base/bind.h

// 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.
//
// RETURN TYPES
//
// 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,
          Args...>::UnboundRunType;
};

}  // 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 =
      internal::TakeTypeListItem<sizeof...(Args),
                                 internal::ExtractArgs<BoundRunType>>;

  // 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.
  static_assert(!internal::HasNonConstReferenceItem<BoundArgs>::value,
                "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");
  static_assert(
      !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),
                    std::forward<Args>(args)...));
}

}  // namespace base

#endif  // BASE_BIND_H_