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stackoverflow-39080189-use-stdvector-for-dynamically-allocated-2d-array.cpp

#include<vector>
#include<iostream>

template<class T>
struct Matrix {
  Matrix(std::size_t n=1, std::size_t m=1)
    : n{n}, m{m}, data(n*m)
  {}
  
  Matrix(std::size_t n, std::size_t m, std::vector<T> const& data)
    : n{n}, m{m}, data{data}
  {}
  
  T const&  operator()(size_t i, size_t j) const {
    return data[i*m + j];
  }
  
  T& operator()(size_t i, size_t j) {
    return data[i*m + j];
  }

  size_t n;
  size_t m;
  std::vector<T> data;
  using ScalarType = T;
};