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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_BIND_H_
#define BASE_BIND_H_
#include "base/bind_internal.h"
// -----------------------------------------------------------------------------
// Usage documentation
// -----------------------------------------------------------------------------
// See base/callback.h for documentation.
// -----------------------------------------------------------------------------
// Implementation notes
// -----------------------------------------------------------------------------
// If you're reading the implementation, before proceeding further, you should
// read the top comment of base/bind_internal.h for a definition of common
// terms and concepts.
// Though Bind()'s result is meant to be stored in a Callback<> type, it
// cannot actually return the exact type without requiring a large amount
// of extra template specializations. The problem is that in order to
// discern the correct specialization of Callback<>, Bind would need to
// unwrap the function signature to determine the signature's arity, and
// whether or not it is a method.
// Each unique combination of (arity, function_type, num_prebound) where
// function_type is one of {function, method, const_method} would require
// one specialization. We eventually have to do a similar number of
// specializations anyways in the implementation (see the Invoker<>,
// classes). However, it is avoidable in Bind if we return the result
// via an indirection like we do below.
// TODO(ajwong): We might be able to avoid this now, but need to test.
// It is possible to move most of the static_assert into BindState<>, but it
// feels a little nicer to have the asserts here so people do not need to crack
// open bind_internal.h. On the other hand, it makes Bind() harder to read.
namespace base {
namespace internal {
// Don't use Alias Template directly here to avoid a compile error on MSVC2013.
template <typename Functor, typename... Args>
struct MakeUnboundRunTypeImpl {
using Type =
typename BindState<
typename FunctorTraits<Functor>::RunnableType,
typename FunctorTraits<Functor>::RunType,
} // namespace internal
template <typename Functor, typename... Args>
using MakeUnboundRunType =
typename internal::MakeUnboundRunTypeImpl<Functor, Args...>::Type;
template <typename Functor, typename... Args>
base::Callback<MakeUnboundRunType<Functor, Args...>>
Bind(Functor functor, Args&&... args) {
// Type aliases for how to store and run the functor.
using RunnableType = typename internal::FunctorTraits<Functor>::RunnableType;
using RunType = typename internal::FunctorTraits<Functor>::RunType;
// Use RunnableType::RunType instead of RunType above because our
// checks below for bound references need to know what the actual
// functor is going to interpret the argument as.
using BoundRunType = typename RunnableType::RunType;
using BoundArgs =
// Do not allow binding a non-const reference parameter. Non-const reference
// parameters are disallowed by the Google style guide. Also, binding a
// non-const reference parameter can make for subtle bugs because the
// invoked function will receive a reference to the stored copy of the
// argument and not the original.
"do not bind functions with nonconst ref");
const bool is_method = internal::HasIsMethodTag<RunnableType>::value;
// For methods, we need to be careful for parameter 1. We do not require
// a scoped_refptr because BindState<> itself takes care of AddRef() for
// methods. We also disallow binding of an array as the method's target
// object.
static_assert(!internal::BindsArrayToFirstArg<is_method, Args...>::value,
"first bound argument to method cannot be array");
!internal::HasRefCountedParamAsRawPtr<is_method, Args...>::value,
"a parameter is a refcounted type and needs scoped_refptr");
using BindState = internal::BindState<RunnableType, RunType, Args...>;
return Callback<typename BindState::UnboundRunType>(
new BindState(internal::MakeRunnable(functor),
} // namespace base
#endif // BASE_BIND_H_