initialize stl valarray in one row - c++

Question

Hello I wannted to build a helper class to initialize a stl valarray. What I would like is to do the following:
std::valarray<float> vec(3);
vlist_of<float>(vec)(2)(3)(5);
So I can just initialize the vectors at runtime using just one row command statement.
For doing the following I have tryed the following structure:
template <typename T>
struct vlist_of {
std::valarray<T>& data;
int i;
vlist_of(std::valarray<T>& _data):data(_data),i(0) {
(*this)(data);
}
vlist_of& operator()(std::valarray<T>& data){return *this;}
vlist_of& operator()(const T& t) {
data [i]=t;
i++;
return *this;
}
};
this structure works if I do the following :
vlist_of<float> tmp(vec);tmp(2)(3)(4);
Is it possible what I am asking ?

Answer 1

Yes. Make vlist_of a factory function:
template <typename T>
vlist_builder<T> vlist_of(std::valarray<T>& data)
{
return vlist_builder<T>(data);
}
Now it works http://liveworkspace.org/code/48aszl$0.
I'd personally prefer uniform initialization:
/*const*/ std::valarray<float> prefer { 2, 3, 5 };
See full sample:
#include <valarray>
#include <vector>
#include <iostream>
template <typename T>
struct vlist_builder
{
std::valarray<T>& data;
int i;
vlist_builder(std::valarray<T>& _data):data(_data),i(0) { }
vlist_builder& operator()(const T& t)
{
data[i++]=t;
return *this;
}
};
template <typename T>
vlist_builder<T> vlist_of(std::valarray<T>& data)
{
return vlist_builder<T>(data);
}
int main()
{
std::valarray<float> vec(3);
vlist_of<float>(vec)(2)(3)(5);
for(auto f : vec)
std::cout << f << "\n";
// prefer uniform initialization:
const std::valarray<float> prefer { 2, 3, 5 };
}