[boost] Re: Re: Re: FOREACH / C++ "for" proposal

From: Thorsten Ottosen (nesotto_at_[hidden])
Date: 2005-05-11 14:28:23

"Peter Dimov" <pdimov_at_[hidden]> wrote in message 
news:011e01c5565c$b9f22000$6401a8c0_at_pdimov2...
| Thorsten Ottosen wrote:
| > "Peter Dimov" <pdimov_at_[hidden]> wrote in message
| > news:007301c55654$256e3bb0$6401a8c0_at_pdimov2...
| >> Thorsten Ottosen wrote:
|
| >>> yep, but the latter approach is harder and takes just about twice as
| >>> much code in C++0x.
| >>
| >> Can you elaborate, please?
| >
| > sure.
| >
| > as a free-standing function we can say
| >
| > template< class T >
| > auto begin( MyType<T>&& r ) -> decltype( r.Begin() )
| > {
| >    return r.Begin();
| > }
|
| And

no.

| template< class T >
| auto begin( MyType<T> const && r ) -> decltype( r.Begin() )
| {
|   return r.Begin();
| }
|
| assuming r.Begin() is const-correct, which it isn't in your example.

why not?

| You
| also forgot the end() overload.

well, It's just the same code again.

| It's debatable whether non-const MyType rvalues should be
| mutably-iteratable, so we might prefer & instead of && in the above.

at least the standard should not dictate whether it is.

| > for( const auto& r : my_obj )
| >    { ... }
|
| > with an adapter class we need to say...
|
| With an adapter we need to say
|
| template<class T> std::range<T*> adapt( MyType<T> & r )
| {
|    return std::range<T*>( r.Begin(), r.End() );
| }
|
| template<class T> std::range<T const *> adapt( MyType<T> const & r )
| {
|    return std::range<T const *>( r.Begin(), r.End() );
| }
|
| for( auto & r: adapt( my_obj ) )
| {
|    // ...
| }
|
| which is twice as short, not twice as long.

your assumptions are wrong; but we should get the same size
if the free-standing adapter is used.

now take into consideration that begin()/end() are also needed for range 
algorithms
and there shouldn't be any doubt that ADL + free.standing functions is 
superior.

-Thorsten