On Wed, Aug 13, 2008 at 1:51 PM, David Abrahams <dave@boostpro.com> wrote:
but after deciding not to use the library because of the first reason I listed, I didn't really feel motivated to look into it further.
Finally, I was concerned about the impact on compile time, which I've seen mentioned on the list a few times. Whether what I have now is any faster is yet to be tested.
I'd be interested in hearing the results.
OK I felt motivated to get some results :-) I tested on the following use case: class named_constructableN { public: // a macro could extend the number of parameters if desired template<typename FM0, typename FM1, typename FM2> named_constructableN(const FM0 &a0, const FM1 &a1, const FM2 &a2) { label = (a0, a1, a2)[labelN]; size = (a0, a1, a2)[sizeN]; volume = (a0, a1, a2)[volumeN]; } std::string label; int size; double volume; }; //... named_constructableN constructed(labelN = "hello", sizeN = 1, volumeN = 1.0); BOOST_CHECK_EQUAL(constructed.label, "hello"); BOOST_CHECK_EQUAL(constructed.size, 1); BOOST_CHECK_EQUAL(constructed.volume, 1.0); I put together three versions of the test - one using the fusion based solution (I'm calling it field_map), one using the same use code but using Boost.Parameter, and then one using the recommended Boost.Parameter method for constructors. I then varied the number of "replicas" of the above code for the test. The following times were reported for 2 replicas in each test case: user: [test_field_map_compilation.o] 0.000249 system: [test_field_map_compilation.o] 0.000028 user: [test_parameter_compilation.o] 0.000102 system: [test_parameter_compilation.o] 0.000013 user: [test_parameter_dispatch_compilation.o] 0.000135 system: [test_parameter_dispatch_compilation.o] 0.000015 And the following for 20 replicas: user: [test_field_map_compilation.o] 0.002128 system: [test_field_map_compilation.o] 0.000139 user: [test_parameter_compilation.o] 0.000195 system: [test_parameter_compilation.o] 0.000030 user: [test_parameter_dispatch_compilation.o] 0.000555 system: [test_parameter_dispatch_compilation.o] 0.000041 I committed the tests to the sandbox, if you're interested in the details (I'm not sure whether I designed the tests adequately): http://svn.boost.org/svn/boost/sandbox/guigl/libs/guigl/test/ So, to revisit my speculations, these results would indicate that the fusion-based approach (as I have it implemented, at least) is significantly slower (in terms of compile-time) than using Boost.Parameter with the same syntax. Since it turned out that Boost.Parameter supports that same syntax, the fusion-based approach also isn't less convoluted, as I suggested it might be. Of course, perhaps I am again being hasty :-) Kind regards, Stjepan