docs/callback.md

# base::Callback<> and base::Bind()

## Introduction

The templated `Callback<>` class is a generalized function object. Together with
the `Bind()` function in base/bind.h, they provide a type-safe method for
performing partial application of functions.

Partial application (or "currying") 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 (`Callback<void()>`) is called a
`Closure`. 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.

### Memory Management And Passing

The Callback objects themselves should be passed by const-reference, and stored
by copy. They internally store their state via a refcounted class and thus do
not need to be deleted.

The reason to pass via a const-reference is to avoid unnecessary AddRef/Release
pairs to the internal state.

## Quick reference for basic stuff

### Binding A Bare Function

```cpp
int Return5() { return 5; }
base::Callback<int()> func_cb = base::Bind(&Return5);
LOG(INFO) << func_cb.Run();  // Prints 5.
```

### 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<Ref> {
 public:
  int Foo() { return 3; }
  void PrintBye() { LOG(INFO) << "bye."; }
};
scoped_refptr<Ref> ref = new Ref();
base::Callback<void()> ref_cb = base::Bind(&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.

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

Callbacks 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::Callback<double(double)>& callback) {
  double myresult = callback.Run(3.14159);
  myresult += callback.Run(2.71828);
}
```

### Passing Unbound Input Parameters

Unbound parameters are specified at the time a callback is `Run()`. They are
specified in the `Callback` template type:

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

### Passing Bound Input Parameters

Bound parameters are specified when you create the callback as arguments to
`Bind()`. 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::Callback<void()> cb = base::Bind(&MyFunc, 23, "hello world");
cb.Run();
```

A callback with no unbound input parameters (`base::Callback<void()>`) is called
a `base::Closure`. So we could have also written:

```cpp
base::Closure cb = base::Bind(&MyFunc, 23, "hello world");
```

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

```cpp
base::Closure cb = base::Bind(&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.

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

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

## Quick reference for advanced binding

### Binding A Class Method With Weak Pointers

```cpp
base::Bind(&MyClass::Foo, GetWeakPtr());
```

The callback will not be run if the object has already been destroyed.
**DANGER**: weak pointers are not threadsafe, so don't use this when passing between
threads!

### Binding A Class Method With Manual Lifetime Management

```cpp
base::Bind(&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

```cpp
MyClass* myclass = new MyClass;
base::Bind(&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.

### 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; }
base::Callback<void(int)> cb =
    base::Bind(base::IgnoreResult(&DoSomething));
```

## Quick reference for binding parameters to Bind()

Bound parameters are specified as arguments to `Bind()` and are passed to the
function. A callback with no parameters or no unbound parameters is called a
`Closure` (`base::Callback<void()>` and `base::Closure` are the same thing).

### Passing Parameters Owned By The Callback

```cpp
void Foo(int* arg) { cout << *arg << endl; }
int* pn = new int(1);
base::Closure foo_callback = base::Bind(&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<Foo> arg) {}
std::unique_ptr<Foo> f(new Foo);
// f becomes null during the following call.
base::Closure cb = base::Bind(&TakesOwnership, base::Passed(&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<Foo> arg) {}
scoped_refptr<Foo> f(new Foo)
base::Closure cb = base::Bind(&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.

### Passing Parameters By Reference

Const references are *copied* unless `ConstRef` is used. Example:

```cpp
void foo(const int& arg) { printf("%d %p\n", arg, &arg); }
int n = 1;
base::Closure has_copy = base::Bind(&foo, n);
base::Closure has_ref = base::Bind(&foo, base::ConstRef(n));
n = 2;
foo(n);                        // Prints "2 0xaaaaaaaaaaaa"
has_copy.Run();                // Prints "1 0xbbbbbbbbbbbb"
has_ref.Run();                 // Prints "2 0xaaaaaaaaaaaa"
```

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

## Implementation notes

### Where Is This Design From:

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

### How The Implementation Works:

There are three main components to the system:
  1) The Callback classes.
  2) The `Bind()` functions.
  3) The arguments wrappers (e.g., `Unretained()` and `ConstRef()`).

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.  Each `Callback` specialization has a templated constructor
that takes an `BindState<>*`.  In the context of the constructor, the static
type of this `BindState<>` pointer uniquely identifies the function it is
representing, all its bound parameters, and a `Run()` method that is capable of
invoking the target.

`Callback`'s constructor takes the `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 `BindState<>*` to a `BindStateBase*`. This is safe as
long as this `BindStateBase` pointer is only used with the stored `Run()`
pointer.

To `BindState<>` objects are created inside the `Bind()` functions.
These 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 an `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 `Bind` functions do the above using type-inference, and template
specializations.

By default `Bind()` 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.,
`Unretained()`, and `ConstRef()`).  These are simple container templates that
are passed by value, and wrap a pointer to argument.  See the file-level comment
in base/bind_helpers.h for more info.

These types are passed to the `Unwrap()` functions, and the `MaybeRefcount()`
functions respectively to modify the behavior of `Bind()`.  The `Unwrap()` and
`MaybeRefcount()` functions change behavior by doing partial specialization
based on whether or not a parameter is a wrapper type.

`ConstRef()` is similar to `tr1::cref`.  `Unretained()` is specific to Chromium.

### Why Not Tr1 Function/Bind?

Direct use of `tr1::function` and `tr1::bind` was considered, but ultimately
rejected because of the number of copy constructors invocations involved in the
binding of arguments during construction, and the forwarding of arguments during
invocation.  These copies will no longer be an issue in C++0x because C++0x will
support rvalue reference allowing for the compiler to avoid these copies.
However, waiting for C++0x is not an option.

Measured with valgrind on gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5), the
`tr1::bind` call itself will invoke a non-trivial copy constructor three times
for each bound parameter.  Also, each when passing a `tr1::function`, each bound
argument will be copied again.

In addition to the copies taken at binding and invocation, copying a
`tr1::function` causes a copy to be made of all the bound parameters and state.

Furthermore, in Chromium, it is desirable for the `Callback` to take a reference
on a target object when representing a class method call. This is not supported
by tr1.

Lastly, `tr1::function` and `tr1::bind` has a more general and flexible
API. This includes things like argument reordering by use of
`tr1::bind::placeholder`, support for non-const reference parameters, and some
limited amount of subtyping of the `tr1::function` object (e.g.,
`tr1::function<int(int)>` is convertible to `tr1::function<void(int)>`).

These are not features that are required in Chromium. Some of them, such as
allowing for reference parameters, and subtyping of functions, may actually
become a source of errors. Removing support for these features actually allows
for a simpler implementation, and a terser Currying API.

### Why Not Google Callbacks?

The Google callback system also does not support refcounting.  Furthermore, its
implementation has a number of strange edge cases with respect to type
conversion of its arguments. In particular, the argument's constness must at
times match exactly the function signature, or the type-inference might
break. Given the above, writing a custom solution was easier.

### Missing Functionality
 - Invoking the return of `Bind`.  `Bind(&foo).Run()` does not work;
 - Binding arrays to functions that take a non-const pointer.
   Example:
```cpp
void Foo(const char* ptr);
void Bar(char* ptr);
Bind(&Foo, "test");
Bind(&Bar, "test");  // This fails because ptr is not const.
```

If you are thinking of forward declaring `Callback` in your own header file,
please include "base/callback_forward.h" instead.