Re: [boost] Re: boos::mpl::for_each: call function with return value

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2003-11-02 03:12:22

David Abrahams wrote:
> Aleksey Gurtovoy <agurtovoy_at_[hidden]> writes:
>
> > <digression>
> > Of course ideally Fusion and MPL should interpolate
>
> Do you mean interoperate?

Yep.

> > so that the above could be written as
> >
> >     struct make_vehicle
> >     {
> >         template< typename T > struct result
> >         {
> >             typedef vehicle* type;
> >         };
> >
> >         template< typename T >
> >         vehicle* operator()(identity<T>, vehicle* ptr) const
> >         {
> >             return new T(ptr);
> >         }
> >     };
> >
> >     int main()
> >     {
> >         vehicle* result = fusion::fold(
> >               fusion::identity_view<
mpl::vector<vehicle,car,bicycle,train>
> >>()
> >             , static_cast<vehicle*>(0)
> >             , make_vehicle
> >             );
> >
> >         return 0;
> >     }
> > </digression>
>
> I'm not sure why that's an improvement.

IMO the way the OP problem is formulated, it literally screams for
'fold'/'accumulate':

>     vehicle* make(vehicle* v0)
>     {
>         vehicle* v1 = new car( v0 );
>         vehicle* v2 = new bicycle( v1 );
>         vehicle* v3 = new train( v2 );
>         return v3;
>     }

And then the 'fold' version requires less code on the user side.

>  Is identity_view supposed to
> default-construct a vehicle,car, bicycle, and train?

Nope, it constructs 'identity<vehicle>', 'identity<car>', etc.

Aleksey