Re: [boost] function.hpp: some ideas

From: Douglas Gregor (gregod_at_[hidden])
Date: 2001-05-07 18:19:44

On Monday 07 May 2001 03:28 pm, you wrote:
> Douglas Gregor wrote:
> > >       I have no idea what 'set, clear, empty' etc
> > > might be for: functions can't be set, cleared,
> > > or emptied. They should act like values, and be
> > > callable.
> > >
> > >       {Have I missed something deep here?]
> >
> > Prefix 'function' with 'boost::' and you've missed something :)
> > boost::function is a generalized function object adaptor that is declared
> > with some given function signature and that can contain any function
> > object with a compatible signature. It overloads the function call
> > operator to call the underlying function object's function call operator.
> > One of its uses (the one we're debating currently) is as a callback.
>
> 	Are you saying that 'set', for example, is to be used to
> change the object to which the adaptor delegates?

Yes.

> So for example, if an adaptor f were registered as a callback,
> the actual callback can be dynamically changed without
> changing the object registered?

I'm tripping over the word 'registered' here. Assuming that it is 'declared' 
in the C++ context, yes. 

Perhaps code would better illustrate what I mean:

int add(int x, int y) { return x+y; }
int mul(long x, long y) { return x+y; }

boost::function<int, int, int> f;

f = &add;
cout << f(2, 3) << endl; // Writes 5
f = &mul;
cout << f(2, 3) << endl; // Writes 6

So essentially boost::function is a function adaptor that gives a common 
static interface (here, a function object taking two ints and returning an 
int) to any of the set of function objects (function pointers are merely a 
type of function object) with compatible signatures. Here, both "add" and 
"mul" have compatible signatures, so they can be targets of the function f.

        Doug