Why structure size is incorrect

Question

I just tried to check how memory is allocated to structure objects, its taking more space what i expected. I am using 64 bit windows OS and Microsoft visual studio 2010(i think its 32 bit), so can some explain why is its printing 52 bytes ?

struct test {
  int year;// should take 4 byte
  string title;// how much bytes would take ? in my case taking 31 bytes ?
  double date;//should take 8 byte
  int month;// should take 4 byte
  } mine;

int main ()
{
 cout << " size is: "<<sizeof(mine);//printing 52 ?
 cout << " size is: "<<sizeof(struct test);//printing 52 ?
  return 0;
}

Is that `string` = `std::string` or what? If it's a custom class / struct, you should post a trimmed-down version of the applicable class (it needn't contain more than the variables - no functions required). — Bernhard Barker, Mar 06 '14 at 11:17
@MarounMaroun Why compiler adds padding, i couldnt understand why compiler find it necessary to add padding bytes ? — User, Mar 06 '14 at 11:18
http://stackoverflow.com/questions/11770451/what-is-the-meaning-of-attribute-packed-aligned4/11772340#11772340 — Jeyaram, Mar 06 '14 at 11:19
Your question is missing a "I expect it to print X", although it could probably be inferred that X = 4 + 31 + 8 + 4 = 47. — Bernhard Barker, Mar 06 '14 at 11:21

score 1 · Accepted Answer · answered Mar 06 '14 at 11:18

1

Note that

sizeof(struct) >= sizeof(its members)

Because each member might be aligned to the lowest address after the previous member that satisfies:

mod(address/sizeof(member)) == 0

For example, consider this struct:

struct s {
   char c;
   int i[2];
   double d;
}

The memory might look like that:

+-------------------------------------------------+
| c |  |  |  | i[0] | i[1] |  |  |  |  | .. v ..  |
+-------------------------------------------------+
     ^  ^  ^                ^  ^  ^  ^

answered Mar 06 '14 at 11:18

Maroun

94,125
30
188
241

1

Does it really have to be aligned at its size? I believe in general, alignment requirements could be different. – Angew is no longer proud of SO Mar 06 '14 at 11:21
@Angew I think you're right.. I'll look into that and will edit my answer, thank you. – Maroun Mar 06 '14 at 11:24
@Angew Obviously not. `struct X { char x[1000000]; }` does not require an alignment of a million (or more). The alignment cannot be greater than the size, however. – James Kanze Mar 06 '14 at 11:51

Why structure size is incorrect

1 Answers1