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

6 Jun 2008


      On Fri, Jun 6, 2008 at 4:51 PM, Eric Niebler <eric@boost-consulting.com> wrote:
...
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.
Haha! Thanks.
...
...
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);
}
Very good! Thanks. I hate "pure" functions. ;-)

Daniel Walker