# 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`) 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. ```cpp // |Foo| just refers to |cb| but doesn't store it nor consume it. bool Foo(const base::OnceCallback& cb) { return cb.is_null(); } // |Bar| takes the ownership of |cb| and stores |cb| into |g_cb|. base::RepeatingCallback g_cb; void Bar(base::RepeatingCallback cb) { g_cb = std::move(cb); } // |Baz| takes the ownership of |cb| and consumes |cb| by Run(). void Baz(base::OnceCallback 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 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::OnceCallback`s together: ```cpp int Floor(float f) { return std::floor(f); } std::string IntToString(int i) { return base::NumberToString(i); } base::OnceCallback first = base::BindOnce(&Floor); base::OnceCallback 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::RepeatingCallback`s together: ```cpp int Floor(float f) { return std::floor(f); } std::string IntToString(int i) { return base::NumberToString(i); } base::RepeatingCallback first = base::BindRepeating(&Floor); base::RepeatingCallback 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 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::OnceCallback`s. However since a `base::RepeatingCallback` can be run multiple times, it can be joined non-destructively as well. ```cpp int Floor(float f) { return std::floor(f); } std::string IntToString(int i) { return base::NumberToString(i); } base::RepeatingCallback first = base::BindRepeating(&Floor); base::RepeatingCallback 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. ```cpp // Returns an integer. base::RepeatingCallback 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()` helper in the [base::PassThrough CL](https://chromium-review.googlesource.com/c/chromium/src/+/2493243). If this would be helpful for you, please let danakj@chromium.org know or ping the CL. ### Chaining callbacks across different task runners ```cpp // The task runner for a different thread. scoped_refptr 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 compute_result_cb = base::BindOnce(&ComputeResult); // Task runner for the current thread. scoped_refptr 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 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`. ```cpp // 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: ```cpp std::unique_ptr CreateFooTask(base::OnceClosure task) { std::pair split = base::SplitOnceCallback(std::move(task)); std::unique_ptr 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: ```cpp using StatusCallback = base::OnceCallback; void DoOperation(StatusCallback done_cb) { std::pair 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`. The `BarrierCallback` 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` 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: ```cpp void Merge(const std::vector& data); void Collect(base::OnceCallback 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(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 ```cpp int Return5() { return 5; } base::OnceCallback func_cb = base::BindOnce(&Return5); LOG(INFO) << std::move(func_cb).Run(); // Prints 5. ``` ```cpp int Return5() { return 5; } base::RepeatingCallback func_cb = base::BindRepeating(&Return5); LOG(INFO) << func_cb.Run(); // Prints 5. ``` ### Binding A Captureless Lambda ```cpp base::RepeatingCallback lambda_cb = base::BindRepeating([] { return 4; }); LOG(INFO) << lambda_cb.Run(); // Print 4. base::OnceCallback lambda_cb2 = base::BindOnce([] { return 3; }); LOG(INFO) << std::move(lambda_cb2).Run(); // Print 3. base::OnceCallback lambda_cb3 = base::BindOnce([] { return 2; }); base::OnceCallback)> lambda_cb4 = base::BindOnce( [](base::OnceCallback 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. ``` cpp #include "base/test/bind.h" int i = 2; base::RepeatingCallback 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. ```cpp class Ref : public base::RefCountedThreadSafe { public: int Foo() { return 3; } }; scoped_refptr ref = new Ref(); base::RepeatingCallback 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. ```cpp void DoSomething(const base::RepeatingCallback& callback) { callback.Run(5, "hello"); } void DoSomethingOther(base::OnceCallback 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). ```cpp void DoSomething(const base::RepeatingCallback& 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.) ```cpp 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: ```cpp using MyCallback = base::OnceCallback; 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()`: ```cpp ... 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: ```cpp // Binds |foo_ptr| to a no-op OnceCallback takes a scoped_refptr. // ANTIPATTERN WARNING: This should likely be changed to ReleaseSoon()! base::BindOnce(base::DoNothingAs)>(), 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: ```cpp void MyFunc(int i, const std::string& str) {} base::RepeatingCallback 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. ```cpp void MyFunc(int i, const std::string& str) {} base::RepeatingCallback cb = base::BindRepeating(&MyFunc, 23, "hello world"); cb.Run(); ``` As described earlier, a callback with no unbound input parameters (`base::RepeatingCallback`) is called a `base::RepeatingClosure`. So we could have also written: ```cpp base::RepeatingClosure cb = base::BindRepeating(&MyFunc, 23, "hello world"); ``` When calling member functions, bound parameters just go after the object pointer. ```cpp 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. ```cpp void ReadIntFromFile(const std::string& filename, base::OnceCallback 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](http://en.wikipedia.org/wiki/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. ```cpp std::vector v = {1, 2, 3}; // |v| is moved into the internal storage without copy. base::BindOnce(&Foo, std::move(v)); ``` ```cpp // The vector is moved into the internal storage without copy. base::BindOnce(&Foo, std::vector({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. ```cpp void Foo(std::unique_ptr) {} auto p = std::make_unique(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 Callbacks to a class method may be bound using a weak pointer as the receiver. A callback bound using a weak pointer receiver will be automatically cancelled (calling `Run()` becomes a no-op) if the weak pointer is invalidated, e.g. its associated class instance is destroyed. The most common way to use this pattern is by embedding a `base::WeakPtrFactory` field, e.g.: ```cpp class MyClass { public: MyClass(); void Foo(); private: std::string data_; // Chrome's compiler toolchain enforces that any `WeakPtrFactory` // fields are declared last, to avoid destruction ordering issues. base::WeakPtrFactory weak_factory_{this}; }; ``` Then use `base::WeakPtrFactory::GetWeakPtr()` as the receiver when binding a callback: ```cpp base::SequencedTaskRunnerHandle::Get()->PostTask( FROM_HERE, base::BindOnce(&MyClass::Foo, weak_factory_.GetWeakPtr()); ``` If `this` is destroyed before the posted callback runs, the callback will simply become a no-op when run, rather than being a use-after-free bug on the destroyed `MyClass` instance. **Sequence safety** Class method callbacks bound to `base::WeakPtr`s must be run on the same sequence on which the object will be destroyed to avoid potential races between object destruction and callback execution. The same caveat applies if a class manually invalidates live `base::WeakPtr`s with `base::WeakPtrFactory::InvalidateWeakPtrs()`. ### Binding A Class Method With Manual Lifetime Management If a callback bound to a class method does not need cancel-on-destroy semantics (because there is some external guarantee that the class instance will always be live when running the callback), then use: ```cpp // base::Unretained() is safe since `this` joins `background_thread_` in the // destructor. background_thread_->PostTask( FROM_HERE, base::BindOnce(&MyClass::Foo, base::Unretained(this))); ``` It is often a good idea to add a brief comment to explain why `base::Unretained()` is safe in this context; if nothing else, for future code archaeologists trying to fix a use-after-free bug. An alternative is `base::WeakPtrFactory::GetSafeRef()`: ```cpp background_thread_->PostTask( FROM_HERE, base::BindOnce(&MyClass::Foo, weak_factory_.GetSafeRef()); ``` Similar to `base::Unretained()`, this disables cancel-on-destroy semantics; unlike `base::Unretained()`, this is guaranteed to terminate safely if the lifetime expectations are violated. ### Binding A Class Method And Having The Callback Own The Class ```cpp 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. ```cpp std::unique_ptr 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. ```cpp int DoSomething(int arg) { cout << arg << endl; return arg; } base::RepeatingCallback 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. ```cpp base::RepeatingCallback 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 ```cpp 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 ```cpp void TakesOwnership(std::unique_ptr arg) {} auto f = std::make_unique(); // 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 ```cpp void TakesOneRef(scoped_refptr arg) {} scoped_refptr 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. ```cpp void DontTakeRef(Foo* arg) {} scoped_refptr 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: ```cpp 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: ```cpp 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: ```cpp 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. ```cpp namespace base { template struct IsWeakReceiver { static constexpr bool value = false; }; template struct UnwrapTraits { template T&& Unwrap(T&& obj) { return std::forward(obj); } }; } // namespace base ``` If `base::IsWeakReceiver::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::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: ```cpp 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: ```cpp 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.