Structure Padding:
Whenever we create classes and structures we should pay special attention to the way we r defining variables insdie, because the size of the struct or class depends on it.
Also frequently used variables shoild be kept on top in order to avoid cache misses.
A class or a struct is always a multiple of the largest sized datatype inside it.
for eg:
struct A
{
double a;
char ch;
};
now sizeof(A) == 16 and not 8 + 1 = 9
even if the struct is rewritten like
struct A
{
double a;
char ch;
char ch1;
char ch2;
char ch3;
};
even then the size vil be 16
All variables defined inside a struct or a class are alligned in memory according to their size. Like an int can only be stored at addresses 4,8 and 12 that are multiples of 4.
6 is an invalid address for an int.Similarly for double it can be strored only at multiples of 8.
NOTE: A char can be stored at any byte coz its nly of 1 byte.
for eg:
struct A
{
char ch;
int x;
char ch1;
double d;
};
1 + 4 + 1 + 8 = 14 and considering double
final size must be 16 but actually
sizeof(A) == 24 .............WHY???
Now lets see WHY, hypothetical starting address 0;
0-1 1 byte for ch
now next valid address for int is 4, hence
1-2 padding
2-3 padding
3-4 padding
4-8 4 bytes for int
8-9 1 byte for ch1
Now next valid address for double vil be 16, hence
9-16 7 bytes for padding
16-24 8 bytes for d
So we wasted 10 bytes in padding dats actually a quite if you use dis struct for creating a 2D array of size 100*100
total objects of struct = 10000
bytes wasted on padding = 10000* 10 bytes
Approx 98 kB is being wasted on just padding.
Lets rearrange d structrure now
struct A
{
double d;
int x;
char ch;
char ch1;
};
0-8 8 bytes for double
8-12 4 bytes for int
12-13 1 byte for ch
13-14 1 byte for ch1
since d structure needs to be a multiple of 8 so final size would be 16
Only 2 bytes required for padding
So for 100*100 array only 19kB is wasted on padding
Simply shuffling d variables inside saved us 78kB
So always keep this thing in mind while creating classes and structure in the future.
