Using std::copy to cast between container types with c++11.

gistfile1.cpp

#include <algorithm>
#include <iostream>
#include <valarray>
#include <vector>

template<typename T>
std::ostream& operator<< (std::ostream& os, std::valarray<T> va)
{
    for (auto&& i : va)
        os << i << " ";
    return os;
}

int main(int argc, char const *argv[])
{
    std::initializer_list<double> a{1, 2, 3};
    std::valarray<int>    b(a.size());
    std::copy(begin(a), end(a), begin(b));
    // std::cout << a << std::endl;
    std::cout << b << std::endl;

    // The only thing you really need to worry about is the 
    // target contaniner to have enough space to store all the 
    // values, for example, this will result in a segmentation fault
    
    //     std::vector<int> v{2, 3, 4};
    //     std::valarray<double> c();
    //     std::copy(begin(v), end(v), begin(c));

    // because the std::valarray has no allocated space.

    std::vector<int> v{2, 3, 4};
    std::valarray<double> c(v.size());
    std::copy(begin(v), end(v), begin(c));

    std::cout << c << std::endl;

    return 0;
}