c++ - Double (two-dimensional) array using std::unique_ptr

Question

I have a double array allocated by pointer to pointer.

  // pointer to pointer
  int **x = new int *[5];   // allocation
  for (i=0; i<5; i++){
      x[i] = new int[2];
  }

  for (i=0; i<5; i++){      // assignment
      for (j=0; j<2; j++){
          x[i][j] = i+j;
      }
  }

  for (i=0; i<5; i++)   // deallocation
      delete x[i];
  delete x;

I am trying to do this using unique_ptr:

std::unique_ptr<std::unique_ptr<int>[]> a(new std::unique_ptr<int>[5]);
  for (i=0; i<5; i++)
      a[i] = new int[2];

but kept getting an error saying that no operator = matches these operands. What I am doing wrong here?

Answer 1

You cannot assign a int* to a std::unique_ptr<int[]>, that is the cause for your error. The correct code is

      a[i] = std::unique_ptr<int[]>(new int[2]);

However, piokuc is correct, that it is highly unusual to use unique_ptr for arrays, as that's what std::vector and std::array are for, depending on if the size is known ahead of time.

//make a 5x2 dynamic jagged array, 100% resizable any time
std::vector<std::vector<int>> container1(5, std::vector<int>(2)); 
//make a 5x2 dynamic rectangular array, can resize the 5 but not the 2
std::vector<std::array<int, 2>> container1(5); 
//make a 5x2 automatic array, can't resize the 2 or 5 but is _really fast_.
std::array<std::array<int, 2>, 5> container;

All of these can be initialized and used just the same as the code you already had, except they're easier to construct, and you don't have to destroy them.