What's the difference between sizeof and alignof?

Question

What\'s the difference between sizeof and alignof?

#include 

#define SIZEOF_ALIGNOF(T) std::cout<< sizeof(T) << \'/\' << a         


        

Answer 1

Old question (although not marked as answered..) but thought this example makes the difference a bit more explicit in addition to Christian Stieber's answer. Also Meluha's answer contains an error as sizeof(S) output is 16 not 12.

// c has to occupy 8 bytes so that d (whose size is 8) starts on a 8 bytes boundary
//            | 8 bytes |  | 8 bytes  |    | 8 bytes |
struct Bad  {   char c;      double d;       int i;     }; 
cout << alignof(Bad) << " " << sizeof(Bad) << endl;   // 8 24

//             | 8 bytes |   |   8 bytes    |    
struct Good {   double d;     int i; char c;          };
cout << alignof(Good) << " " << sizeof(Good) << endl; // 8 16

It also demonstrates that it is best ordering members by size with largest first (double in this case), as the others members are constrained by that member.