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