On Tue, Jun 3, 2008 at 5:40 AM, Marco Costalba <mcostalba@gmail.com> wrote:
Hi all,
following code fails to compile:
#include <boost/fusion/functional/invocation/invoke.hpp> #include <boost/fusion/include/make_vector.hpp>
namespace fusion = boost::fusion;
struct foo_t { int operator()() { return 1; } int operator()(int) { return 2; }
template<typename>struct result { typedef int type; }; };
int main(int, char**) { foo_t foo;
assert( fusion::invoke(foo, fusion::make_vector(3)) == 2 );
assert( fusion::invoke(foo, fusion::make_vector()) == 1 ); // COMPILE ERROR HERE ! }
Boost is released 1.35, compiler gcc 4.2
This is actual a problem with the legacy/pre-C++0x result_of heuristic. If you use boost trunk and gcc 4.3 with -std=c++0x, your example will compile without error due to the new decltype-based result_of. For C++98, the only solution is to specialize result_of for nullary calls of your functor. namespace boost { template<> struct result_of<foo_t()> { typedef int type; }; } The reason result_of couldn't deduce argument dependent return types for nullary invocations in C++98 is that according to the heuristic only the result_type member is checked for nullary calls, and for non-nullary calls if the result_type is present, result<> is never checked. So, you can't have both. Daniel Walker