RE: [boost] any_function (callback) library

From: Geurt Vos (G.Vos_at_[hidden])
Date: 2001-04-09 09:38:56

First, sorry for not replying sooner. I just realized you were
working on this.

I've been working on a callback class for some time now, and 
next to the obvious goals, there are two more requirements
I'm trying to meet:
1. the class should be extendable
2. it should be easy to extend

Both these criteria are not met by the any_function
implementation.

> 
> ------------------------------
> Interface summary:
> 
>   template<typename Result, typename Arg1, typename Arg2, 
> ..., typename ArgN>

The main problem with this approach is that it isn't extendable.
I mean, how many parameters to support? as I view the code, it's
implemented for up to 6 parameters, which is in my opinion enough
(my 'previous version' had this approach for up to 5), but I 
guarantee you someone will require 7. The largest implementation
I've seen so far is Alexandrescu's Functor class: a massive 15 
parameters (the one in Modern C++ Design). 

Imagine the error message you'll get when the number of required
parameters isn't met, and any_function is called with either one
argument more or one less.......

>   class any_function

why not 'class callback', and why only functions and functors,
and not also function members? 

>     any_function& operator=(const int);  // assignment to 
> zero clears target

This one seems quite superfluous. I mean,
just clear() seems enough to me.

>     bool empty() const; // true if no target
>     operator const void*() const; // evaluates true if non-empty
> 

Again, I'm not certain whether both are required.
I personally would lose the operator...

Lastly, I'm not sure whether or not the distinction between
invoker and void_invoker is done for compatibility with
older compilers. Anyway, note that the following code is
legal C++ code:

void f1();
void f2()
{
    return f1();
}

--------------------------------------------------
Up until now I only said what's 'wrong' with any_function,
of course I should also provide some solutions. First, I
can make my solution available, should I simply upload it
to groups.yahoo.com/boost?

Anyway, the declaration of the callback is as following
(omitted namespaces for brevity):

Callback<CxParam<Param0,...,Paramx,Result,validate> > instance;

'x' is the implementation for a specific number of
parameters. I have it implemented for 0 to 5.
Result defaults to 'void' and validate to 'true'. 
If 'validate' is true, a call to operator() will 
first check whether a function is attached and if
not, return Result();

Callback will inherit from CxParam<>, and CxParam will
inherit from CallbackHolder<StoragexParam>. StoragexParam
is a class template, which has two nested classes: one for
the interface (with only operator() as virtual func) and
one for storing a pointer to the actual function (which is
either a function, function object or a function member).

I'm not sure whether this all makes sense until you've
seen the code....just let me know where I should make it
available...

The two criteria I mentioned at the beginning of this mail
I think have been met quite nicely with this implementation.
I can make it even easier by using virtual inheritance at 
two place, but I didn't want to touch that yet :) 

Geurt