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Majority of programmers are not aware of this padding, but in my opinion one of the important things to consider while designing your class, which being a game programmer you will have to do eventually
Posted by codinpsycho on Feb 29th, 2012
Advanced Client Side Coding.
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.
now sizeof(A) == 16 and not 8 + 1 = 9
even if the struct is rewritten like
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.
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
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
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.