Eric Niebler wrote:
But the hint expands to the derived classes and the derived classes "must" customize 'has_cheap_copy'.
I agree that's not good. I've reimplemented the has_cheap_copy customization point again. See below.
Is it portability-issue the reason why you avoid metafunctions?
You mean, why isn't there a has_cheap_copy<T> trait? I had originally wanted the cheap_copy optimization to automatically apply to derived types, which is the reason for the particular form of the customization point. I no longer think that's a good idea, so I can now add a has_cheap_copy<> trait, however ...
... on compilers without partial template specialization, there isn't a good way of saying that for all types T, boost::iterator_range<T> is cheap to copy. So the free function template is still the way to go on those compilers.
Here is the new-and-improved(-and-please-god-let-this-be-the-last) way to optimize FOREACH for cheap-to-copy types:
- For a user-defined range type Foo that is cheap to copy, you should specialize has_cheap_copy<> in the boost::foreach namespace, as:
namespace boost { namespace foreach { template<> struct has_cheap_copy<Foo> : mpl::true_ {}; }}
- For maximum portability, you could achieve the same effect by overloaded boost_foreach_has_cheap_copy() at global scope like this:
inline boost::mpl::true_ * boost_foreach_has_cheap_copy(Foo *&, boost::foreach::tag) { return 0; }
(The slightly strange Foo *& type prevents this overload from being selected for types derived from Foo.)
By default FOREACH defines overloads to recognize std::pair<T,T>, iterator_range<T> and sub_range<T> as cheap-to-copy, but NOT types derived from them.
I hope this is satisfactory.
I think you are right. My misgiving was how common a range which is noncopyable but derived from iterator_range is. My sixth range was such a one. Though I still wonder what range is cheap to copy, ('iterator_range' of 'spirit::file_iterator' that has 'shared_ptr' is cheap to copy?) optimization-issues now don't surprise any legal ranges. Regards, MB http://p-stade.sourceforge.net/