Re: [Boost-users] [fusion] filter_if at run-time???

Subject: Re: [Boost-users] [fusion] filter_if at run-time???
From: Jeff Flinn (Jeffrey.Flinn_at_[hidden])
Date: 2013-03-12 15:20:46

On 3/12/2013 11:43 AM, Nathan Ridge wrote:
>
>> I've got a fusion set say:
>>
>> set<int, optional<double>, bool> s(...)
>>
>> I'd like to filter out empty optional<double> values when streaming out:
>>
>> std::cout << s;
>>
>> so for s(1, 2.0, true) ==> (1,2.0,true)
>>
>> and for s(3, optional<double>(), false) ==> (2,false)
>>
>> filter_if's predicate is an a Metafunction returning mpl::bool_ so it
>> looks to me like this won't support run time?
>>
>> Is there another facility that would achieve the above?
>
> Perhaps you can use a polymorphic function object like:
>
> struct output_if_not_none
> {
>      output_if_not_none(ostream& os) : os_(os) {}
>
>      ostream& os_;
>
>      template <typename T>
>      void operator()(const T& t)
>      {
>          os_ << t;
>      }
>
>      template <typename T>
>      void operator()(const optional<T>& t)
>      {
>          if (t) os_ << *t;
>      }
> };
>
> for_each(s, output_if_not_none(cout));
>
> It doesn't do the formatting that you desire but it might be a
> starting point.
>
> The problem with trying to do it with the syntax
>
> cout << some_function(s);
>
> is that the expression some_function(s) needs to have a type
> determined at compile time.

Yep that's what I'm running into. some_function will need to return a 
proxy that needs to grab the manipulator values tuple_open, *_close and 
*_delimiter and use the output_if_not_none function you show above to 
deal with leading, internal and trailing cases.

Thanks, Jeff