allocation with reserve

#include <iostream>
    #include <vector>
    #include <numeric>

    int main() {
        constexpr int size = 1024*1024;
        
        std::vector<double> x;
        x.reserve(size);
        
        #pragma omp parallel for
        for(int i = 0; i<size; ++i)
            x[i] = 0.0;
        
        x.resize(size);

        double sum = 0.0;
        for(int i = 0; i<size; ++i)
            sum += x[i];
        
        if(sum > 1.0)
            std::cout << sum << std::endl;
        else
            std::cout << "sum is zero!!!!!" << std::endl;
    }