Re: [Boost-users] How to use boost::function with boost::phoenix?

3 May 2010

      Joel de Guzman wrote:
...
On 5/1/2010 5:18 PM, Jens Mueller wrote:
...
Hi,
I'm trying out boost:phoenix. Until now I used boost::lambda. I have code
written using boost::lambda that returns a boost::function. E.g.
boost::function  flog() {
     using namespace boost::lambda;
     return bind(static_cast(std::log), _1);
}
And with boost::lambda I can reuse the boost::function like this
boost::function  func = -bind(flog(), _1);
to build bigger blocks.
But I cannot make it work with boost::phoenix.
boost::function  func = -bind(flog(), arg1);
does not compile.
How do I use a boost::function within phoenix? Or what do I need to return
instead of a boost::function to reuse it in phoenix? Basically I want to
build more complex functions out of simple ones which are returned by some
function to hide the internals of the function.
I'm using Boost version 1.41.0 and g++ (Debian 4.3.2-1.1) 4.3.2.
It is a known deficiency with phoenix that it does not work well
with boost::function the way you are doing it, due to result_of
protocol incompatibility. This has been bugging me for some time
now. Let me think about an easy fix. In the meantime, I suggest
wrapping the boost function call in a generic phoenix::function.
Please see this thread:
http://tinyurl.com/232t7yk
Such a wrapper should work any boost function if you make it generic.
I see. Thanks. Can I make the wrapper more generic than:

template<typename func>
struct boostWrapper {
    template <typename T>
    struct result { typedef typename func::result_type type; };

    // TODO
    // can't I use an reference here
    func f1;

    boostWrapper(func const& f1)
        : f1(f1) {}

    typename func::result_type operator()(typename func::arg1_type i) const {
        return f1(i);
    }
};  

template<typename func>
boost::phoenix::function
wrap(func const& f1)
{
    return boostWrapper<func>(f1);
}

Specifically: How do I handle general n-ary functions? And is it safe to
just store a const reference for the boost function?

Jens