c - Memory allocation for structure extension -

suppose have 2 structures 1 of extends other 1 (as below). what's correct procedure dealing memory when creating extended structure existing structure? should malloc called again new structure? doubling memory allocation shouldn't be?

typedef struct grid {   int rows;   int cols;   int * grid; } grid;  typedef struct supergrid {      grid;      char * property;     int id; } supergrid;  static grid * new_grid(int rows, int cols) {   grid * grid = (grid *)(malloc(sizeof(grid)));    grid->rows= rows;   grid->cols= cols;   grid->grid = (int *)(calloc(rows*cols, sizeof(int))); }  static supergrid * new_super_grid(int rows, int cols) {     /* needs happen here? */     return (supergrid *)new_grid(rows, cols);  } 

your structure extension mechanism similar primitive c++ class derivation system. achieve goal, need separate structure allocation , initialization:

typedef struct grid {     int rows;     int cols;     int *grid; } grid;  typedef struct supergrid {      grid;      char *property;     int id; } supergrid;  static grid *init_grid(grid *grid, int rows, int cols) {     if (grid) {         grid->rows = rows;         grid->cols = cols;         grid->grid = (int *)calloc(rows * cols, sizeof(int));     }     return grid; }  static grid *new_grid(int rows, int cols) {     return init_grid((grid *)malloc(sizeof(grid)), rows, cols); }  static supergrid *init_super_grid(supergrid *sg, int rows, int cols,                                   const char *property, int id) {     if (sg) {         init_grid((grid *)sg, rows, cols);         sg->property = strdup(property);         sg->id = id;     }     return sg; }  static supergrid *new_super_grid(supergrid *sg, int rows, int cols,                                  const char *property, int id) {     return init_super_grid((supergrid *)malloc(sizeof(supergrid)),                            rows, cols, property, id); } 

the allocators may fail , return null in case of memory allocation failure. may want them fail in more explicit ways such exiting program error message via abort. similarly, delete_xxx functions should gracefully ignore null pointers free , c++ delete operator.

for consistency, separate delete_grid , finalize_grid functions in order call finalize_grid finalize_super_grid. not strictly necessary simple example, may prove handy if instantiate grid structures not allocated (with static or automatic storage or embedded in more complex superstructures).

this poor man's c++ class system show limitations. may consider using c++ directly.