id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
8704	Using Phoenix lambdas on top of custom Proto expressions	ddemidov@…	Thomas Heller	"Some prehistory:

I am working on VexCL (https ://github.com/ddemidov/vexcl) - a vector expression template library for OpenCL. I have recently added support for automatic generation of OpenCL kernels from generic functors (description of the feature: https ://github.com/ddemidov/vexcl#function-generator). 

In order to implement this, I pass the instances of vex::symbolic<T> class to the given functor. vex::symbolic<T> dumps to output stream any arithmetic operations it is being subjected to, and that's how I am able to construct the OpenCL kernel source. The symbolic<T> class is of course a Boost.Proto terminal.



Now, it seems natural to use Boost.Phoenix to provide the generic functors for the function generator. And indeed it is possible with simple expressions (see https ://github.com/ddemidov/vexcl/blob/a95dfdd68/tests/generator.cpp#L124). However, if I try to bring cmath functions overloads from <boost/phoenix/stl/cmath.hpp> and use those (https ://github.com/ddemidov/vexcl/commit/b95da14e51), I get compilation errors. The reason apparently is that boost::phoenix_impl::<function>_impl assumes that result of <function>(expr) has the same type as expr itself. This of course is not true if expr is a Boost.Proto expression.

Everything works as intended with the following patch:

{{{
--- a/cmath.hpp
+++ b/cmath.hpp
@@ -25,11 +25,11 @@ namespace boost {
         struct result<This(BOOST_PHOENIX_A(n))>                         \
         {                                                               \
             typedef                                                     \
-                typename proto::detail::uncvref<A0>::type               \
+                decltype( name(typename proto::detail::uncvref<A0>::type()) )               \
                 type;                                                   \
         };                                                              \
         template<BOOST_PHOENIX_typename_A(n)>                           \
-        A0 operator()(BOOST_PHOENIX_A_const_ref_a(n)) const {           \
+        auto operator()(BOOST_PHOENIX_A_const_ref_a(n)) const -> decltype( name(BOOST_PHOENIX_a(n)) ) {           \
             using namespace std;                                        \
             return name(BOOST_PHOENIX_a(n));                            \
         }                                                               \
}}}
I understand this is unacceptable in its current form (because it uses c++11 features), but I could not come up with other solution (e.g. in terms of boost::result_of).

Sorry for the broken links, Trac did not allow me to submit otherwise.
Best regards,
Denis"	Patches	closed	To Be Determined	phoenix	Boost 1.53.0	Problem	fixed	phoenix proto