Re: [Boost-Users] Getting the hang of Boost::Bind

From: Douglas Gregor (gregod_at_[hidden])
Date: 2002-08-12 07:48:09

On Monday 12 August 2002 05:10 am, Sam Partington wrote:
> Am I right in thinking that boost::bind will automatically detect whether
> the function call should be a->f(), a.f() or f(a)?

Bind deals with either function objects or pointers to member functions, so 
the forms are: (a.*f)() and (a->*f)() for pointers to member functions, or 
f(a) for function objects.

> bool f(const A& a);
>
> std::vector<A>	v;
>
>  std::vector<A>::const_iterator iter =
>     std::find_if(v.begin(), v.end(), boost::bind(&A::f, _1));
>
> Presumably it does the right thing and calls A::f, but how does it do that?

When bind has a pointer-to-member function, it has the return type and the 
argument types. Most importantly, it has the type of the object it will be 
called with: 'A'

The trick is this: when you call the bind object as f(x), bind is going to try 
to determine if 'x' is an A or is derived from A. If so, it can proceed with 
the call as (x.*f)(). Otherwise, bind assumes that 'x' is some form of 
pointer and uses a  'get_pointer' facility to extract a raw pointer to call 
via (get_pointer(x)->*f)().

[All this logic is actually part of mem_fn, not bind; bind just wraps mem_fn 
in these cases]

        Doug