Move documentation of base::Callback from base/callback.h to docs/callback.md

base/callback.h

Index: base/callback.h
diff --git a/base/callback.h b/base/callback.h
index e67b3181d5924af8388b840e337a95c2550562ad..15e84086f30279a143736c58cfbada1bd4f6843e 100644
--- a/base/callback.h
+++ b/base/callback.h
@@ -12,337 +12,10 @@
 // Closure should #include "base/callback_forward.h" instead of this file.
 
 // -----------------------------------------------------------------------------
-// Introduction
+// Usage documentation
 // -----------------------------------------------------------------------------
 //
-// The templated Callback class is a generalized function object. Together
-// with the Bind() function in 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 (base::Callback<void()>)
-// is called a base::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
-//
-//   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.
-//
-//   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.
-//
-//   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).
-//
-//   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:
-//
-//   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.
-//
-//   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:
-//
-//   base::Closure cb = base::Bind(&MyFunc, 23, "hello world");
-//
-//   When calling member functions, bound parameters just go after the object
-//   pointer.
-//
-//   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.
-//
-//   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
-//
-//   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
-//
-//   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
-//
-//   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.
-//
-//   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
-//
-//   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 scoped_ptr
-//
-//   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
-//
-//   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:
-//
-//   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:
-//      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.
+// See //docs/callback.md for documentation.
 
 namespace base {