#include <iostream>
#include <vector>
#include <numeric>
#include <algorithm>
 
class Compare
{
public:
    Compare (const std::vector<double> &v) : m_v (v) {}
    bool operator () (int i, int j) const { return m_v [i] < m_v [j]; }
 
private:
    const std::vector<double> &m_v;
};
 
std::vector<int> order_ideal(const std::vector<double>& x) {
    std::vector<int> idx(x.size());
    std::iota(idx.begin(), idx.end(), 0);
    Compare c (x);
    std::sort(idx.begin(), idx.end(), c);
    return idx;
}
 
int main ()
{
    std::vector <double> x (20);
    order_ideal (x);
}

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