c++ - is there a way to pass nested initializer lists in C++11 to construct a 2D matrix?

Question

Imagine you have a simple matrix class

template <typename T = double>
class Matrix {

  T* data;
  size_t row, col;

public:

  Matrix(size_t m, size_t n) : row(m), col(n), data(new T[m*n]) {}
  //...       

  friend std::ostream& operator<<(std::ostream& os, const Matrix& m) {
    for (int i=0; i<m.row; ++i) {
      for (int j=0; j<m.col; ++j)
        os<<" "<<m.data[i + j*m.row];
      os<<endl;
    }
    return os;
  }
};      

Is there a way that I can initialize this matrix with an initializer list? I mean to obtain the sizes of the matrix and the elements from an initializer list. Something like the following code:

Matrix m = { {1., 3., 4.}, {2., 6, 2.}};

would print

 1 3 4
 2 6 2

Looking forward to your answers. Thank you all. aa

EDIT

So I worked on your suggestions to craft a somewhat generic array that initializes elements using initializer lists. But this is the most generic I could obtain. I would appreciate if any of you have any suggestions as to make it a more generic class. Also, a couple of questions:

• Is it fine that a derived class initializes the state of the base class? I'm not calling the base constructor because of this, but should I call it anyways?
• I defined the destructor a the Generic_base class as protected, is this the right way to do it?
• Is there any foreseeable way to carry out the code that belongs to the constructor of the initializer in a more generic way? I mean to have one general constructor that takes care of all cases?

I included just the necessary code to illustrate the use of initializer lists in construction. When going to higher dimensions it gets messy, but I did one just to check the code.

#include <iostream>
#include <cassert>

using std::cout;
using std::endl;


template <int d, typename T>
class Generic_base {

protected:

  typedef T value_type;

  Generic_base() : n_(), data_(nullptr){}

  size_t n_[d] = {0};
  value_type* data_;
};



template <int d, typename T>
class Generic_traits;


template <typename T>
class Generic_traits<1,T> : public Generic_base<1,T> {

protected:

  typedef T value_type;
  typedef Generic_base<1,T> base_type;
  typedef std::initializer_list<T> initializer_type;

  using base_type::n_;
  using base_type::data_;


public:

  Generic_traits(initializer_type l) {

    assert(l.size() > 0);
    n_[0] = l.size();
    data_ = new T[n_[0]];

    int i = 0;
    for (const auto& v : l)
      data_[i++] = v;
  }
};


template <typename T>
class Generic_traits<2,T> : public Generic_base<2,T> {

protected:

  typedef T value_type;
  typedef Generic_base<2,T> base_type;

  typedef std::initializer_list<T> list_type;
  typedef std::initializer_list<list_type> initializer_type;

  using base_type::n_;
  using base_type::data_;

public:

  Generic_traits(initializer_type l) {

    assert(l.size() > 0);
    n_[0] = l.size();
    n_[1] = l.begin()->size();

    data_ = new T[n_[0]*n_[1]];

    int i = 0, j = 0;
    for (const auto& r : l) {

      assert(r.size() == n_[1]);
      for (const auto& v : r) {
        data_[i + j*n_[0]] = v;
        ++j;
      }
      j = 0;
      ++i;
    }
  }
};


template <typename T>
class Generic_traits<4,T> : public Generic_base<4,T> {

protected:

  typedef T value_type;
  typedef Generic_base<4,T> base_type;

  typedef std::initializer_list<T> list_type;
  typedef std::initializer_list<list_type> llist_type;
  typedef std::initializer_list<llist_type> lllist_type;
  typedef std::initializer_list<lllist_type> initializer_type;

  using base_type::n_;
  using base_type::data_;

public:

  Generic_traits(initializer_type l) {

    assert(l.size() > 0);
    assert(l.begin()->size() > 0);
    assert(l.begin()->begin()->size() > 0);
    assert(l.begin()->begin()->begin()->size() > 0);

    size_t m = n_[0] = l.size();
    size_t n = n_[1] = l.begin()->size();
    size_t o = n_[2] = l.begin()->begin()->size();
    n_[3] = l.begin()->begin()->begin()->size();

    data_ = new T[m*n*o*n_[3]];

    int i=0, j=0, k=0, p=0;
    for (const auto& u : l) {
      assert(u.size() == n_[1]);
      for (const auto& v : u) {
        assert(v.size() == n_[2]);
        for (const auto& x : v) {
          assert(x.size() == n_[3]);
          for (const auto& y : x) {
            data_[i + m*j + m*n*k + m*n*o*p] = y;
            ++p;
          }
          p = 0;
          ++k;
        }
        k = 0;
        ++j;
      }
      j = 0;
      ++i;
    }
  }
};



template <int d, typename T>
class Generic : public Generic_traits<d,T> {

public:

  typedef Generic_traits<d,T> traits_type;
  typedef typename traits_type::base_type base_type;

  using base_type::n_;
  using base_type::data_;

  typedef typename traits_type::initializer_type initializer_type;

  // initializer list constructor
  Generic(initializer_type l) : traits_type(l) {}

  size_t size() const {
    size_t n = 1;
    for (size_t i=0; i<d; ++i)
      n *= n_[i];
    return n;
  }

  friend std::ostream& operator<<(std::ostream& os, const Generic& a) {
    for (int i=0; i<a.size(); ++i)
      os<<" "<<a.data_[i];
    return os<<endl;
  }      
};


int main()
{

  // constructors for initializer lists

  Generic<1, double> y = { 1., 2., 3., 4.};
  cout<<"y -> "<<y<<endl;

  Generic<2, double> C = { {1., 2., 3.}, {4., 5., 6.} };
  cout<<"C -> "<<C<<endl;

  Generic<4, double> TT = { {{{1.}, {7.}, {13.}, {19}}, {{2}, {8}, {14}, {20}}, {{3}, {9}, {15}, {21}}}, {{{4.}, {10}, {16}, {22}}, {{5}, {11}, {17}, {23}}, {{6}, {12}, {18}, {24}}} };
  cout<<"TT -> "<<TT<<endl;

  return 0;
}

Which prints as expected:

y ->  1 2 3 4

C ->  1 4 2 5 3 6

TT ->  1 4 2 5 3 6 7 10 8 11 9 12 13 16 14 17 15 18 19 22 20 23 21 24

Answer 1

Why not?

  Matrix(std::initializer_list<std::initializer_list<T>> lst) :
  Matrix(lst.size(), lst.size() ? lst.begin()->size() : 0)
  {
     int i = 0, j = 0;
     for (const auto& l : lst)
     {
        for (const auto& v : l)
        {
           data[i + j * row] = v;
           ++j;
        }
        j = 0;
        ++i;
     }
  }

And as stardust_ suggests - you should use vectors, not arrays here.