Question

I have a matrix declared like int **matrix, and I know that the proper way to pass it to a function to allocate memory should be like this:

void AllocMat(int ***mat, int size);

But now I need to delete these memory in another function and am not sure about what to pass:

void DeallocMat(int **mat, int size);

or

void DeallocMat(int ***mat, int size);

I think the second one should be right, but neither way gives me segmentation fault as I tried.

Answer 1

The question is tagged C++, and yet the answers only use the C subset…

Well, first of all, I would recommend against the whole thing. Create a class that encapsulates your matrix and allocate it in a single block, offer operator()(int,int) to gain access to the elements…

But back to the problem. In C++ you should use references rather than pointers to allow the function to change the argument, so your original allocate signature should be:

void AllocMat(int **&mat, int size);

And call it like:

int **matrix = 0;
AllocMat( matrix, 5 );

Or better, just return the pointer:

int **AllocMat( int size );
int **matrix = AllocMat( 5 );

For the deallocation function, since you don’t need to modify the outer pointer, you can just use:

void DeallocMat( int**mat, int size ); // size might be required to release the 
                                       // internal pointers

Now, for a sketch of the C++ solution:

template <typename T>                   // no need to limit this to int
class square_matrix {
   const unsigned size;
   T * data;
public:
   square_matrix( unsigned size ) : size(size), data( new T[size*size]() ) {}
   square_matrix( matrix const & m ) : size( m.size ), data( new T[m.size*m.size] ) {
      std::copy( m.data, m.data+size*size, data );
   }
   ~matrix() {
      delete [] data;
   }
   T const & operator()( unsigned x, unsigned y ) const {
      // optional range check and throw exception
      return data[ x + y*size ];
   }
   void set( unsigned x, unsigned y, T const & value ) {
      // optional range check and throw exception
      data[ x + y*size ] = value;
   }
};