Jeremiah Willcock wrote:
On 1/25/2012 6:42 PM, Jeremiah Willcock wrote:
At least on GCC 4.6 in C++0x (C++11) mode, BOOST_RESULT_OF_USE_DECLTYPE is not set by default, preventing classes such as boost::transform_iterator from working on built-in lambda expressions. <snip> So far I'm using a -D on the compile line to turn it on;
BOOST_RESULT_OF_USE_DECLTYPE might break some codes of Boost, so how about wrapping a function object without tr1 result_of support into a tr1 function object (i.e. a function object with tr1 result_of support)? This approach enables us to use transform_iterator with C++11 lambda without defining BOOST_RESULT_OF_USE_DECLTYPE. I just wrote a code which wraps into a tr1 function object. Additionally, I made wrapped functors to be DefaultConstructible and CopyAssignable. (Since C++11 lambda is not DefaultConstructible nor CopyAssignable, transform_iterator with it is not a "decent" iterator. Like oven::regular, the wrapper solves this problem.) Here is a code: (Complete code is attached in this mail) template <typename Func> class tr1_functor { public: // tr1 result_of support template <typename Sig> struct result; template <typename This, typename... Args> struct result<This(Args...)> { typedef decltype(boost::declval<This>()(boost::declval<Args>()...)) type; }; // construction and assignment /* … */ // invoke // precondition: f_ is initialized template <typename... Args> typename result<Func(Args&&...)>::type operator()(Args&&... args) { return (*f_)(static_cast<Args&&>(args)...); } // const version of operator() /* … */ private: // data boost::optional<Func> f_; }; template <typename Func> tr1_functor<Func> make_tr1_functor(Func const& f) { return tr1_functor<Func>(f); } /* Usage */ int main (int argc, char* argv[]) { using boost::adaptors::transformed; BOOST_FOREACH(int i , boost::irange(0,5) | transformed(make_tr1_functor([](int i){ return i * 100; })) ) { std::cout << i << " "; } // Output: "0 100 200 300 400" return 0; } Regards, Michel