How to initialize a multi-dimensional array in a C++ constructor

Question

I have a class that contains a few multi-dimensional arrays. I am trying to initialize these arrays in the constructor, but I am having trouble figuring our how to do it. The array is always of a fixed size. Here's what I have so far:

class foo {
  private: 
    int* matrix; //a 10x10 array

  public:
    foo();

  foo:foo() {
    matrix = new int[10][10]; //throws error
  }

the error I get is:

cannot convert `int (*)[10]' to `int*' in assignment

how can I accomplish this? preferably, I would like the array to default to a 10x10 array of all 0s.

Demian Brecht · Accepted Answer · 2011-04-30 05:13:15Z

4

#include <memory.h>
class foo
{
    public:
        foo()
        {
            memset(&matrix, 100*sizeof(int), 0);
        }
    private:
        int matrix[10][10];
};

That is, if you're not binding yourself to doing it with pointers (otherwise you can just pass in the pointer to memset, rather than a reference to the array).

answered Apr 30, 2011 at 5:13

Demian Brecht

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1 Comment

Potatoswatter Over a year ago

or memset(matrix, sizeof matrix / sizeof **matrix, 0); or std::fill_n( &matrix[0][0], sizeof matrix / sizeof **matrix, 0 );.

Sarfaraz Nawaz · Accepted Answer · 2011-04-30 05:09:05Z

Do this:

int **matrix; //note two '**'

//allocation
matrix = new int*[row]; //in your case, row = 10. also note single '*'
for(int i = 0 ; i < row ; ++i)
   matrix[i] = new int[col]; //in your case, col = 10


 //deallocation
 for(int i = 0 ; i < row ; ++i)
   delete [] matrix[i];
 delete matrix;

Advice: instead of using int**, you can using std::vector as:

 std::vector<std::vector<int> > matrix;

//then in the constructor initialization list
foo() : matrix(10, std::vector<int>(10))
{  // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this is called initialization list
}

If you follow this approach, you don't need to use new and delete in your code. Also, the size of matrix is 10x10; you can access them as matrix[i][j] where 0<=i<10 and 0<=j<10; also note that all the elements in the matrix is initialized with 0.

andand · Accepted Answer · 2011-04-30 05:04:17Z

0

Try this:

class foo
{
private:
  int **matrix;

public:
  foo()
  {
    matrix = new int*[10];
    for (size_t i=0; i<10; ++i) 
      matrix[i] = new int[10];
  }

  virtual ~foo()
  {
    for (size_t i=0; i<10; ++i)
      delete[] matrix[i];
    delete[] matrix;
  }
};

answered Apr 30, 2011 at 5:04

andand

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Comments

xDD · Accepted Answer · 2011-04-30 05:11:50Z

0

Until your compiler supports C++0x uniform intialization, I'm afraid you'll have to initialize each entry in the array separately if you want to do so in the initialization list.

What you can do however, is to not initialize but assign to the array inside your constructor (simple for loop).

In your code you have a pointer, not an array. It looks like you may want to use std::vector if you need a collection of elements that takes care of memory management for you.

edited Apr 30, 2011 at 5:11

answered Apr 30, 2011 at 5:05

xDD

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