Re: [boost] [local] Help for the Alternatives section

Subject: Re: [boost] [local] Help for the Alternatives section
From: lcaminiti (lorcaminiti_at_[hidden])
Date: 2011-05-04 14:43:48

Mathias Gaunard-2 wrote:
> 
> On 03/05/2011 21:44, lcaminiti wrote:
> 
>> For example, it would be useful to program a polymorphic global functor
>> to
>> add a vector of doubles and ints using the same global_add algorithm
>> (without duplicating the code to implement two different algorithms):
> 
> Or for more advanced things.
> Consider a generic algorithm such as fold (accumulate). Now consider 
> another generic algorithm (transform, for_each, whatever) defined in 
> terms of fold.
> It needs to pass a polymorphic function object to fold in order to be 
> generic.
> 

I see.

>> template<typename T>
>> struct global_add {
>>      global_add(double&  _sum, const int&  _factor): sum(_sum),
>> factor(_factor)
>> {}
>>      inline void operator()(T num) {
>>          sum += factor * num;
>>      }
>> private:
>>      double&  sum;
>>      const int&  factor;
>> };
> 
> I'd put the template<typename T> on the operator() to avoid having to 
> specify it.
> 

Yes, I did think of it but only after I sent the email :)

>> int main() {
>>      double sum = 0.0;
>>      int factor = 10;
>>
>>      __some_local_template_type__ local_add = let(_f = cref(factor))[
>>          ref(sum) += _f * _1
>>      ];
>>
>>      std::vector<double>  v(3);
>>      v[0] = 1.0; v[1] = 2.0; v[2] = 3.0;
>>      std::for_each(v.begin(), v.end(), local_add<double>(sum, factor));
>>
>>      std::vector<int>  w(3);
>>      w[0] = -1; w[1] = -2; w[2] = -3;
>>      std::for_each(w.begin(), w.end(), local_add<int>(sum, factor));
>>
>>      std::cout<<  sum<<  std::endl;
>>      return 0;
>> }
>>
> 
>> Is there a way to do this or something similar with Boost.Phoenix (or
>> Boost.Lambda)?
> 
> std::vector<double> v(3);
> std::for_each(v.begin(), v.end(), ref(sum) += factor * _1);
> 
> std::vector<int> w(3);
> std::for_each(w.begin(), w.end(), ref(sum) += factor * _1);
> 
> or if you want to avoid repetition,
> 
> auto local_add = ref(sum) += factor * _1;
> 
> std::vector<double> v(3);
> std::for_each(v.begin(), v.end(), local_add);
> 
> std::vector<int> w(3);
> std::for_each(w.begin(), w.end(), local_add);
> 
> Note you need auto, since using type erasure would force the function 
> object to be monomorphic.
> 

Can I do this without auto? Otherwise, the local Phoenix function retains
its polymorphic behavior only on C++0x (but not on ISO C++) when it is given
a name by assigning it to a local variable like local_add.

Thanks a lot again.
--Lorenzo

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