Styjun

My task is to generate a square matrix of zeros in a function and return it. There are plenty ways to do this, but I decided not to go with returning the matrix by value for efficiency. I went for a pointer approach like in this answer, but since it requires manual cleaning memory (and also as far as I know it's better to use smart pointers), I decided to turn it into std::unique_ptr, but I can't get it to work. This is my code:

#include <iostream>
#include <memory>

std::unique_ptr<std::unique_ptr<int>[] > GenerateMatrix(const int &n) 
 std::unique_ptr<std::unique_ptr<int>[] > matrix(new std::unique_ptr<int>[n]);
 for (int i = 0; i < n; i++) 
 for (int j = 0; j < n; j++) 
 matrix[i].get()[j] = 0;
 
 

 return matrix;


int main() 
 int n = 4;
 auto matrix = GenerateMatrix(n);
 for (int i = 0; i < n; i++) 
 for (int j = 0; j < n; j++) 
 std::cout<<matrix[j].get()[i]<<" ";
 
 std::cout<<std::endl;
 

 return 0;

What do I do wrong here? Is this approach even correct?

Why not just make your life easier by

vector<vector<int>> generate (int m, int n)

 return vector<vector<int>>(m ,vector<int>(n));


int main()

 int m = 3, n = 4;
 auto matrix = generate(m, n); // a 3-by-4 matrix of zeros
 return 0;

Just rely on guarenteed copy elision or return value optimization:

std::vector<int> GenerateMatrix(const int &n) 
 return std::vector<int>(n*n, 0);//, 0 can be omitted (as elements will then be zero-initialized)

You might create and initialize a matrix at compile time. For example:

template<int RowCount, int ColumnCount, int DefaultValue = 0>
struct Matrix

 static_assert(RowCount >= 0 && ColumnCount >=0,
 "The number of rows and columns should be positive");
 struct Row
 
 int column[ColumnCount] = DefaultValue ;
 ;
 Row row[RowCount];
;

And use it like:

Matrix<2, 2, 33> matrix;
auto val = matrix.row[0].column[0]; // val == 33
matrix.row[0].column[0] = 55;
val = matrix.row[0].column[0]; // val == 55

Beware the matrix dimensions, when refer to its elements by row and column.

You are not allocating enough memory for your matrix. Change this line:

std::unique_ptr<std::unique_ptr<int>[] > matrix(new std::unique_ptr<int>[n*n]);

Also, I would just use i*n + j for your accesses since you are really dealing with a 1D array:

#include <iostream>
#include <memory>

std::unique_ptr<std::unique_ptr<int>[] > GenerateMatrix(const int &n) 
 std::unique_ptr<std::unique_ptr<int>[] > matrix(new std::unique_ptr<int>[n*n]);
 for (int i = 0; i < n; i++) 
 for (int j = 0; j < n; j++) 
 matrix.get()[i*n+j] = 0;
 
 

 return matrix;


int main() 
 int n = 4;
 auto matrix = GenerateMatrix(n);
 for (int i = 0; i < n; i++) 
 for (int j = 0; j < n; j++) 
 std::cout<<matrix.get()[i*n+j]<<" ";
 
 std::cout<<std::endl;
 

 return 0;