[boost] Re: Formal Review: FOREACH macro

From: Andy Little (andy_at_[hidden])
Date: 2005-05-02 11:11:45

"Eric Niebler" <eric_at_[hidden]> wrote

> What about nested invocations for BOOST_FOREACH:
>
>     int a[100][200];
>     int sum = 0;
>
>     BOOST_FOREACH(int (&x)[200], a)
>         BOOST_FOREACH(int i, x)
>             sum += i;
>
> ?

Its a fair point but  just as  possible the other way:

        FOR_EACH(a){
            BOOST_AUTO(& x, *_); // or int (&x)[200]  = *_;
            FOR_EACH(x)
                sum += *_;
        }

>> I dont know of a reason to require conversions as in the example:
>> short array_short[] = {1,2,3};
>> BOOST_FOREACH( int i, array_short )
>> {
>>    // The short was implicitly converted to an int
>> }
>> What is the point of the example?
>
>
> Conversions aren't "required", they are allowed. The point is to show that the 
> loop variable doesn't have to have the same type as the element type, it only 
> must be convertible.

OK.

>> However maybe this is an advantage... because the iterator is exposed then 
>> more loop information is avaialable eg if _ == my_container::begin() etc., 
>> which is an advantage over simply having the element available so making 
>> things more versatile.

> It makes BOOST_FOREACH harder to extend. Not every sequence that users may 
> want to iterate has anything like an iterator which is dereferencable. It is 
> currently possible to make such non-STL sequences work with FOREACH with 
> little effort, but after this change it would be impossible.

Ok.. but   the main use would be with standard containers I would guess. I cant 
argue about what  users may require.  I think that other collections (e.g 
boost::graph) work quite hard to try to be consistent with the std algorithms. 
Is there a motivating example in the documentation?

regards
Andy Little