[boost] Re: A mem_fn thought

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2003-09-16 19:56:07

On Sep 16, 2003, at 5:48 PM, David Abrahams wrote:

> It would be convenient for me if mem_fn(pm) could be *called* to set
> the data member, as in:
>
>      X x;
>
>      mem_fn(&X::member)(x, value);
>
> That way, bind(&X::member, _1, value) would be useful.

Could you expound on your convenience factor with respect to what you 
have proposed, and the following:

template <class T>
struct assign
{
        typedef T& result_type;

        template <class U>
        T& operator()(T& t, const U& u) const
                {return t = u;}
};

...

bind(assign<T>(), bind(&X::member, _1), value)(x);

Clearly what you have proposed is more convenient, but it seems less 
consistent.  And if assign<T> was commonly available, the inconvenience 
appears to be small.

The motivation for my request is that bind(f, a1, a2, ... an) currently 
has an invariant that f is a function-like object taking n parameters, 
except if f is a member pointer, then a1 is a (smart) pointer or 
reference to the object, and the function itself (if it is a function) 
takes n-1 parameters.  The degenerate case of f being a member data 
pointer coincides nicely with a member function pointer taking 0 
parameters.

So the motivation for my question is:  Is there sufficient motivation 
to make an exception out of member data pointers, thus complicating the 
interface?

-Howard