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OnceCallback<> and BindOnce(), RepeatingCallback<> and BindRepeating()

[TOC]

Introduction

The templated base::{Once, Repeating}Callback<> classes are generalized function objects. Together with the base::Bind{Once, Repeating}() functions in base/bind.h, they provide a type-safe method for performing partial application of functions.

Partial application is the process of binding a subset of a function's arguments to produce another function that takes fewer arguments. This can be used to pass around a unit of delayed execution, much like lexical closures are used in other languages. For example, it is used in Chromium code to schedule tasks on different MessageLoops.

A callback with no unbound input parameters (base::OnceCallback<void()>) is called a base::OnceClosure. The same pattern exists for base::RepeatingCallback, as base::RepeatingClosure. Note that this is NOT the same as what other languages refer to as a closure -- it does not retain a reference to its enclosing environment.

OnceCallback<> And RepeatingCallback<>

base::OnceCallback<> is created by base::BindOnce(). This is a callback variant that is a move-only type and can be run only once. This moves out bound parameters from its internal storage to the bound function by default, so it's easier to use with movable types. This should be the preferred callback type: since the lifetime of the callback is clear, it's simpler to reason about when a callback that is passed between threads is destroyed.

base::RepeatingCallback<> is created by base::BindRepeating(). This is a callback variant that is copyable that can be run multiple times. It uses internal ref-counting to make copies cheap. However, since ownership is shared, it is harder to reason about when the callback and the bound state are destroyed, especially when the callback is passed between threads.

Prefer base::OnceCallback<> where possible, and use base::RepeatingCallback<> otherwise.

base::RepeatingCallback<> is convertible to base::OnceCallback<> by the implicit conversion.

Memory Management And Passing

Pass base::{Once,Repeating}Callback objects by value if ownership is transferred; otherwise, pass it by const-reference.

// |Foo| just refers to |cb| but doesn't store it nor consume it.
bool Foo(const base::OnceCallback<void(int)>& cb) {
  return cb.is_null();
}

// |Bar| takes the ownership of |cb| and stores |cb| into |g_cb|.
base::RepeatingCallback<void(int)> g_cb;
void Bar(base::RepeatingCallback<void(int)> cb) {
  g_cb = std::move(cb);
}

// |Baz| takes the ownership of |cb| and consumes |cb| by Run().
void Baz(base::OnceCallback<void(int)> cb) {
  std::move(cb).Run(42);
}

// |Qux| takes the ownership of |cb| and transfers ownership to PostTask(),
// which also takes the ownership of |cb|.
void Qux(base::RepeatingCallback<void(int)> cb) {
  PostTask(FROM_HERE, base::BindOnce(cb, 42));
  PostTask(FROM_HERE, base::BindOnce(std::move(cb), 43));
}

When you pass a base::{Once,Repeating}Callback object to a function parameter, use std::move() if you don't need to keep a reference to it, otherwise, pass the object directly. You may see a compile error when the function requires the exclusive ownership, and you didn't pass the callback by move. Note that the moved-from base::{Once,Repeating}Callback becomes null, as if its Reset() method had been called. Afterward, its is_null() method will return true and its operator bool() will return false.

Chaining callbacks

When you have 2 callbacks that you wish to run in sequence, they can be joined together into a single callback through the use of Then().

Calling Then() on a base::OnceCallback joins a second callback that will be run together with, but after, the first callback. The return value from the first callback is passed along to the second, and the return value from the second callback is returned at the end. More concretely, calling a.Then(b) produces a new base::OnceCallback that will run b(a());, returning the result from b.

This example uses Then() to join 2 base::OnceCallbacks together:

int Floor(float f) { return std::floor(f); }
std::string IntToString(int i) { return base::NumberToString(i); }

base::OnceCallback<int(float)> first = base::BindOnce(&Floor);
base::OnceCallback<std::string(int)> second = base::BindOnce(&IntToString);

// This will run |first|, run and pass the result to |second|, then return
// the result from |second|.
std::string r = std::move(first).Then(std::move(second)).Run(3.5f);
// |r| will be "3". |first| and |second| are now both null, as they were
// consumed to perform the join operation.

Similarly, Then() also works with base::RepeatingCallback; however, the joined callback must also be a base::RepeatingCallback to ensure the resulting callback can be invoked multiple times.

This example uses Then() to join 2 base::RepeatingCallbacks together:

int Floor(float f) { return std::floor(f); }
std::string IntToString(int i) { return base::NumberToString(i); }

base::RepeatingCallback<int(float)> first = base::BindRepeating(&Floor);
base::RepeatingCallback<std::string(int)> second = base::BindRepeating(&IntToString);

// This creates a RepeatingCallback that will run |first|, run and pass the
// result to |second|, then return the result from |second|.
base::RepeatingCallback<std::string(float)> joined =
    std::move(first).Then(std::move(second));
// |first| and |second| are now both null, as they were consumed to perform
// the join operation.

// This runs the functor that was originally bound to |first|, then |second|.
std::string r = joined.Run(3.5);
// |r| will be "3".

// It's valid to call it multiple times since all callbacks involved are
// base::RepeatingCallbacks.
r = joined.Run(2.5);
// |r| is set to "2".

In the above example, casting the base::RepeatingCallback to an r-value with std::move() causes Then() to destroy the original callback, in the same way that occurs for joining base::OnceCallbacks. However since a base::RepeatingCallback can be run multiple times, it can be joined non-destructively as well.

int Floor(float f) { return std::floor(f); }
std::string IntToString(int i) { return base::NumberToString(i); }

base::RepeatingCallback<int(float)> first = base::BindRepeating(&Floor);
base::RepeatingCallback<std::string(int)> second = base::BindRepeating(&IntToString);

// This creates a RepeatingCallback that will run |first|, run and pass the
// result to |second|, then return the result from |second|.
std::string r = first.Then(second).Run(3.5f);
// |r| will be 3, and |first| and |second| are still valid to use.

// Runs Floor().
int i = first.Run(5.5);
// Runs IntToString().
std::string s = second.Run(9);

If the second callback does not want to receive a value from the first callback, you may use base::IgnoreResult to drop the return value in between running the two.

// Returns an integer.
base::RepeatingCallback<int()> first = base::BindRepeating([](){ return 5; });
// Does not want to receive an integer.
base::RepeatingClosure second = base::BindRepeating([](){});

// This will not compile, because |second| can not receive the return value from
// |first|.
//   first.Then(second).Run();

// We can drop the result from |first| before running second.
base::BindRepeating(base::IgnoreResult(first)).Then(second).Run();
// This will effectively create a callback that when Run() will call
// `first(); second();` instead of `second(first());`.

Note that the return value from |first| will be lost in the above example, and would be destroyed before |second| is run. If you want the return value from |first| to be preserved and ultimately returned after running both |first| and |second|, then you would need a primitive such as the base::PassThrough<T>() helper in the base::PassThrough CL. If this would be helpful for you, please let danakj@chromium.org know or ping the CL.

Chaining callbacks across different task runners

// The task runner for a different thread.
scoped_refptr<base::SequencedTaskRunner> other_task_runner = ...;

// A function to compute some interesting result, except it can only be run
// safely from `other_task_runner` and not the current thread.
int ComputeResult();

base::OnceCallback<int()> compute_result_cb = base::BindOnce(&ComputeResult);

// Task runner for the current thread.
scoped_refptr<base::SequencedTaskRunner> current_task_runner =
    base::SequencedTaskRunnerHandle::Get();

// A function to accept the result, except it can only be run safely from the
// current thread.
void ProvideResult(int result);

base::OnceCallback<void(int)> provide_result_cb =
    base::BindOnce(&ProvideResult);

Using Then() to join compute_result_cb and provide_result_cb directly would be inappropriate. ComputeResult() and ProvideResult() would run on the same thread which isn't safe. However, base::BindPostTask() can be used to ensure provide_result_cb will run on current_task_runner.

// The following two statements post a task to `other_task_runner` to run
// `task`. This will invoke ComputeResult() on a different thread to get the
// result value then post a task back to `current_task_runner` to invoke
// ProvideResult() with the result.
OnceClosure task =
    std::move(compute_result_cb)
        .Then(base::BindPostTask(current_task_runner,
                                 std::move(provide_result_cb)));
other_task_runner->PostTask(FROM_HERE, std::move(task));

Splitting a OnceCallback in two

If a callback is only run once, but two references need to be held to the callback, using a base::OnceCallback can be clearer than a base::RepeatingCallback, from an intent and semantics point of view. base::SplitOnceCallback() takes a base::OnceCallback and returns a pair of callbacks with the same signature. When either of the returned callback is run, the original callback is invoked. Running the leftover callback will result in a crash. This can be useful when passing a base::OnceCallback to a function that may or may not take ownership of the callback. E.g, when an object creation could fail:

std::unique_ptr<FooTask> CreateFooTask(base::OnceClosure task) {
  std::pair<base::OnceClosure,base::OnceClosure> split
                                    = base::SplitOnceCallback(std::move(task));

  std::unique_ptr<FooTask> foo = TryCreateFooTask(std::move(split.first));
  if (foo)
    return foo;

  return CreateFallbackFooTask(std::move(split.second));
}

While it is best to use a single callback to report success/failure, some APIs already take multiple callbacks. base::SplitOnceCallback() can be used to split a completion callback and help in such a case:

using StatusCallback = base::OnceCallback<void(FooStatus)>;
void DoOperation(StatusCallback done_cb) {
  std::pair<StatusCallback, StatusCallback> split
                                 = base::SplitOnceCallback(std::move(done_cb));

  InnerWork(BindOnce(std::move(split.first), STATUS_OK),
            BindOnce(std::move(split.second), STATUS_ABORTED));
}

void InnerWork(base::OnceClosure work_done_cb,
               base::OnceClosure work_aborted_cb);

BarrierCallback

Sometimes you might need to request data from several sources, then do something with the collective results once all data is available. You can do this with a BarrierCallback<T>. The BarrierCallback<T> is created with two parameters:

  • num_callbacks: The number of times the BarrierCallback can be run, each time being passed an object of type T.
  • done_callback: This will be run once the BarrierCallback has been run num_callbacks times.

The done_callback will receive a std::vector<T> containing the num_callbacks parameters passed in the respective Run calls. The order of Ts in the vector is unspecified.

Note that

  • barrier callback must not be run more than num_callback times,
  • done_callback will be called on the same thread as the final call to the barrier callback. done_callback will also be cleared on the same thread.

Example:

void Merge(const std::vector<Data>& data);

void Collect(base::OnceCallback<void(Data)> collect_and_merge) {
  // Do something, probably asynchronously, and at some point:
  std::move(collect_and_merge).Run(data);
}

CollectAndMerge() {
  const auto collect_and_merge =
      base::BarrierCallback<Image>(sources_.size(), base::BindOnce(&Merge));
  for (const auto& source : sources_) {
    // Copy the barrier callback for asynchronous data collection.
    // Once all sources have called `collect_and_merge` with their respective
    // data, |Merge| will be called with a vector of the collected data.
    source.Collect(collect_and_merge);
  }
}

Quick reference for basic stuff

Binding A Bare Function

int Return5() { return 5; }
base::OnceCallback<int()> func_cb = base::BindOnce(&Return5);
LOG(INFO) << std::move(func_cb).Run();  // Prints 5.
int Return5() { return 5; }
base::RepeatingCallback<int()> func_cb = base::BindRepeating(&Return5);
LOG(INFO) << func_cb.Run();  // Prints 5.

Binding A Captureless Lambda

base::RepeatingCallback<int()> lambda_cb = base::BindRepeating([] { return 4; });
LOG(INFO) << lambda_cb.Run();  // Print 4.

base::OnceCallback<int()> lambda_cb2 = base::BindOnce([] { return 3; });
LOG(INFO) << std::move(lambda_cb2).Run();  // Print 3.

base::OnceCallback<int()> lambda_cb3 = base::BindOnce([] { return 2; });
base::OnceCallback<int(base::OnceCallback<int()>)> lambda_cb4 =
    base::BindOnce(
        [](base::OnceCallback<int()> callback) {
            return std::move(callback).Run(); },
        std::move(lambda_cb3));
LOG(INFO) << std::move(lambda_cb4).Run();  // Print 2.

Binding A Capturing Lambda (In Tests)

When writing tests, it is often useful to capture arguments that need to be modified in a callback.

#include "base/test/bind.h"

int i = 2;
base::RepeatingCallback<void()> lambda_cb = base::BindLambdaForTesting([&]() { i++; });
lambda_cb.Run();
LOG(INFO) << i;  // Print 3;

Binding A Class Method

The first argument to bind is the member function to call, the second is the object on which to call it.

class Ref : public base::RefCountedThreadSafe<Ref> {
 public:
  int Foo() { return 3; }
};
scoped_refptr<Ref> ref = new Ref();
base::RepeatingCallback<void()> ref_cb = base::BindRepeating(&Ref::Foo, ref);
LOG(INFO) << ref_cb.Run();  // Prints out 3.

By default the object must support RefCounted or you will get a compiler error. If you're passing between threads, be sure it's RefCountedThreadSafe! See "Advanced binding of member functions" below if you don't want to use reference counting.

Running A Callback

Callbacks can be run with their Run method, which has the same signature as the template argument to the callback. Note that base::OnceCallback::Run consumes the callback object and can only be invoked on a callback rvalue.

void DoSomething(const base::RepeatingCallback<void(int, std::string)>& callback) {
  callback.Run(5, "hello");
}

void DoSomethingOther(base::OnceCallback<void(int, std::string)> callback) {
  std::move(callback).Run(5, "hello");
}

RepeatingCallbacks can be run more than once (they don't get deleted or marked when run). However, this precludes using base::Passed (see below).

void DoSomething(const base::RepeatingCallback<double(double)>& callback) {
  double myresult = callback.Run(3.14159);
  myresult += callback.Run(2.71828);
}

If running a callback could result in its own destruction (e.g., if the callback recipient deletes the object the callback is a member of), the callback should be moved or copied onto the stack before it can be safely invoked. (Note that this is only an issue for RepeatingCallbacks, because a OnceCallback always has to be moved for execution.)

void Foo::RunCallback() {
  std::move(&foo_deleter_callback_).Run();
}

Creating a Callback That Does Nothing

Sometimes you need a callback that does nothing when run (e.g. test code that doesn't care to be notified about certain types of events). It may be tempting to pass a default-constructed callback of the right type:

using MyCallback = base::OnceCallback<void(bool arg)>;
void MyFunction(MyCallback callback) {
  std::move(callback).Run(true);  // Uh oh...
}
...
MyFunction(MyCallback());  // ...this will crash when Run()!

Default-constructed callbacks are null, and thus cannot be Run(). Instead, use base::DoNothing():

...
MyFunction(base::DoNothing());  // Can be Run(), will no-op

base::DoNothing() can be passed for any OnceCallback or RepeatingCallback that returns void.

Implementation-wise, base::DoNothing() is actually a functor which produces a callback from operator(). This makes it unusable when trying to bind other arguments to it. Normally, the only reason to bind arguments to DoNothing() is to manage object lifetimes, and in these cases, you should strive to use idioms like DeleteSoon(), ReleaseSoon(), or RefCountedDeleteOnSequence instead. If you truly need to bind an argument to DoNothing(), or if you need to explicitly create a callback object (because implicit conversion through operator()() won't compile), you can instantiate directly:

// Binds |foo_ptr| to a no-op OnceCallback takes a scoped_refptr<Foo>.
// ANTIPATTERN WARNING: This should likely be changed to ReleaseSoon()!
base::BindOnce(base::DoNothing::Once<scoped_refptr<Foo>>(), foo_ptr);

Passing Unbound Input Parameters

Unbound parameters are specified at the time a callback is Run(). They are specified in the base::{Once, Repeating}Callback template type:

void MyFunc(int i, const std::string& str) {}
base::RepeatingCallback<void(int, const std::string&)> cb = base::BindRepeating(&MyFunc);
cb.Run(23, "hello, world");

Passing Bound Input Parameters

Bound parameters are specified when you create the callback as arguments to base::Bind{Once, Repeating}(). They will be passed to the function and the Run()ner of the callback doesn't see those values or even know that the function it's calling.

void MyFunc(int i, const std::string& str) {}
base::RepeatingCallback<void()> cb = base::BindRepeating(&MyFunc, 23, "hello world");
cb.Run();

As described earlier, a callback with no unbound input parameters (base::RepeatingCallback<void()>) is called a base::RepeatingClosure. So we could have also written:

base::RepeatingClosure cb = base::BindRepeating(&MyFunc, 23, "hello world");

When calling member functions, bound parameters just go after the object pointer.

base::RepeatingClosure cb = base::BindRepeating(&MyClass::MyFunc, this, 23, "hello world");

Partial Binding Of Parameters

You can specify some parameters when you create the callback, and specify the rest when you execute the callback.

When calling a function bound parameters are first, followed by unbound parameters.

void ReadIntFromFile(const std::string& filename,
                     base::OnceCallback<void(int)> on_read);

void DisplayIntWithPrefix(const std::string& prefix, int result) {
  LOG(INFO) << prefix << result;
}

void AnotherFunc(const std::string& file) {
  ReadIntFromFile(file, base::BindOnce(&DisplayIntWithPrefix, "MyPrefix: "));
};

This technique is known as partial application. It should be used in lieu of creating an adapter class that holds the bound arguments. Notice also that the "MyPrefix: " argument is actually a const char*, while DisplayIntWithPrefix actually wants a const std::string&. Like normal function dispatch, base::Bind, will coerce parameter types if possible.

Avoiding Copies With Callback Parameters

A parameter of base::BindRepeating() or base::BindOnce() is moved into its internal storage if it is passed as a rvalue.

std::vector<int> v = {1, 2, 3};
// |v| is moved into the internal storage without copy.
base::BindOnce(&Foo, std::move(v));
// The vector is moved into the internal storage without copy.
base::BindOnce(&Foo, std::vector<int>({1, 2, 3}));

Arguments bound with base::BindOnce() are always moved, if possible, to the target function. A function parameter that is passed by value and has a move constructor will be moved instead of copied. This makes it easy to use move-only types with base::BindOnce().

In contrast, arguments bound with base::BindRepeating() are only moved to the target function if the argument is bound with base::Passed().

DANGER: A base::RepeatingCallback can only be run once if arguments were bound with base::Passed(). For this reason, avoid base::Passed(). If you know a callback will only be called once, prefer to refactor code to work with base::OnceCallback instead.

Avoid using base::Passed() with base::BindOnce(), as std::move() does the same thing and is more familiar.

void Foo(std::unique_ptr<int>) {}
auto p = std::make_unique<int>(42);

// |p| is moved into the internal storage of BindOnce(), and moved out to |Foo|.
base::BindOnce(&Foo, std::move(p));
base::BindRepeating(&Foo, base::Passed(&p)); // Ok, but subtle.
base::BindRepeating(&Foo, base::Passed(std::move(p))); // Ok, but subtle.

Quick reference for advanced binding

Binding A Class Method With Weak Pointers

If MyClass has a base::WeakPtr<MyClass> weak_this_ member (see below) then a class method can be bound with:

base::BindOnce(&MyClass::Foo, weak_this_);

The callback will not be run if the object has already been destroyed.

Note that class method callbacks bound to base::WeakPtrs may only be run on the same sequence on which the object will be destroyed, since otherwise execution of the callback might race with the object's deletion.

To use base::WeakPtr with base::Bind{Once, Repeating}() as the this pointer to a method bound in a callback, MyClass will typically look like:

class MyClass {
public:
  MyClass() {
    weak_this_ = weak_factory_.GetWeakPtr();
  }
private:
  base::WeakPtr<MyClass> weak_this_;
  // MyClass member variables go here.
  base::WeakPtrFactory<MyClass> weak_factory_{this};
};

weak_factory_ is the last member variable in MyClass so that it is destroyed first. This ensures that if any class methods bound to weak_this_ are Run() during teardown, then they will not actually be executed.

If MyClass only ever binds and executes callbacks on the same sequence, then it is generally safe to call weak_factory_.GetWeakPtr() at the base::Bind{Once, Repeating}() call, rather than taking a separate weak_this_ during construction.

Binding A Class Method With Manual Lifetime Management

base::BindOnce(&MyClass::Foo, base::Unretained(this));

This disables all lifetime management on the object. You're responsible for making sure the object is alive at the time of the call. You break it, you own it!

Binding A Class Method And Having The Callback Own The Class

MyClass* myclass = new MyClass;
base::BindOnce(&MyClass::Foo, base::Owned(myclass));

The object will be deleted when the callback is destroyed, even if it's not run (like if you post a task during shutdown). Potentially useful for "fire and forget" cases.

Smart pointers (e.g. std::unique_ptr<>) are also supported as the receiver.

std::unique_ptr<MyClass> myclass(new MyClass);
base::BindOnce(&MyClass::Foo, std::move(myclass));

Ignoring Return Values

Sometimes you want to call a function that returns a value in a callback that doesn't expect a return value.

int DoSomething(int arg) {
  cout << arg << endl;
  return arg;
}
base::RepeatingCallback<void(int)> cb =
    base::BindRepeating(IgnoreResult(&DoSomething));

Similarly, you may want to use an existing callback that returns a value in a place that expects a void return type.

base::RepeatingCallback<int()> cb = base::BindRepeating([](){ return 5; });
base::RepeatingClosure void_cb = base::BindRepeating(base::IgnoreResult(cb));

Quick reference for binding parameters to BindOnce() and BindRepeating()

Bound parameters are specified as arguments to base::Bind{Once, Repeating}() and are passed to the functions.

Passing Parameters Owned By The Callback

void Foo(int* arg) { cout << *arg << endl; }
int* pn = new int(1);
base::RepeatingClosure foo_callback = base::BindRepeating(&foo, base::Owned(pn));

The parameter will be deleted when the callback is destroyed, even if it's not run (like if you post a task during shutdown).

Passing Parameters As A unique_ptr

void TakesOwnership(std::unique_ptr<Foo> arg) {}
auto f = std::make_unique<Foo>();
// f becomes null during the following call.
base::OnceClosure cb = base::BindOnce(&TakesOwnership, std::move(f));

Ownership of the parameter will be with the callback until the callback is run, and then ownership is passed to the callback function. This means the callback can only be run once. If the callback is never run, it will delete the object when it's destroyed.

Passing Parameters As A scoped_refptr

void TakesOneRef(scoped_refptr<Foo> arg) {}
scoped_refptr<Foo> f(new Foo);
base::RepeatingClosure cb = base::BindRepeating(&TakesOneRef, f);

This should "just work." The closure will take a reference as long as it is alive, and another reference will be taken for the called function.

void DontTakeRef(Foo* arg) {}
scoped_refptr<Foo> f(new Foo);
base::RepeatingClosure cb = base::BindRepeating(&DontTakeRef, base::RetainedRef(f));

base::RetainedRef holds a reference to the object and passes a raw pointer to the object when the Callback is run.

Binding Const Reference Parameters

If the callback function takes a const reference parameter then the value is copied when bound unless std::ref or std::cref is used. Example:

void foo(const int& arg) { printf("%d %p\n", arg, &arg); }
int n = 1;
base::OnceClosure has_copy = base::BindOnce(&foo, n);
base::OnceClosure has_ref = base::BindOnce(&foo, std::cref(n));
n = 2;
foo(n);                                   // Prints "2 0xaaaaaaaaaaaa"
std::move(has_copy).Run();                // Prints "1 0xbbbbbbbbbbbb"
std::move(has_ref).Run();                 // Prints "2 0xaaaaaaaaaaaa"

Normally parameters are copied in the closure. DANGER: std::ref and std::cref store a (const) reference instead, referencing the original parameter. This means that you must ensure the object outlives the callback!

Binding Non-Const Reference Parameters

If the callback function takes a non-const reference then the bind statement must specify what behavior is desired. If a reference that can mutate the original value is desired then std::ref is used. If the callback should take ownership of the value, either by making a copy or moving an existing value, then base::OwnedRef is used. If neither is used the bind statement will fail to compile. Example:

void foo(int& arg) {
  printf("%d\n", arg);
  ++arg;
}

int n = 0;
base::RepeatingClosure has_ref = base::BindRepeating(&foo, std::ref(n));
base::RepeatingClosure has_copy = base::BindRepeating(&foo, base::OwnedRef(n));

foo(n);                        // Prints "0"
has_ref.Run();                 // Prints "1"
has_ref.Run();                 // Prints "2"
foo(n);                        // Prints "3"

has_copy.Run();                // Prints "0"
has_copy.Run();                // Prints "1"

// This will fail to compile.
base::RepeatingClosure cb = base::BindRepeating(&foo, n);

Normally parameters are copied in the closure. DANGER: std::ref stores a reference instead, referencing the original parameter. This means that you must ensure the object outlives the callback!

If the callback function has an output reference parameter but the output value isn't needed then base::OwnedRef() is a convenient way to handle it. The callback owned value will be mutated by the callback function and then deleted along with the callback. Example:

bool Compute(size_t index, int& output);

// The `output` parameter isn't important for the callback, it only cares about
// the return value.
base::OnceClosure cb = base::BindOnce(&Compute, index, base::OwnedRef(0));
bool success = std::move(cb).Run();

Implementation notes

Where Is This Design From:

The design is heavily influenced by C++'s tr1::function / tr1::bind, and by the "Google Callback" system used inside Google.

Customizing the behavior

There are several injection points that controls binding behavior from outside of its implementation.

namespace base {

template <typename Receiver>
struct IsWeakReceiver {
  static constexpr bool value = false;
};

template <typename Obj>
struct UnwrapTraits {
  template <typename T>
  T&& Unwrap(T&& obj) {
    return std::forward<T>(obj);
  }
};

}  // namespace base

If base::IsWeakReceiver<Receiver>::value is true on a receiver of a method, base::Bind checks if the receiver is evaluated to true and cancels the invocation if it's evaluated to false. You can specialize base::IsWeakReceiver to make an external smart pointer as a weak pointer.

base::UnwrapTraits<BoundObject>::Unwrap() is called for each bound argument right before the callback calls the target function. You can specialize this to define an argument wrapper such as base::Unretained, base::Owned, base::RetainedRef and base::Passed.

How The Implementation Works:

There are three main components to the system:

  1. The base::{Once, Repeating}Callback<> classes.
  2. The base::BindOnce() and base::BindRepeating() functions.
  3. The arguments wrappers (e.g., base::Unretained() and base::Owned()).

The Callback classes represent a generic function pointer. Internally, it stores a refcounted piece of state that represents the target function and all its bound parameters. The base::{Once, Repeating}Callback constructor takes a base::BindStateBase*, which is upcasted from a base::BindState<>. In the context of the constructor, the static type of this base::BindState<> pointer uniquely identifies the function it is representing, all its bound parameters, and a Run() method that is capable of invoking the target.

base::BindOnce() or base::BindRepeating() creates the base::BindState<> that has the full static type, and erases the target function type as well as the types of the bound parameters. It does this by storing a pointer to the specific Run() function, and upcasting the state of base::BindState<>* to a base::BindStateBase*. This is safe as long as this BindStateBase pointer is only used with the stored Run() pointer.

These bind functions, along with a set of internal templates, are responsible for

  • Unwrapping the function signature into return type, and parameters
  • Determining the number of parameters that are bound
  • Creating the BindState storing the bound parameters
  • Performing compile-time asserts to avoid error-prone behavior
  • Returning a Callback<> with an arity matching the number of unbound parameters and that knows the correct refcounting semantics for the target object if we are binding a method.

The base::Bind functions do the above using type-inference and variadic templates.

By default base::Bind{Once, Repeating}() will store copies of all bound parameters, and attempt to refcount a target object if the function being bound is a class method. These copies are created even if the function takes parameters as const references. (Binding to non-const references is forbidden, see bind.h.)

To change this behavior, we introduce a set of argument wrappers (e.g., base::Unretained()). These are simple container templates that are passed by value, and wrap a pointer to argument. Each helper has a comment describing it in base/bind.h.

These types are passed to the Unwrap() functions to modify the behavior of base::Bind{Once, Repeating}(). The Unwrap() functions change behavior by doing partial specialization based on whether or not a parameter is a wrapper type.

base::Unretained() is specific to Chromium.

Missing Functionality

  • Binding arrays to functions that take a non-const pointer. Example:
void Foo(const char* ptr);
void Bar(char* ptr);
base::BindOnce(&Foo, "test");
base::BindOnce(&Bar, "test");  // This fails because ptr is not const.
  • In case of partial binding of parameters a possibility of having unbound parameters before bound parameters. Example:
void Foo(int x, bool y);
base::BindOnce(&Foo, _1, false); // _1 is a placeholder.

If you are thinking of forward declaring base::{Once, Repeating}Callback in your own header file, please include "base/callback_forward.h" instead.