Boost logo

Boost :

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:

            BOOST_AUTO(& x, *_); // or int (&x)[200] = *_;
                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.


>> 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?

Andy Little

Boost list run by bdawes at, gregod at, cpdaniel at, john at