CODEWITHSUNDEEP
c
HAL9000
HAL9000

Reputation: 3751

Is it correct to cast a pointer to pointer to pointer to int to a pointer to pointer to pointer to double?

I have a function which deallocates a cubic matrix of double:

free_matrix(double ***U, int n) {
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            free(U[i][j]);
        }
        free(U[i]);
    }
    free(U);
}

Then I have a cubic matrix of integer int ***matrix;. Compiler gives me an error when I call free_matrix(matrix,n) but I don't want to duplicate the previous function which only deallocates elements. Is it correct/good/safe to simply cast it like this?

free_matrix((double ***)matrix,n);

Upvotes: 1

Views: 98

Answers (2)

Massimiliano
Massimiliano

Reputation: 8032

What you propose is in general not safe, as noted by @ecatmur. I would rather try to change the interface of your C function to something like the following (I assume 2D for simplicity):

int    ** imatrix_alloc(size_t nrows, size_t ncols);
double ** dmatrix_alloc(size_t nrows, size_t ncols);
void      matrix_free  (void * matrix);

and use it in client code like:

int    ** imat = imatrix_alloc(10,20);
double ** fmat = dmatrix_alloc(30,20);
...
matrix_free(imat);
matrix_free(fmat);

The tricky point in the allocation will be to pre-compute all the space you need and allocate it with a single call to malloc. Then you should initialized pointers appropriately. If you do this matrix_free can be implemented as a simple wrapper to free:

void matrix_free  (void * matrix) {
  free(matrix);
}

Upvotes: 0

ecatmur
ecatmur

Reputation: 157374

Unfortunately not; per 6.2.5p28:

[...] Pointers to other types need not have the same representation or alignment requirements.

So there is no guarantee that a int ** can be indexed the same way as a double **, because int * and double * might e.g. have different sizes.

Upvotes: 8

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