Login | Register   
LinkedIn
Google+
Twitter
RSS Feed
Download our iPhone app
TODAY'S HEADLINES  |   ARTICLE ARCHIVE  |   FORUMS  |   TIP BANK
Browse DevX
Sign up for e-mail newsletters from DevX


Tip of the Day
Language: C++
Expertise: All
May 8, 2000

A class's size can change simply by playing with the order of its members' decla

A class's size can change simply by playing with the order of its members' declaration. For example:

 
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 six bytes come from? The compiler inserted three padding bytes after each bool member to make it align on a word boundary. You can reduce A's size by reorganizing its data members as follows:

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

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

As a rule, group data members are smaller than a word's size together.

Danny Kalev
 
Comment and Contribute

 

 

 

 

 


(Maximum characters: 1200). You have 1200 characters left.

 

 

Sitemap
Thanks for your registration, follow us on our social networks to keep up-to-date