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# CALLBACKS IN C++ USING TEMPLATE FUNCTORS
## 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 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 button(&cd,&CDPlayer::play);
button.click(); //calls cd.play()
A `ButtonThatCallsBack` 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 `is of a different type than a `ButtonThatCallsBack`, 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 //include the callback library header
#include
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"<
Functor2
Functor3
Functor4
Functor0wRet