On Tue, May 4, 2010 at 11:59 PM, Jeffrey Lee Hellrung, Jr. <jhellrung@ucla.edu> wrote:
On 05/04/2010 06:20 PM, Daniel Walker wrote:
On Tue, May 4, 2010 at 6:55 PM, Jeffrey Lee Hellrung, Jr. <jhellrung@ucla.edu> wrote:
On 5/4/2010 3:49 PM, Daniel Walker wrote:
On Tue, May 4, 2010 at 1:49 PM, Peter Dimov<pdimov@pdimov.com> wrote:
Samuel Debionne:
Anyway, in my opinion, using bind/mem_fn/lambda with transform_iterator is a common use case. That would be great to have an option to add a default constructor in those libs... or is it to risky ?
The proper fix would be to make transform_iterator not require a default-constructible function object, by using boost::optional or an equivalent.
That's a great idea! I took a look at it, and the required changes seem to be minor. I made a ticket and attached a patch. https://svn.boost.org/trac/boost/ticket/4189 I ran Boost.Iterator regression tests with gcc 4.2 and it passed without error.
Daniel Walker
Maybe you should only conditionally wrap in a boost::optional, e.g., if has_trivial_constructor<F> is false
What would that gain? It's a little hairy to implement on old compilers. I'm not sure that it's worth it.
The gain is you don't have to pay the cost of a boolean in space. The boolean also has to be checked upon assignment and copy construction, so there is some amount of additional runtime overhead.
Thanks for the clarification. That seems like reasonable justification for the change.
What is "it" in "it's a little hairy to implement on old compilers"? The has_trivial_constructor metafunction, or a has_trivial_constructor-aware transform_iterator? For the former, I'd believe it, but I would expect the Boost.TypeTraits version to be good enough. For the latter, (and I could be very very wrong here, but) seems like the only thing you have to do is put an indirection layer when calling the function held by the transform_iterator, e.g., unwrap_optional(f)( ...args... ). In fact, there may be such a function provided by Boost.Optional...
Boost.Optional doesn't have an unwrap that I could find. I uploaded a new patch that conditionally uses optional based on has_trivial_constructor per your suggestion. It has a simple unwrap that should suffice. It passes regressions with gcc 4.2. My concern with older compilers is that I know the authors of Boost.Iterator went to some effort to support deficient compilers, but, unfortunately, I am unable to test this patch on nearly as many. Still, this is a start at least. Daniel Walker