Memory allocation of struct in C++ - struct

Question

My struct is as follows:
typedef struct KeypointSt {
float row, col;
float scale, ori;
unsigned char *descrip; /* Vector of descriptor values */
struct KeypointSt *next;
} *Keypoint;
The following is a part of a code in C. How can I translate it to C++, considering allocation and de-allocation of heap.
Keypoint k, keys = NULL;
for (i = 0; i < num; i++) {
/* Allocate memory for the keypoint. */
k = (Keypoint) malloc(sizeof(struct KeypointSt));
k->next = keys;
keys = k;
k->descrip = malloc(len);
for (j = 0; j < len; j++) {
k->descrip[j] = (unsigned char) val;
}
}

Answer 1

One possible way of converting to C++ is:
#include <cstring> // memset()
typedef struct KeypointSt
{
float row, col;
float scale, ori;
size_t len;
unsigned char *descrip; /* Vector of descriptor values */
KeypointSt *next;
KeypointSt(int p_len, int p_val) : row(0.0), col(0.0), scale(0.0),
ori(0.0), len(p_len),
descrip(new unsigned char[len]), next(0)
{ memset(descrip, len, p_val); }
~KeypointSt() { delete descrip; }
} *Keypoint;
extern KeypointSt *init_keypoints(size_t num, size_t len, unsigned char val);
extern void free_keypoints(KeypointSt *list);
KeypointSt *init_keypoints(size_t num, size_t len, unsigned char val)
{
KeypointSt *keys = NULL;
for (size_t i = 0; i < num; i++)
{
/* Allocate memory for the keypoint. */
KeypointSt *k = new KeypointSt(len, val);
k->next = keys;
keys = k;
}
return keys;
}
void free_keypoints(KeypointSt *list)
{
while (list != 0)
{
KeypointSt *next = list->next;
delete list;
list = next;
}
}
int main(void)
{
KeypointSt *keys = init_keypoints(4, 5, 6);
free_keypoints(keys);
return 0;
}
The only reason I've kept the typedef in place is because you have existing code; the C++ code would be better using KeypointSt * everywhere — or renaming the structure tag to Keypoint and using Keypoint * in place of your original Keypoint. I don't like non-opaque types where the typedef conceals a pointer. If I see a declaration XYZ xyz;, and it is a structure or class type, I expect to use xyz.pqr and not xyz->pqr.
We can debate code layout of the constructor code, the absence of a default constructor (no arrays), and the absence of a copy constructor and an assignment operator (both needed because of the allocation for descrip). The code of init_keypoints() is not exception safe; a memory allocation failure will leak memory. Fixing that is left as an exercise (it isn't very hard, I think, but I don't claim exception-handling expertise). I've not attempted to consider any extra requirements imposed by C++11. Simply translating from C to C++ is 'easy' until you look at the extra demands that C++ makes — demands that make your life easier in the long run, but at a short-term cost in pain.