Warning(s):

In file included from /usr/include/x86_64-linux-gnu/bits/_G_config.h:19:0,
                 from /usr/include/x86_64-linux-gnu/bits/libio.h:35,
                 from /usr/include/stdio.h:41,
                 from 1487884015/source.cpp:2:
1487884015/source.cpp: In function ‘int main()’:
1487884015/source.cpp:17:47: warning: offsetof within non-standard-layout type ‘A’ is undefined [-Winvalid-offsetof]
     printf("offsetof(A, a) : %zu\n", offsetof(A, a));
                                               ^
1487884015/source.cpp:18:47: warning: offsetof within non-standard-layout type ‘B’ is undefined [-Winvalid-offsetof]
     printf("offsetof(B, b) : %zu\n", offsetof(B, b));
                                               ^