On Dec 5, 2011, at 11:39 AM, Jeffrey Lee Hellrung, Jr. wrote:
On Mon, Dec 5, 2011 at 11:30 AM, Jeremiah Willcock <jewillco@osl.iu.edu>wrote:
On Mon, 5 Dec 2011, Marshall Clow wrote:
I have a function that returns a pair of iterators.
There's also a version that takes a comparison predicate.
[ snip]
I want to provide a range-based version of it.
[ snip ]
Ok. There's a problem. If I call: foo ( first, last ) I get an error, because there are two perfectly good two argument candidates.
Fine. Been there, seen that. I can use disable_if to make sure that the second range based version is only "active" when the arguments are different types. It's verbose, but it (usually) works
[ more snippage ]
But this time it does not! I get a compile error telling me that the compiler can't deal with boost::range_iterator<XXX>::**type, when XXX = some random iterator. It seems that the compiler wants to evaluate all the parameters of disable_if_c before deciding whether or not to SFINAE it (I guess that's not unreasonable, but it's not what I want)
Apparently, all the other times that I did this, the return type of the function was not a dependent type of the template arguments.
Any suggestions for a workaround here?
Look at lazy_disable_if -- it doesn't get the nested "type" member of the type you give it unless the condition evaluates to false.
Just to supplement: also use an extra level of indirection, a metafunction that maps Range to the return type of foo. So will look like boost::lazy_disable_if_c< condition, foo_result< Range > >::type.
Yeah, that didn't seem to work for me. So - here's some code. And here's some error messages (from clang 3.0): $ clang++ -I /Volumes/EyeFive/Marshall/Sources/boost/trunk disable_if.cpp In file included from disable_if.cpp:5: In file included from /Volumes/EyeFive/Marshall/Sources/boost/trunk/boost/range/begin.hpp:24: In file included from /Volumes/EyeFive/Marshall/Sources/boost/trunk/boost/range/iterator.hpp:23: /Volumes/EyeFive/Marshall/Sources/boost/trunk/boost/mpl/eval_if.hpp:60:26: error: no type named 'type' in 'boost::range_const_iterator<__gnu_cxx::__normal_iterator<int *, std::vector<int, std::allocator<int> > > >' typedef typename f_::type type; ~~~~~~~~~~~~~^~~~ /Volumes/EyeFive/Marshall/Sources/boost/trunk/boost/range/iterator.hpp:61:18: note: in instantiation of template class 'boost::mpl::eval_if_c<true, boost::range_const_iterator<__gnu_cxx::__normal_iterator<int *, std::vector<int, std::allocator<int> > > >, boost::range_mutable_iterator<const __gnu_cxx::__normal_iterator<int *, std::vector<int, std::allocator<int> > > > >' requested here mpl::eval_if_c< is_const<C>::value, ^ disable_if.cpp:29:37: note: in instantiation of template class 'boost::range_iterator<const __gnu_cxx::__normal_iterator<int *, std::vector<int, std::allocator<int> > > >' requested here typedef std::pair<typename boost::range_iterator<const Range>::type, ^ disable_if.cpp:45:20: note: in instantiation of template class 'detail::range_pair<__gnu_cxx::__normal_iterator<int *, std::vector<int, std::allocator<int> > > >' requested here typename detail::range_pair<Range>::type> ^ disable_if.cpp:47:1: note: while substituting deduced template arguments into function template 'Foo' [with Range = __gnu_cxx::__normal_iterator<int *, std::vector<int, std::allocator<int> > >, Compare = ] Foo ( const Range &r, Compare comp ) ^ disable_if.cpp:83:2: note: in instantiation of function template specialization 'test_sequence_no_comparison<std::vector<int, std::allocator<int> > >' requested here test_sequence_no_comparison ( v ); // even # of elements ^ 1 error generated. Thanks! -- Marshall Marshall Clow Idio Software <mailto:mclow.lists@gmail.com> A.D. 1517: Martin Luther nails his 95 Theses to the church door and is promptly moderated down to (-1, Flamebait). -- Yu Suzuki