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

From: Daniel Walker (daniel.j.walker_at_[hidden])
Date: 2008-06-06 16:58:56

On Fri, Jun 6, 2008 at 4:51 PM, Eric Niebler <eric_at_[hidden]> 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