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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 <functional>
#include <memory>
#include <type_traits>
#include <utility>
#include "base/bind_internal.h"
#include "base/compiler_specific.h"
#include "build/build_config.h"
#if defined(OS_MACOSX) && !HAS_FEATURE(objc_arc)
#include "base/mac/scoped_block.h"
// -----------------------------------------------------------------------------
// Usage documentation
// -----------------------------------------------------------------------------
// Overview:
// base::BindOnce() and base::BindRepeating() are helpers for creating
// base::OnceCallback and base::RepeatingCallback objects respectively.
// For a runnable object of n-arity, the base::Bind*() family allows partial
// application of the first m arguments. The remaining n - m arguments must be
// passed when invoking the callback with Run().
// // The first argument is bound at callback creation; the remaining
// // two must be passed when calling Run() on the callback object.
// base::OnceCallback<long(int, long)> cb = base::BindOnce(
// [](short x, int y, long z) { return x * y * z; }, 42);
// When binding to a method, the receiver object must also be specified at
// callback creation time. When Run() is invoked, the method will be invoked on
// the specified receiver object.
// class C : public base::RefCounted<C> { void F(); };
// auto instance = base::MakeRefCounted<C>();
// auto cb = base::BindOnce(&C::F, instance);
// std::move(cb).Run(); // Identical to instance->F()
// base::Bind is currently a type alias for base::BindRepeating(). In the
// future, we expect to flip this to default to base::BindOnce().
// See //docs/ for the full 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.
namespace base {
namespace internal {
// IsOnceCallback<T> is a std::true_type if |T| is a OnceCallback.
template <typename T>
struct IsOnceCallback : std::false_type {};
template <typename Signature>
struct IsOnceCallback<OnceCallback<Signature>> : std::true_type {};
// Helper to assert that parameter |i| of type |Arg| can be bound, which means:
// - |Arg| can be retained internally as |Storage|.
// - |Arg| can be forwarded as |Unwrapped| to |Param|.
template <size_t i,
typename Arg,
typename Storage,
typename Unwrapped,
typename Param>
struct AssertConstructible {
static constexpr bool param_is_forwardable =
std::is_constructible<Param, Unwrapped>::value;
// Unlike the check for binding into storage below, the check for
// forwardability drops the const qualifier for repeating callbacks. This is
// to try to catch instances where std::move()--which forwards as a const
// reference with repeating callbacks--is used instead of base::Passed().
param_is_forwardable ||
!std::is_constructible<Param, std::decay_t<Unwrapped>&&>::value,
"Bound argument |i| is move-only but will be forwarded by copy. "
"Ensure |Arg| is bound using base::Passed(), not std::move().");
"Bound argument |i| of type |Arg| cannot be forwarded as "
"|Unwrapped| to the bound functor, which declares it as |Param|.");
static constexpr bool arg_is_storable =
std::is_constructible<Storage, Arg>::value;
static_assert(arg_is_storable ||
!std::is_constructible<Storage, std::decay_t<Arg>&&>::value,
"Bound argument |i| is move-only but will be bound by copy. "
"Ensure |Arg| is mutable and bound using std::move().");
"Bound argument |i| of type |Arg| cannot be converted and "
"bound as |Storage|.");
// Takes three same-length TypeLists, and applies AssertConstructible for each
// triples.
template <typename Index,
typename Args,
typename UnwrappedTypeList,
typename ParamsList>
struct AssertBindArgsValidity;
template <size_t... Ns,
typename... Args,
typename... Unwrapped,
typename... Params>
struct AssertBindArgsValidity<std::index_sequence<Ns...>,
: AssertConstructible<Ns, Args, std::decay_t<Args>, Unwrapped, Params>... {
static constexpr bool ok = true;
// The implementation of TransformToUnwrappedType below.
template <bool is_once, typename T>
struct TransformToUnwrappedTypeImpl;
template <typename T>
struct TransformToUnwrappedTypeImpl<true, T> {
using StoredType = std::decay_t<T>;
using ForwardType = StoredType&&;
using Unwrapped = decltype(Unwrap(std::declval<ForwardType>()));
template <typename T>
struct TransformToUnwrappedTypeImpl<false, T> {
using StoredType = std::decay_t<T>;
using ForwardType = const StoredType&;
using Unwrapped = decltype(Unwrap(std::declval<ForwardType>()));
// Transform |T| into `Unwrapped` type, which is passed to the target function.
// Example:
// In is_once == true case,
// `int&&` -> `int&&`,
// `const int&` -> `int&&`,
// `OwnedWrapper<int>&` -> `int*&&`.
// In is_once == false case,
// `int&&` -> `const int&`,
// `const int&` -> `const int&`,
// `OwnedWrapper<int>&` -> `int* const &`.
template <bool is_once, typename T>
using TransformToUnwrappedType =
typename TransformToUnwrappedTypeImpl<is_once, T>::Unwrapped;
// Transforms |Args| into `Unwrapped` types, and packs them into a TypeList.
// If |is_method| is true, tries to dereference the first argument to support
// smart pointers.
template <bool is_once, bool is_method, typename... Args>
struct MakeUnwrappedTypeListImpl {
using Type = TypeList<TransformToUnwrappedType<is_once, Args>...>;
// Performs special handling for this pointers.
// Example:
// int* -> int*,
// std::unique_ptr<int> -> int*.
template <bool is_once, typename Receiver, typename... Args>
struct MakeUnwrappedTypeListImpl<is_once, true, Receiver, Args...> {
using UnwrappedReceiver = TransformToUnwrappedType<is_once, Receiver>;
using Type = TypeList<decltype(&*std::declval<UnwrappedReceiver>()),
TransformToUnwrappedType<is_once, Args>...>;
template <bool is_once, bool is_method, typename... Args>
using MakeUnwrappedTypeList =
typename MakeUnwrappedTypeListImpl<is_once, is_method, Args...>::Type;
// Used below in BindImpl to determine whether to use Invoker::Run or
// Invoker::RunOnce.
// Note: Simply using `kIsOnce ? &Invoker::RunOnce : &Invoker::Run` does not
// work, since the compiler needs to check whether both expressions are
// well-formed. Using `Invoker::Run` with a OnceCallback triggers a
// static_assert, which is why the ternary expression does not compile.
// TODO( Remove this indirection once we have `if constexpr`.
template <typename Invoker>
constexpr auto GetInvokeFunc(std::true_type) {
return Invoker::RunOnce;
template <typename Invoker>
constexpr auto GetInvokeFunc(std::false_type) {
return Invoker::Run;
template <template <typename> class CallbackT,
typename Functor,
typename... Args>
decltype(auto) BindImpl(Functor&& functor, Args&&... args) {
// This block checks if each |args| matches to the corresponding params of the
// target function. This check does not affect the behavior of Bind, but its
// error message should be more readable.
static constexpr bool kIsOnce = IsOnceCallback<CallbackT<void()>>::value;
using Helper = internal::BindTypeHelper<Functor, Args...>;
using FunctorTraits = typename Helper::FunctorTraits;
using BoundArgsList = typename Helper::BoundArgsList;
using UnwrappedArgsList =
internal::MakeUnwrappedTypeList<kIsOnce, FunctorTraits::is_method,
using BoundParamsList = typename Helper::BoundParamsList;
std::make_index_sequence<Helper::num_bounds>, BoundArgsList,
UnwrappedArgsList, BoundParamsList>::ok,
"The bound args need to be convertible to the target params.");
using BindState = internal::MakeBindStateType<Functor, Args...>;
using UnboundRunType = MakeUnboundRunType<Functor, Args...>;
using Invoker = internal::Invoker<BindState, UnboundRunType>;
using CallbackType = CallbackT<UnboundRunType>;
// Store the invoke func into PolymorphicInvoke before casting it to
// InvokeFuncStorage, so that we can ensure its type matches to
// PolymorphicInvoke, to which CallbackType will cast back.
using PolymorphicInvoke = typename CallbackType::PolymorphicInvoke;
PolymorphicInvoke invoke_func =
GetInvokeFunc<Invoker>(std::integral_constant<bool, kIsOnce>());
using InvokeFuncStorage = internal::BindStateBase::InvokeFuncStorage;
return CallbackType(BindState::Create(
std::forward<Functor>(functor), std::forward<Args>(args)...));
} // namespace internal
// Bind as OnceCallback.
template <typename Functor, typename... Args>
inline OnceCallback<MakeUnboundRunType<Functor, Args...>> BindOnce(
Functor&& functor,
Args&&... args) {
static_assert(!internal::IsOnceCallback<std::decay_t<Functor>>() ||
(std::is_rvalue_reference<Functor&&>() &&
"BindOnce requires non-const rvalue for OnceCallback binding."
" I.e.: base::BindOnce(std::move(callback)).");
return internal::BindImpl<OnceCallback>(std::forward<Functor>(functor),
// Bind as RepeatingCallback.
template <typename Functor, typename... Args>
inline RepeatingCallback<MakeUnboundRunType<Functor, Args...>>
BindRepeating(Functor&& functor, Args&&... args) {
"BindRepeating cannot bind OnceCallback. Use BindOnce with std::move().");
return internal::BindImpl<RepeatingCallback>(std::forward<Functor>(functor),
// Unannotated Bind.
// TODO(tzik): Deprecate this and migrate to OnceCallback and
// RepeatingCallback, once they get ready.
template <typename Functor, typename... Args>
inline Callback<MakeUnboundRunType<Functor, Args...>>
Bind(Functor&& functor, Args&&... args) {
return base::BindRepeating(std::forward<Functor>(functor),
// Special cases for binding to a base::Callback without extra bound arguments.
template <typename Signature>
OnceCallback<Signature> BindOnce(OnceCallback<Signature> callback) {
return callback;
template <typename Signature>
OnceCallback<Signature> BindOnce(RepeatingCallback<Signature> callback) {
return callback;
template <typename Signature>
RepeatingCallback<Signature> BindRepeating(
RepeatingCallback<Signature> callback) {
return callback;
template <typename Signature>
Callback<Signature> Bind(Callback<Signature> callback) {
return callback;
// Unretained() allows binding a non-refcounted class, and to disable
// refcounting on arguments that are refcounted objects.
// EXAMPLE OF Unretained():
// class Foo {
// public:
// void func() { cout << "Foo:f" << endl; }
// };
// // In some function somewhere.
// Foo foo;
// OnceClosure foo_callback =
// BindOnce(&Foo::func, Unretained(&foo));
// std::move(foo_callback).Run(); // Prints "Foo:f".
// Without the Unretained() wrapper on |&foo|, the above call would fail
// to compile because Foo does not support the AddRef() and Release() methods.
template <typename T>
static inline internal::UnretainedWrapper<T> Unretained(T* o) {
return internal::UnretainedWrapper<T>(o);
// RetainedRef() accepts a ref counted object and retains a reference to it.
// When the callback is called, the object is passed as a raw pointer.
// EXAMPLE OF RetainedRef():
// void foo(RefCountedBytes* bytes) {}
// scoped_refptr<RefCountedBytes> bytes = ...;
// OnceClosure callback = BindOnce(&foo, base::RetainedRef(bytes));
// std::move(callback).Run();
// Without RetainedRef, the scoped_refptr would try to implicitly convert to
// a raw pointer and fail compilation:
// OnceClosure callback = BindOnce(&foo, bytes); // ERROR!
template <typename T>
static inline internal::RetainedRefWrapper<T> RetainedRef(T* o) {
return internal::RetainedRefWrapper<T>(o);
template <typename T>
static inline internal::RetainedRefWrapper<T> RetainedRef(scoped_refptr<T> o) {
return internal::RetainedRefWrapper<T>(std::move(o));
// Owned() transfers ownership of an object to the callback resulting from
// bind; the object will be deleted when the callback is deleted.
// EXAMPLE OF Owned():
// void foo(int* arg) { cout << *arg << endl }
// int* pn = new int(1);
// RepeatingClosure foo_callback = BindRepeating(&foo, Owned(pn));
// foo_callback.Run(); // Prints "1"
// foo_callback.Run(); // Prints "1"
// *pn = 2;
// foo_callback.Run(); // Prints "2"
// foo_callback.Reset(); // |pn| is deleted. Also will happen when
// // |foo_callback| goes out of scope.
// Without Owned(), someone would have to know to delete |pn| when the last
// reference to the callback is deleted.
template <typename T>
static inline internal::OwnedWrapper<T> Owned(T* o) {
return internal::OwnedWrapper<T>(o);
template <typename T, typename Deleter>
static inline internal::OwnedWrapper<T, Deleter> Owned(
std::unique_ptr<T, Deleter>&& ptr) {
return internal::OwnedWrapper<T, Deleter>(std::move(ptr));
// Passed() is for transferring movable-but-not-copyable types (eg. unique_ptr)
// through a RepeatingCallback. Logically, this signifies a destructive transfer
// of the state of the argument into the target function. Invoking
// RepeatingCallback::Run() twice on a callback that was created with a Passed()
// argument will CHECK() because the first invocation would have already
// transferred ownership to the target function.
// Note that Passed() is not necessary with BindOnce(), as std::move() does the
// same thing. Avoid Passed() in favor of std::move() with BindOnce().
// EXAMPLE OF Passed():
// void TakesOwnership(std::unique_ptr<Foo> arg) { }
// std::unique_ptr<Foo> CreateFoo() { return std::make_unique<Foo>();
// }
// auto f = std::make_unique<Foo>();
// // |cb| is given ownership of Foo(). |f| is now NULL.
// // You can use std::move(f) in place of &f, but it's more verbose.
// RepeatingClosure cb = BindRepeating(&TakesOwnership, Passed(&f));
// // Run was never called so |cb| still owns Foo() and deletes
// // it on Reset().
// cb.Reset();
// // |cb| is given a new Foo created by CreateFoo().
// cb = BindRepeating(&TakesOwnership, Passed(CreateFoo()));
// // |arg| in TakesOwnership() is given ownership of Foo(). |cb|
// // no longer owns Foo() and, if reset, would not delete Foo().
// cb.Run(); // Foo() is now transferred to |arg| and deleted.
// cb.Run(); // This CHECK()s since Foo() already been used once.
// We offer 2 syntaxes for calling Passed(). The first takes an rvalue and is
// best suited for use with the return value of a function or other temporary
// rvalues. The second takes a pointer to the scoper and is just syntactic sugar
// to avoid having to write Passed(std::move(scoper)).
// Both versions of Passed() prevent T from being an lvalue reference. The first
// via use of enable_if, and the second takes a T* which will not bind to T&.
template <typename T,
std::enable_if_t<!std::is_lvalue_reference<T>::value>* = nullptr>
static inline internal::PassedWrapper<T> Passed(T&& scoper) {
return internal::PassedWrapper<T>(std::move(scoper));
template <typename T>
static inline internal::PassedWrapper<T> Passed(T* scoper) {
return internal::PassedWrapper<T>(std::move(*scoper));
// IgnoreResult() is used to adapt a function or callback with a return type to
// one with a void return. This is most useful if you have a function with,
// say, a pesky ignorable bool return that you want to use with PostTask or
// something else that expect a callback with a void return.
// EXAMPLE OF IgnoreResult():
// int DoSomething(int arg) { cout << arg << endl; }
// // Assign to a callback with a void return type.
// OnceCallback<void(int)> cb = BindOnce(IgnoreResult(&DoSomething));
// std::move(cb).Run(1); // Prints "1".
// // Prints "2" on |ml|.
// ml->PostTask(FROM_HERE, BindOnce(IgnoreResult(&DoSomething), 2);
template <typename T>
static inline internal::IgnoreResultHelper<T> IgnoreResult(T data) {
return internal::IgnoreResultHelper<T>(std::move(data));
#if defined(OS_MACOSX) && !HAS_FEATURE(objc_arc)
// RetainBlock() is used to adapt an Objective-C block when Automated Reference
// Counting (ARC) is disabled. This is unnecessary when ARC is enabled, as the
// BindOnce and BindRepeating already support blocks then.
// EXAMPLE OF RetainBlock():
// // Wrap the block and bind it to a callback.
// OnceCallback<void(int)> cb =
// BindOnce(RetainBlock(^(int n) { NSLog(@"%d", n); }));
// std::move(cb).Run(1); // Logs "1".
template <typename R, typename... Args>
base::mac::ScopedBlock<R (^)(Args...)> RetainBlock(R (^block)(Args...)) {
return base::mac::ScopedBlock<R (^)(Args...)>(block,
#endif // defined(OS_MACOSX) && !HAS_FEATURE(objc_arc)
} // namespace base
#endif // BASE_BIND_H_