Optimizing Class Member Alignment

The size of a class can be changed simply by playing with the order of its members' declaration:

Code:

struct A
 {
 bool a;
 int b;
 bool c;
 }; /*sizeof (A) == 12*/

On my machine, sizeof (A) equals 12. This result might seem surprising because the total size of A's members is only 6 bytes: 1+4+1 bytes. Where did the remaining 6 bytes come from? The compiler inserted 3 padding bytes after each bool member to make it align on a four-byte boundary. You can reduce A's size by reorganizing its data members as follows:

Code:

struct B
 {
 bool a;
 bool c;
 int b;
 }; // sizeof (B) == 8

This time, the compiler inserted only 2 padding bytes after the member c. Because b occupies four bytes, it naturally aligns on a word boundary without necessitating additional padding bytes.

 

optimization is not so necessary but efficient programming is