to trigger unless someone manually
// specifies type of the bind parameters. However, in case they do,
// this will guard against us accidentally storing a reference parameter.
// The ForwardType should only be used for unbound arguments.
template <typename T>
struct CallbackParamTraitsForNonMoveOnlyType<T&> {
using ForwardType = T&;
using StorageType = T;
// Note that for array types, we implicitly add a const in the conversion. This
// means that it is not possible to bind array arguments to functions that take
// a non-const pointer. Trying to specialize the template based on a "const
// T[n]" does not seem to match correctly, so we are stuck with this
// restriction.
template <typename T, size_t n>
struct CallbackParamTraitsForNonMoveOnlyType<T[n]> {
using ForwardType = const T*;
using StorageType = const T*;
// See comment for CallbackParamTraits<T[n]>.
template <typename T>
struct CallbackParamTraitsForNonMoveOnlyType<T[]> {
using ForwardType = const T*;
using StorageType = const T*;
// Parameter traits for movable-but-not-copyable scopers.
// Callback<>/Bind() understands movable-but-not-copyable semantics where
// the type cannot be copied but can still have its state destructively
// transferred (aka. moved) to another instance of the same type by calling a
// helper function. When used with Bind(), this signifies transferal of the
// object's state to the target function.
// For these types, the ForwardType must not be a const reference, or a
// reference. A const reference is inappropriate, and would break const
// correctness, because we are implementing a destructive move. A non-const
// reference cannot be used with temporaries which means the result of a
// function or a cast would not be usable with Callback<> or Bind().
template <typename T>
struct CallbackParamTraitsForMoveOnlyType {
using ForwardType = T;
using StorageType = T;
// CallbackForward() is a very limited simulation of C++11's std::forward()
// used by the Callback/Bind system for a set of movable-but-not-copyable
// types. It is needed because forwarding a movable-but-not-copyable
// argument to another function requires us to invoke the proper move
// operator to create a rvalue version of the type. The supported types are
// whitelisted below as overloads of the CallbackForward() function. The
// default template compiles out to be a no-op.
// In C++11, std::forward would replace all uses of this function. However, it
// is impossible to implement a general std::forward without C++11 due to a lack
// of rvalue references.
// In addition to Callback/Bind, this is used by PostTaskAndReplyWithResult to
// simulate std::forward() and forward the result of one Callback as a
// parameter to another callback. This is to support Callbacks that return
// the movable-but-not-copyable types whitelisted above.
template <typename T>
typename std::enable_if<!IsMoveOnlyType<T>::value, T>::type& CallbackForward(
T& t) {
return t;
template <typename T>
typename std::enable_if<IsMoveOnlyType<T>::value, T>::type CallbackForward(
T& t) {
return std::move(t);
} // namespace internal
} // namespace base