I have a struct like:

typedef struct {
    int row;
    int col;
    double *entries;
} Matrix;

I want to write functions to create, free, etc. For example:

Matrix *mtalloc(typeof((Matrix) {}.row) row, typeof((Matrix) {}.col) col)
{
    Matrix *p = malloc(sizeof(Matrix));

    if (p == NULL) {
        fprintf(stderr, "mtalloc(%x, %x): malloc returned NULLn", row, col);
        exit(EXIT_FAILURE);
    }

    p->row = row;
    p->col = col;
    p->entries = malloc(sizeof(*((Matrix) {}.entries)) * row * col);
    return p;
}

This way the function won’t have to be changed if the type of the fields in struct change but is it fine or bad practice?