Re: [boost] A few questions about Phoenix, Boost.Bind, and FC++

6 Jun 2008


      Daniel Walker wrote:
...
OK, but I was thinking more of user defined function objects. Something like ...
struct f_ {
    template<class> struct result {
        typedef void type;
    };
    template<class T>
    void operator()(T& t) { ++t; }
};
int main()
{
    phoenix::function<f_> f;
    int i = 0;
    f(i);
That creates a lazy function invocation, but doesn't execute it.
...
assert(i == 1);
}
Actually, I just tried this and the assertion failed. So, I assume
user defined function objects are required to be "pure." Why is that?
#include <boost/phoenix/core.hpp>
#include <boost/phoenix/operator.hpp>
#include <boost/phoenix/function.hpp>

namespace phoenix = boost::phoenix;

struct f_
{
     template<class> struct result {
         typedef void type;
     };
     template<class T>
     void operator()(T& t) { ++t; }
};

int main()
{
     using phoenix::arg_names::_1;
     phoenix::function<f_> f;

     int i = 0;
     f(_1)(i); // OK
     assert(i == 1);

     int j = 0;
     //f(j)(); // by value, doesn't compile
     //assert(j == 0);

     f(phoenix::ref(j))(); // by ref, OK
     assert(j == 1);
}

HTH,

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com