Subject: Re: [boost] [local] Help for the Alternatives section
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2011-05-04 13:31:43
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.
> 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.
> 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.