Le 05/05/2010 00:55, Jeffrey Lee Hellrung, Jr. a écrit :
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 (I don't think there's a standard is_default_constructible trait, right?).
I add a boost::optional use case in my "benchmark" and, performance wise, boost::optional is a much better option than boost::function to wrap non Default Constructible object function. I would say the game isn't worth the candle to wrap it conditionnaly, at least under vc10. I used the following wrapper : template <class UnaryFunc> struct optional_function { optional_function() {} optional_function(UnaryFunc _func) : func_(_func) {} template <class Arg1> struct result {typedef typename boost::result_of<UnaryFunc(Arg1)>::type type;}; template <class Arg1> typename result<Arg1>::type operator()(Arg1 _a1) const {return (*func_)(_a1);} boost::optional<UnaryFunc> func_; }; template <class UnaryFunc> optional_function<UnaryFunc> make_optional_function(UnaryFunc _func) {return optional_function<UnaryFunc>(_func);} Note that, to use a polymorphic object function like optional_function with transform_iterator, you need to apply the patch I suggested in https://svn.boost.org/trac/boost/ticket/1427 Finally, for my use case, the definitive solution is fast_mem_fn, an example of the function_type library. It's inlinable (and correctly inlined by vc10) and Default Constructible without the draw back of undefined runtime behaviour. I believe that this example could be a seed of a premium Boost library. Samuel Debionne