Re: [Boost-users] Why not overloading && and || in bind?

From: Peter Dimov (pdimov_at_[hidden])
Date: 2008-02-20 13:54:18

Richard:

> boost-users_at_[hidden] spake the secret code
> <019801c8721d$e1612fc0$6407a80a_at_pdimov2> thusly:
>
>>It is impossible to overload && in such a way so that in e1 && e2, e2 is 
>>not
>>evaluated when e1 is false. But it's possible to overload f && g so that 
>>in
>>(f && g)(x) :- f(x) && g(x), g(x) is not evaluated when f(x) is false.
>
> I guess I'm still a little lost... why does it work for f && g and not
> e1 && e2?  Is it because f and g are functors and don't evaluate their
> expressions until their operator() is called?

Yes, exactly. As an example, boost::bind( f, 4 ) creates a function object 
that will call f(4) when its operator() is called; and with the operator&& 
defined, bind( f, 4 ) && bind( g, 5 ) will call f(4) && g(5) when called, 
which will correctly not call g(5) when f(4) returns false.