Re: [Boost-users] Two questions on boost::function

Hi, what you write looks great. You are definitely sending me back to the drawing board here. I am going to need some time out in order to study your example and tie this up with the doc. I will write again later. What I have in mind is pretty much what I wrote. That example is a typical application. When I wrote this, what I had in mind was defining an objective function for use in an optimization problem. So there h and g would be constant functions of a vector. Thanks for your help, ChrisK

Date: Sun, 29 Nov 2009 19:14:52 -0700 From: overminddl1@gmail.com To: boost-users@lists.boost.org Subject: Re: [Boost-users] Two questions on boost::function

On Sun, Nov 29, 2009 at 10:27 AM, CR Koudella wrote:

...

Thanks for your reply on this, too. Yes, I am beginning to look into phoenix. Here too, it would be of great help if you could indicate to me how you would handle my example with phoenix.

Since you have code in this one I should be able to come up with a phoenix example:

// Assume this line everywhere: using namespace boost::phoenix;

// For doing h*(f-g)^2 generically in phoenix: auto func1 = _3*(_1-_2)*(_1-_2); // Or more 'optimized' as this (which the compiler may optimize the above just fine as well, but this demonstrates local vars): auto func1 = let(_a=(_1-_2))[_3*_a*_a];

// Results in this: func1(3,2,1) -> 1 func1(6,4,2) -> 8 ... etc

That barely demonstrates the surface of phoenix. Phoenix practically rewrites C++ in C++ in a lazy form, very powerful, but that should give you an example. As for your nasty binds in binds in binds in etc... Maybe this?

// Supposing f, g, and h are static functions and there is only one variable passed in (phoenix cares not what type it is, as long as f/g/h can resolve it), you were not really clear on exactly what your code is supposed to do. auto bfoWRF = let(_a=(bind(&f, _1)-bind(&g, _1)))[bind(&h, _1)*_a*_a]

// In which case if you have this: std::vector v = {...} // and you call this: auto value = bfoWRF(v); // then it does the same thing as this non-lazy code: auto a = f(v)-g(v); auto value = h(v)*a*a;

And yes, you can combine different returns just fine as long as they obey the normal non-lazy C++ rules.

P.S. If you want to assign bfoWRF to a boost function, it will be as you may think: boost::function bfoWRF = let(_a=(bind(&f, _1)-bind(&g, _1)))[bind(&h, _1)*_a*_a]

However, if you leave it as auto (or use BOOST_AUTO for old compiler support) then it will work on any arbitrary type that would work in the non-lazy version of the code without requiring that everything be doubles.

If you clarify your example some more then I can re-demonstrate as well. _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users