Callbacks in C++ using template functors – Rich Hickey (1994)
CALLBACKS IN C++ USING TEMPLATE FUNCTORS

Copyright 1994 Rich Hickey
INTRODUCTION
One of the many promises of Object-Oriented programming is that it
will allow for plug-and-play software design with re-usable components.
Designers will pull objects from their library 'shelves' and hook them
together to make software.  In C++, this hooking together of components
can be tricky, particulary if they are separately designed.  We are
still a long way from interoperable libraries and application
components.  Callbacks provide a mechanism whereby independently
developed objects may be connected together.  They are vital for plug
and play programming, since the likelihood of Vendor A implementing
their library in terms of Vendor B's classes, or your home-brewed
classes, is nil.
Callbacks are in wide use, however current implementations
differ and most suffer from shortcomings, not the least of which is
their lack of generality.  This article describes what callbacks are,
how they are used, and the criteria for a good callback mechanism.  It
summarizes current callback methods and their weaknesses.  It then
describes a flexible, powerful and easy-to-use callback technique based
on template functors - objects that behave like functions.
CALLBACK FUNDAMENTALS

What Are Callbacks?
When designing application or sub-system specific components we
often know all of the classes with which the component will interact and
thus explicity code interfaces in terms of those classes.  When
designing general purpose or library components however, it is often
necessary or desirable to put in hooks for calling unknown objects. What
is required is a way for one component to call another without having
been written in terms of, or with knowledge of, the other component's
type.  Such a 'type-blind' call mechanism is often referred to as a
callback.
A callback might be used for simple notification, two-way
communication, or to distribute work in a process.  For instance an
application developer might want to have a Button
component in a GUI library call an application-specific object when
clicked upon.  The designer of a data entry component might want to
offer the capability to call application objects for input validation.
Collection classes often offer an apply() function, which
'applies' a member function of an application object to the items they
contain.
A callback, then, is a way for a component designer to offer a
generic connection point which developers can use to establish
communication with application objects.  At some subsequent point, the
component 'calls back' the application object.  The communication takes
the form of a function call, since this is the way objects interact in
C++.
Callbacks are useful in many contexts.  If you use any
commercial class libraries you have probably seen at least one mechanism
for providing callbacks.  All callback implementations must address a
fundamental problem posed by the C++ type system: How can you build a
component such that it can call a member function of another object
whose type is unknown at the time the component is designed?  C++'s type
system requires that we know something of the type of any object whose
member functions we wish to call, and is often criticized by fans of
other OO languages as being too inflexible to support true
component-based design, since all the components have to 'know' about
each other.  C++'s strong typing has too many advantages to abandon, but
addressing this apparent lack of flexibility may encourage the
proliferation of robust and interoperable class libraries.        
C++ is in fact quite flexible, and the mechanism presented here
leverages its flexibility to provide this functionality without language
extension.  In particular, templates supply a powerful tool for solving
problems such as this.  If you thought templates were only for container
classes, read on!               Callback Terminology
             There are three elements in any callback mechanism
- the caller, the callback function, and the callee.
The caller is usually an instance of some class, for
instance a library component (although it could be a function, like
qsort()), that provides or requires the callback; i.e.  it
can, or must, call some third party code to perform its work, and uses
the callback mechanism to do so.  As far as the designer of the caller
is concerned, the callback is just a way to invoke a process, referred
to here as the callback function. The caller determines the
signature of the callback function i.e.  its argument(s) and return
types.  This makes sense, because it is the caller that has the work to
do, or the information to convey.  For instance, in the examples above,
the Button class may want a callback function with no
arguments and no return.  It is a simple notification function used by
the Button to indicate it has been clicked upon.  The
DataEntryField component might want to pass a String
to the callback function and get a Boolean return.
A caller may require the callback for just the duration of one
function, as with ANSI C's qsort(), or may want to hold on
to the callback in order to call back at some later time, as with the
Button class.        The callee is
usually a member function of an object of some class, but it can also be
a stand-alone function or static member function, that the application
designer wishes to be called by the caller component. Note that in the
case of a non-static member function a particular object/member-function
pair is the callee.  The function to be called must be compatible with
the signature of the callback function specified by the caller.
Criteria for a Good Callback Mechanism         A
callback mechanism in the object oriented model should support both
component and application design. Component designers should have a
standard, off-the-shelf way of providing callback services, requiring no
invention on their part.  Flexibility in specifying the number and types
of argument and return values should be provided. Since the component
may be designed for use in as-yet-unthought-of applications, the
component designer should neither need to know, nor dictate, the types
of the objects which may be 'called back' by the component.
               Application developers, given a component with
this standard callback mechanism and some instance of a class with a
member function compatible with the callback function signature, should
have to do no custom 'glue' coding in order to connect the two together.
 Nor should they have to modify the callee class or hand-derive a new
class.  If they want to have the callback invoke a stand-alone,
non-member function, that should be supported as well.
                 To support this behavior the callback mechanism
should be:                  Object Oriented -
Our applications are built with objects.  In a C++ application most
functionality is contained in member functions, which cannot be invoked
via normal ptr-to-functions.  Non-static member functions operate upon
objects, which have state.  Calling such functions is more than just
invoking a process, it is operating upon a particular object, thus an
object-oriented callback must contain information about which object to
call.                                                  Type
Safe - Type safety is a fundamental feature and benefit of C++
and any robust C++ callback mechanism must be type safe.  That means we
must ensure that objects are used in compliance with their specified
interfaces, and that type rules are enforced for arguments, return
values, and conversions.  The best way to ensure this is to have the
compiler do the work at compile time.
Non-Coupling - This is the fundamental goal of
callbacks - to allow components designed in ignorance of each other to
be connected together. If the mechanism somehow introduces a dependancy
between caller and callee it has failed in its basic mission.
Non-Type-Intrusive - Some mechanisms for doing
callbacks require a modification to, or derivation of, the caller or
callee types.  The fact that an object is connected to another object in
a particular application often has nothing to do with its type.  As
we'll see below, mechanisms that are type intrusive can reduce the
flexibility and increase the complexity of application code.   
Generic - The primary differences between different callback
situations are the types involved.  This suggests that the callback
mechanism should be parameterized using templates. Templates insure
consistent interfaces and names in all callback situations, and provide
a way to have any necessary support code be generated by the compiler,
not the user.                      Flexible -
Experience has shown that callback systems that require an exact match
between callback function and callee function signatures are too rigid
for real-world use. For instance you may encounter a callback that
passes a Derived * that you want to connect to a callee
function that takes a Base *.
CURRENT MECHANISMS
Function Model
The simplest callback mechanism is a pointer-to-function, a la
ANSI C's
qsort().  Getting a stand-alone function to act upon a
particular object, however, usually involves kludges like using static
or global pointers to indicate the target object, or having the callback
function take an extra parameter (usually a pointer to the object to act
upon). The static/global pointer method breaks down when the callback
relationship exists across calls, i.e.  'I want to connect this Button
to this X and this other Button to this other X, for the duration of the
app'.  The extra paramter method, if done type-safely, introduces
undesirable coupling between the caller and callee types.        qsort() achieves its genericity by foregoing type safety.  i.e.,
in order for it to be ignorant of the types it is manipulating it takes
untyped (void *) arguments.  There is nothing to prevent
someone from calling qsort() on an array of apples and
passing a pointer to a function that compares oranges!
An example of this typeless mechanism you'll frequently see is
the 'apply' function in collections.  The purpose of an apply function
is to allow a developer to pass a callback to a collection and have it
be 'applied' to (called on) each item in the collection.  Unfortunately
it often looks like this:                      void apply(void (*func)(T &theItem,void *extraStuff),void *theStuff);
                            Chances are really good you don't
have a function like func sitting around, so you'll have
to write one (lots of casting required). And make sure you pass it the
right stuff. Ugh.                                    Single
Rooted Hierarchy
Beware of callback mechanisms that appear type safe but are in
fact not. These mechanisms usually involve some base-of-all-classes like
Object or EventHandler, and utilize casts from ptr-to-member-of-derived
to ptr-to-member-of-base.  Experience has indicated that single-rooted
systems are unworkable if components are to come from multiple sources.
Parameterize the Caller
The component designer could parameterize the component on the
type of the callee.  Such parameterization is inappropriate in many
situations and callbacks are one of them.  Consider:

class Button{
public:
	virtual void click();
//...
};

template <class T>
class ButtonThatCallsBack:public class Button{
public:
	ButtonThatCalls(T *who,void (T::*func)(void)):
		callee(who),callback(func){}
	void click()
		{
		(callee->*callback)();
		}
private:
	T *callee;
        void (T::*callback)(void);
};

class CDPlayer{
public:
	void play();
	//...
};

//Connect a CDPlayer and a Button
CDPlayer cd;
ButtonThatCallsBack<CDPlayer> button(&cd,&CDPlayer::play);
button.click();	//calls cd.play()

A ButtonThatCallsBack<CDPlayer> would thus 'know'
about CDPlayer and provides an interface explicitly based
on it.  The problem is that this introduces rigidity in the system in
that the callee type becomes part of the caller type, i.e.  it is
'type-intrusive'.  All code that creates
ButtonThatCallsBack objects must be made aware of the
callee relationship, increasing coupling in the system.  A
ButtonThatCallsBack<X> is of a different type than a
ButtonThatCallsBack<Y>, thus preventing by-value
manipulation.
If a component has many callback relationships it quickly
becomes unworkable to parameterize them all. Consider a Button
that wants to maintain a dynamic list of callees to be notified upon a
click event. Since the callee type is built into the Button
class type, this list must be either homogeneous or typeless.
Library code cannot even create ButtonThatCallsBack
objects because their instantiation depends on application types.  This
is a severe constraint.  Consider GUI library code that reads a dialog
description from a resource file and creates a Dialog
object. How can it know that you want the Buttons in that
Dialog to call back CDPlayers? It can't,
therefore it can't create the Buttons for you.
Callee Mix-In
The caller component designer can invent an abstract base class
to be the target of the callback, and indicate to application developers
that they mix-in this base in order to connect their class with the
component.  I call this the "callee mix-in."
Here the designer of the Button class wants to
offer a click notification callback, and so defines a nested class
Notifiable with a pure virtual function notify()
that has the desired signature.  Clients of the Button
class will have to pass to its constructor a pointer to a
Notifiable, which the Button will use (at
some point later on) for notification of clicks:

class Button{
public:
	class Notifiable{
	public:
		virtual void notify()=0;
		};
	Button(Notifiable *who):callee(who){}
	void click()
		{callee->notify();}
private:
	Notifiable *callee;
};

Given :

class CDPlayer{
public:
	void play();
	//...
};

an application developer wishing to have a Button call
back a CDPlayer would have to derive a new class from both
CDPlayer and
Button::Notifiable, overriding the pure virtual function
to do the desired work:

class MyCDPlayer:public CDPlayer,public Button::Notifiable{
public:
	void notify()
		{play();}
};

and use this class rather than CDPlayer in the
application:

MyCDPlayer cd;
Button button(&cd);
button.click();	//calls cd.play()

This mechanism is type safe, achieves the decoupling of Button
and
CDPlayer, and is good magazine article fodder.  It is
almost useless in practice, however.
The problem with the callee mix-in is that it, too, is
type-intrusive, i.e.  it impacts the type of the callee, in this case by
forcing derivation.  This has three major flaws.  First, the use of
multiple inheritance, particularly if the callee is a callee of multiple
components, is problematic due to name clashes etc.  Second, derivation
may be impossible, for instance if the application designer gets
CDPlayers from an unchangeable, untouchable API (library
designers note: this is a big problem with mix-in based mechanisms in
general).  The third problem is best demonstrated.  Consider this
version of CDPlayer:

class CDPlayer{
public:
	void play();
	void stop();
	//...
};

It doesn't seem unreasonable to have an application where one Button
calls CDPlayer::play() and another CDPlayer::stop().
 The mix-in mechanism fails completely here, since it can only support a
single mapping between caller/callee/member-function, i.e. MyCDPlayer
can have only one notify().
CALLBACKS USING TEMPLATE FUNCTORS
When I first thought about the inter-component callback
problem I decided that what was needed was a language extension to
support 'bound-pointers', special pointers representing information
about an object and a member function of that object, storable and
callable much like regular pointers to functions.  ARM 5.5 commentary
has a brief explanation of why bound pointers were left out.
How would bound pointers work? Ideally you would initialize them
with either a regular pointer-to-function or a reference to an object
and a pointer-to-member-function. Once initialized, they would behave
like normal pointer-to-functions. You could apply the function call
operator() to them to invoke the function. In order to be
suitable for a callback mechanism, the information about the type of the
callee would _not_ be part of the type of the bound-pointer. It might
look something like this:

// Warning - NOT C++

class Fred{
public:
	void foo();
};

Fred fred;
void (* __bound fptr)() = &fred.foo;

Here fptr is a bound-pointer to a function that takes
no arguments and returns void.  Note that Fred
is not part of fptr's type.  It is initialized with the
object fred and a pointer-to-member-function-of-Fred,
foo.  Saying:

fptr();

would invoke foo on fred.
Such bound-pointers would be ideal for callbacks:

// Warning - NOT C++

class Button{
public:
	Button(void (* __bound uponClickDoThis)() )
		:notify(uponClickDoThis)
		{}
	void click()
		{
		notify();
		}
private:
	void (* __bound notify)();
};

class CDPlayer{
public:
	void play();
};

CDPlayer cd;
Button button(&cd.play);
button.click();	    //calls cd.play()

Bound-pointers would require a non-trivial language extension and
some tricky compiler support.  Given the extreme undesirability of any
new language features I'd hardly propose bound-pointers now.
Nevertheless I still consider the bound-pointer concept to be the
correct solution for callbacks, and set out to see how close I could get
in the current and proposed language.  The result is the Callback
library described below. As it turns out, the library solution can not
only deliver the functionality shown above (albeit with different
syntax), it proved more flexible than the language extension would have
been!
Returning from the fantasy world of language extension, the
library must provide two things for the user.  The first is some
construct to play the role of the 'bound-pointer'.  The second is some
method for creating these 'bound-pointers' from either a regular
pointer-to-function or an object and a pointer-to-member-function.
In the 'bound-pointer' role we need an object that behaves like
a function.  Coplien has used the term functor to describe
such objects. For our purposes a functor is simply an object that
behaves like a pointer-to-function.  It has an operator()
(the function call operator) which can be used to invoke the function to
which it points.  The library provides a set of template Functor
classes.  They hold any necessary callee data and provide
pointer-to-function like behavior. Most important, their type has no
connection whatsoever to the callee type.  Components define their
callback interface using the Functor classes.
The construct provided by the library for creating functors is
an overloaded template function, makeFunctor(), which
takes as arguments the callee information (either an object and a
ptr-to-member-function, or a ptr-to-function) and returns something
suitable for initializing a
Functor object.
The resulting mechanism is very easy to use.  A complete
example:

#include <callback.h>	//include the callback library header
#include <iostream.h>

class Button{
public:
	Button(const Functor0 &uponClickDoThis)
		:notify(uponClickDoThis)
		{}
	void click()
		{
		notify();	//a call to operator()
		}
private:
	Functor0 notify;	//note - held by value
};

//Some application stuff we'd like to connect to Button:

class CDPlayer{ public:
	void play(){cout<<"Playing"<<endl;}
	void stop(){cout<<"Stopped"<<endl;}
};

void wow()
	{cout<<"Wow!"<<endl;}

void main()
	{
	CDPlayer cd;

	//makeFunctor from object and ptr-to-member-function

	Button playButton(makeFunctor(cd,&CDPlayer::play));
	Button stopButton(makeFunctor(cd,&CDPlayer::stop));

	//makeFunctor from pointer-to-function

	Button wowButton(makeFunctor(&wow));

	playButton.click();	//calls cd.play()
	stopButton.click();	//calls cd.stop()
	wowButton.click();	//calls wow()
	}

Voila!  A component (Button) has been connected to
application objects and functions it knows nothing about and that know
nothing about Button, without any custom coding,
derivation or modification of the objects involved.  And it's type safe.
The Button class designer specifies the callback
interface in terms of
Functor0, a functor that takes no arguments and returns
void.  It stores the functor away in its member notify.
 When it comes time to call back, it simply calls operator()
on the functor.  This looks and feels just like a call via a
pointer-to-function.
Connecting something to a component that uses callbacks is
simple.  You can just initialize a Functor with the result
of an appropriate call to
makeFunctor().  There are two flavors of makeFunctor().
 You can call it with a ptr-to-stand-alone function:

	makeFunctor(&wow)

OR with an object and a pointer-to-member function:

	makeFunctor(cd,&CDPlayer::play)

I must come clean at this point, and point out that the syntax above
for
makeFunctor() is possible only in the proposed language,
because it requires template members (specifically, the Functor
constructors would have to be templates).  In the current language the
same result can be achieved by passing to makeFunctor() a
dummy parameter of type ptr-to-the-Functor-type-you-want-to-create. This
iteration of the callback library requires you pass makeFunctor()
the dummy as the first parameter.  Simply cast 0 to
provide this argument:

	makeFunctor((Functor0 *)0,&wow)

	makeFunctor((Functor0 *)0,cd,&CDPlayer::play);

I will use this current-language syntax from here on.
The Button class above only needs a callback
function with no arguments that returns void.  Other
components may want to pass data to the callback or get a return back.
The only things distinguishing one functor from another are the number
and types of the arguments to operator() and its return
type, if any. This indicates that functors can be represented in the
library by (a set of) templates:

//Functor classes provided by the Callback library:

Functor0	//not a template - nothing to parameterize
Functor1<P1>
Functor2<P1,P2>
Functor3<P1,P2,P3>
Functor4<P1,P2,P3,P4>
Functor0wRet<RT>
Functor1wRet<P1,RT>
Functor2wRet<P1,P2,RT>
Functor3wRet<P1,P2,P3,RT>
Functor4wRet<P1,P2,P3,P4,RT>

These are parameterized by the types of their arguments (P1
etc) and return value (RT) if any.  The numbering is
necessary because we can't overload template class names on number of
parameters.  'wRet' is appended to distinguish those with
return values.  Each has an operator() with the
corresponding signature, for example:

template <class P1>
class Functor1{
public:
	void operator()(P1 p1)const;
	//...
};

template <class P1,class P2,class RT>
class Functor2wRet{
public:
	RT operator()(P1 p1,P2 p2)const;
	//...
};

These Functor classes are sufficient to meet the
callback needs of component designers, as they offer a standard and
consistent way to offer callback services, and a simple mechanism for
invoking the callback function.  Given these templates in the library, a
component designer need only pick one with the correct number of
arguments and specify the desired types as parameters.  Here's the
DataEntryField that wants a validation callback that takes
a const String & and returns a
Boolean:

#include <callback.h>

class DataEntryField{
public:
	DataEntryField(const Functor1wRet<const String &,Boolean> &v):
		validate(v){}
	void keyHit(const String & stringSoFar)
		{
		if(validate(stringSoFar))
			// process it etc...
		}
private:
	Functor1wRet<const String &,Boolean> validate;
	//validate has a
	//Boolean operator()(const String &)
};

These trivial examples just scratch the surface of what you can do
given a general purpose callback library such as this.  Consider their
application to state machines, dispatch tables etc.
The callback library is 100% compile-time type safe.  (Where
compile time includes template-instantiation time).  If you try to make
a functor out of something that is not compatible with the functor type
you will get a compiler error.  All correct virtual function behavior is
preserved.
The system is also type flexible.  You'll note that throughout
this article I have said 'type compatible' rather than
'exactly-matching' when talking about the relationship between the
callback function and the callee function.  Experience has shown that
requiring an exact match makes callbacks too rigid for practical use. If
you have done much work with pointer-to-function based interfaces you've
probably experienced the frustration of having a pointer to a function
'that would work' yet was not of the exact type required for a match.
To provide flexibility the library supports building a functor
out of a callee function that is 'type compatible' with the target
functor - it need not have an exactly matching signature.  By type
compatible I mean a function with the same number of arguments, of types
reachable from the functor's argument types by implicit conversion.  The
return type of the function must be implicitly convertible to the return
type of the functor.  A functor with no return can be built from a
function with a return - the return value is safely ignored.

//assumes Derived publicly derived from Base
void foo(Base &);
long bar(Derived &);

Functor1<Derived&> f1 =
        makeFunctor((Functor1<Derived&> *)0,&foo);
	//ok - will implicitly convert

f1 = makeFunctor((Functor1<Derived&> *)0,&bar);
	//ok - ignores return

Any necessary argument conversions or ignoring of returns is done by
the compiler, i.e. there is no coercion done inside the mechanism or by
the user.  If the compiler can't get from the arguments passed to the
functor to the arguments required by the callee function, the code is
rejected at compile time. By allowing the compiler to do the work we get
all of the normal conversions of arguments - derived to base, promotion
and conversion of built-in types, and user-defined conversions.
The type-flexibility of the library is something that would not
have been available in a language extension rendition of bound pointers.
Rounding out the functionality of the Functor
classes are a default constructor that will also accept 0
as an initializer, which puts the
Functor in a known 'unset' state, and a conversion to
Boolean which can be used to test whether the Functor
is 'set'.  The Functor classes do not rely on any virtual
function behavior to work, thus they can be held and copied by-value.
Thus a Functor has the same ease-of-use as a regular
pointer-to-function.
At this point you know everything you need to use the callback
library. All of the code is in one file, callback.h.  To
use a callback in a component class, simply instantiate a Functor
with the desired argument types.  To connect some stuff to a component
that uses Functors for callbacks, simply call makeFunctor()
on the stuff.  Easy.
Power Templates
As usual, what is easy for the user is often tricky for the
implementor. Given the black-box descriptions above of the Functor
classes and makeFunctor() it may be hard to swallow the
claims of type-safety, transparent conversions, correct virtual function
behavior etc.  A look behind the curtain reveals not only how it works,
but also some neat template techniques.  Warning: most people find the
pointer-to-member and template syntax used in the implementation
daunting at first.
Obviously some sort of magic is going on.  How can the Functor
class, with no knowledge of the type or signature of the callee, ensure
a type safe call to it, possibly with implicit conversions of the
arguments?  It can't, so it doesn't.  The actual work must be performed
by some code that knows both the functor callback signature and
everything about the callee.  The trick is to get the compiler to
generate that code, and have the Functor to point to it.
Templates can help out all around.
The mechanism is spread over three components - the Functor
class, a Translator class, and the makeFunctor()
function.  All are templates.
The Functor class is parameterized on the types of
the callback function signature, holds the callee data in a typeless
manner, and defines a typed operator() but doesn't
actually perform the work of calling back. Instead it holds a pointer to
the actual callback code. When it comes time to call back, it passes the
typeless data (itself actually), as well as the callback arguments, to
this pointed-to function.
The Translator class is derived from Functor
but is parameterized on both the Functor type _and_ the
callee types.  It knows about everything, and is thus able to define a
fully type-safe static 'thunk' function that takes the typeless Functor
data and the callback arguments.  It constructs its Functor
base class with a pointer to this static function.  The thunk function
does the work of calling back, turning the typeless Functor
data back into a typed callee and calling the callee.  Since the Translator
does the work of converting the callee data to and from untyped data the
conversions are considered 'safe'. The
Translator isA Functor, so it can be used to
initialize a Functor.
The makeFunctor() function takes the callee data,
creates a
Translator out of it and returns the Translator.
 Thus the Translator object exists only briefly as the
return value of makeFunctor(), but its creation is enough
to cause the compiler to lay down the static 'thunk' function, the
address of which is carried in the Functor that has been
initialized with the Translator.
All of this will become clearer with the details.
For each of the 10 Functor classes there are 2
Translator classes and 3 versions of makeFunctor().
 We'll examine a slice of the library here, Functor1 and
its associated Translators and makeFunctors.
The other Functors differ only in the number of args and
return values.
The Functors
Since the Functor objects are the only entities
held by the caller, they must contain the data about the callee.  With
some care we can design a base class which can hold, in a typeless
manner, the callee data, regardless of whether the callee is a
ptr-to-function or object/ptr-to-member-function combo:

//typeless representation of a function or object/mem-func

class FunctorBase{
public:
	typedef void (FunctorBase::*_MemFunc)();
	typedef void (*_Func)();
	FunctorBase():callee(0),func(0){}
	FunctorBase(const void *c,const void *f,size_t sz)
		{
		if(c)	//must be callee/memfunc
			{
			callee = (void *)c;
			memcpy(memFunc,f,sz);
			}
		else	//must be ptr-to-func
			{
			func = f;
			}
		}
	//for evaluation in conditions
	//will be changed to bool when bool exists
	operator int()const{return func||callee;}

	class DummyInit{
	};
////////////////////////////////////////////////////////////////
// Note: this code depends on all ptr-to-mem-funcs being same size
// If that is not the case then make memFunc as large as largest
////////////////////////////////////////////////////////////////

	union{
	const void *func;
	char memFunc[sizeof(_MemFunc)];
	};
	void *callee;
};

All Functors are derived (protected) from this base.
FunctorBase provides a constructor from typeless args,
where if c is 0 the callee is a
pointer-to-function and f is that pointer, else c
is pointer to the callee object and f is a pointer to a
pointer-to-member function and sz is that
ptr-to-member-function's size (in case an implementation has
pointer-to-members of differing sizes).  It has a default constructor
which inits to an 'unset' state, and an operator int to
allow for testing the state (set or unset).
The Functor class is a template.  It has a default
constructor and the required operator() corresponding to
its template parameters. It uses the generated copy constructor and
assignment operators.

/************************* one arg - no return *******************/
template <class P1>
class Functor1:protected FunctorBase{
public:
	Functor1(DummyInit * = 0){}
	void operator()(P1 p1)const
		{
		thunk(*this,p1);
		}
	FunctorBase::operator int;
protected:
	typedef void (*Thunk)(const FunctorBase &,P1);
	Functor1(Thunk t,const void *c,const void *f,size_t sz):
		FunctorBase(c,f,sz),thunk(t){}
private:
	Thunk thunk;
};

The Functor class has a protected constructor that
takes the same typeless args as FunctorBase, plus an
additional first argument.  This argument is a pointer to function (the
thunk function) that takes the same arguments as the operator(),
plus an additional first argument of type const FunctorBase &.
 The Functor stores this away (in thunk) and implements
operator() by calling thunk(), passing
itself and the other arguments.  Thus it is this thunk()
function that does the work of 'calling back'.
A key issue at this point is whether operator()
should be virtual. In the first iteration of my mechanism the Functor
classes were abstract and the operator()'s pure virtual.
To use them for callbacks a set of derived template classes
parameterized on the callee type was provided. This required that
functors always be passed and held by reference or pointer and never by
value. It also required the caller component or the client code maintain
the derived object for as long as the callback relationship existed. I
found the maintenance and lifetime issues of these functor objects to be
problematic, and desired by-value syntax.
In the current mechanism the Functor classes are
concrete and the operator() is non-virtual.  They can be
treated and used just like ptr-to-functions.  In particular, they can be
stored by value in the component classes.
The Translators
Where does the thunk() come from?  It is generated
by the compiler as a static member of a template 'translator' class.
For each
Functor class there are two translator classes, one for
stand-alone functions (FunctionTranslator) and one for
member functions (MemberTranslator). The translator
classes are parameterized by the type of the Functor as
well as the type(s) of the callee.  With this knowledge they can, in a
fully type-safe manner, perform two important tasks.
First, they can initialize the Functor data.  They
do this by being publicly derived from the Functor.  They
are constructed with typed callee information and which they pass
(untyped) to the functor's protected constructor.
Second, they have a static member function thunk(),
which, when passed a FunctorBase, converts its callee data
back into typed information, and executes the callback on the callee.
It is a pointer to this static function which is passed to the Functor
constructor.

template <class P1,class Func>
class FunctionTranslator1:public Functor1<P1>{
public:
	FunctionTranslator1(Func f):Functor1<P1>(thunk,0,f,0){}
	static void thunk(const FunctorBase &ftor,P1 p1)
		{
		(Func(ftor.func))(p1);
		}
};

FunctionTranslator is the simpler of the two.  It is
parameterized by the argument type of the Functor and some
ptr-to-function type (Func). Its constructor takes an
argument of type Func and passes it and a pointer to its
static thunk() function to the base class constructor. The
thunk function, given a FunctorBase ftor, casts ftor's
func member back to its correct type (Func) and calls it.
There is an assumption here that the FunctorBase ftor is
one initialized by the constructor (or a copy).  There is no danger of
it being otherwise, since the functors are always initialized with
matching callee data and thunk functions. This is what is called a
'safe' cast, since the same entity that removed the type information
also re-instates it, and can guarantee a match.  If
Func's signature is incompatible with the call, i.e.  if
it cannot be called with a single argument of type P1,
then thunk() will not compile. If implicit conversions are
required the compiler will perform them. Note that if func
has a return it is safely ignored.

template <class P1,class Callee, class MemFunc>
class MemberTranslator1:public Functor1<P1>{
public:
	MemberTranslator1(Callee &c,const MemFunc &m):
		Functor1<P1>(thunk,&c,&m,sizeof(MemFunc)){}
	static void thunk(const FunctorBase &ftor,P1 p1)
		{
		Callee *callee = (Callee *)ftor.callee;
		MemFunc &memFunc(*(MemFunc*)(void *)(ftor.memFunc));
		(callee->*memFunc)(p1);
		}
};

MemberTranslator is parameterized by the argument type
of the
Functor, some class type (Callee), and some
ptr-to-member-function type (MemFunc).  Not surprisingly
it's constructor is passed 2 arguments, a
Callee object (by reference) and a ptr-to-member-function,
both of which are passed, along with the thunk function, to the base
class constructor.  Once again, the thunk function casts
the typeless info back to life, and then calls the member function on
the object, with the passed parameter.
Since the Translator objects are Functor
objects, and fully 'bound' ones at that, they are suitable initializers
for their corresponding Functor, using the Functor's
copy constructor.  We needn't worry about the 'chopping' effect since
the data is all in the base class portion of the Translator
class and there are no virtual functions involved.  Thus they are
perfect candidates for the return value of
makeFunctor()!
The makeFunctor Functions
For each Functor class there are three versions of
makeFunctor(), one for ptr-to-function and a const and
non-const version for the object/ptr-to-member-function pair.

template <class P1,class TRT,class TP1>
inline FunctionTranslator1<P1,TRT (*)(TP1)>
makeFunctor(Functor1<P1>*,TRT (*f)(TP1))
	{
	return FunctionTranslator1<P1,TRT (*)(TP1)>(f);
	}

The function version is straightforward. It uses the dummy argument
to tell it the type of the functor and merely returns a corresponding
FunctionTranslator. I mentioned above that the Func
type parameter of
FunctionTranslator was invariably a ptr-to-function type.
This version of makeFunctor() ensures that by explicity
specifying it as such.

template <class P1,class Callee,class TRT,class CallType,class TP1>
inline MemberTranslator1<P1,Callee,TRT (CallType::*)(TP1)>
makeFunctor(Functor1<P1>*,Callee &c,TRT (CallType::* const &f)(TP1))
	{
	typedef TRT (CallType::*MemFunc)(TP1);
	return MemberTranslator1<P1,Callee,MemFunc>(c,f);
	}

This is the gnarliest bit.  Here makeFunctor is
parameterized with the type of the argument to the Functor,
the type of the callee, the type of the class of which the
member-function is a member, the argument and return types of the member
function.  Whew!  We're a long way from
Stack<T> land!  Like the ptr-to-function version, it
uses the dummy first argument of the constructor to determine the type
of the Functor. The second argument is a Callee
object (by reference).  The third argument is this thing:

TRT (CallType::* const &f)(TP1)

Here f is a reference to a constant pointer to a
member function of
CallType taking TP1 and returning TRT.
 You might notice that pointer-to-member-functions are all handled by
reference in the library. On some implementations they can be expensive
to pass by value and copy. The significant feature here is that the
function need not be of type pointer-to-member-of-Callee.  This allows
makeFunctor to match on (and ultimately work with) a
ptr-to-member-function of some base of Callee. It then
typedefs that bit and returns an appropriate MemberTranslator.

template <class P1,class Callee,class TRT,class CallType,class TP1>
inline MemberTranslator1<P1,const Callee,TRT (CallType::*)(TP1)const>
makeFunctor(Functor1<P1>*,const Callee &c,TRT (CallType::* const &f)(TP1)const)
	{
	typedef TRT (CallType::*MemFunc)(TP1)const;
	return MemberTranslator1<P1,const Callee,MemFunc>(c,f);
	}

This last variant just ensures that if the Callee is
const the member function is also (note the const at the
end of the third argument to the constructor - that's where it goes!).
That, for each of ten Functors, is the whole
implementation.
Can Your Compiler Do This?
The callback library has been successfully tested with IBM
CSet++ 2.01, Borland C++ 4.02 (no, its not twice as good ;-), and Watcom
C++32 10.0. It is ARM compliant with the exception of expecting trivial
conversions of template function arguments, which is the behavior of
most compilers. I am interested in feedback on how well it works with
other implementations.
Summary
Callbacks are a powerful and necessary tool for component based
object-oriented development in C++.  They can be a tremendous aid to the
interoperability of libraries.  The template functor system presented
here meets all the stated criteria for a good callback mechanism - it is
object-oriented, compile-time type-safe, generic, non-type-intrusive,
flexible and easy to use.  It is sufficiently general to be used in any
situation calling for callbacks.  It can be implemented in the current
language, and somewhat more elegantly in the proposed language.
This implementation of callbacks highlights the power of C++
templates - their type-safety, their code-generation ability and the
flexibility they offer by accepting ptr-to-function and
ptr-to-member-function type parameters.
Ultimately the greatest benefit is gained when class libraries
start using a standard callback system.  If callbacks aren't in the
components, they can't be retrofitted.  Upon publication of this article
I am making this Callback library freely available in the hope that it
will be adopted by library authors and serve as a starting point for
discussion of a standard callback system.


References
Stroustrup, B. The Design and Evolution of C++, Addison-Wesley,
Reading, MA 1994
Coplien, J.O. Advanced C++ Programming Styles and Idioms,
Addison-Wesley, Reading, MA 1992
Ellis, M.A. and B. Stroustrup. The Annotated C++ Reference
Manual, Addison-Wesley, Reading, MA 1990
Lippman, S.B. C++ Primer 2nd Edition, Addison-Wesley, Reading,
MA 1991
Acknowledgments
Thanks to my fellow developers at RCS and to
Greg Comeau
 for
reviewing and commenting on this article.
About the Author
Rich is Technical Design Lead at Radio Computing Services, a
leading software vendor in the radio industry.  He designed and teaches
the Advanced C++ course at New York University's Information
Technologies Institute.
 He can be reached at:
rhickey@bestweb.net
 

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