// 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.
#ifndef BASE_CALLBACK_INTERNAL_H_
#define BASE_CALLBACK_INTERNAL_H_
#include <stddef.h>
#include "base/base_export.h"
#include "base/memory/ref_counted.h"
#include "base/memory/scoped_ptr.h"
template <typename T>
class ScopedVector;
namespace base {
namespace internal {
// 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."
class BindStateBase : public RefCountedThreadSafe<BindStateBase> {
protected:
friend class RefCountedThreadSafe<BindStateBase>;
virtual ~BindStateBase() {}
};
// Holds the Callback methods that don't require specialization to reduce
// template bloat.
class BASE_EXPORT CallbackBase {
public:
// Returns true if Callback is null (doesn't refer to anything).
bool is_null() const;
// Returns the Callback into an uninitialized state.
void Reset();
protected:
// 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.
typedef void(*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.
~CallbackBase();
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 .Pass() in a
// destructive way.
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) &&
!is_const<T>::value;
};
// 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, bool is_move_only = IsMoveOnlyType<T>::value>
struct CallbackParamTraits {
typedef const T& ForwardType;
typedef T StorageType;
};
// 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 CallbackParamTraits<T&, false> {
typedef T& ForwardType;
typedef T StorageType;
};
// 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 CallbackParamTraits<T[n], false> {
typedef const T* ForwardType;
typedef const T* StorageType;
};
// See comment for CallbackParamTraits<T[n]>.
template <typename T>
struct CallbackParamTraits<T[], false> {
typedef const T* ForwardType;
typedef const T* StorageType;
};
// 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 CallbackParamTraits<T, true> {
typedef T ForwardType;
typedef T StorageType;
};
// 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 with 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 enable_if<!IsMoveOnlyType<T>::value, T>::type& CallbackForward(T& t) {
return t;
}
template <typename T>
typename enable_if<IsMoveOnlyType<T>::value, T>::type CallbackForward(T& t) {
return t.Pass();
}
} // namespace internal
} // namespace base
#endif // BASE_CALLBACK_INTERNAL_H_