offsetof 例子「建议收藏」

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/* offsetof example */

#include “stdafx.h” #include <stdio.h> #include <stddef.h>

/************************************************************************/ /* Macro OBJECT_HEAD_ADDRESS : calculate the struct’s address according /* to one of its member’s address /************************************************************************/ #define OBJECT_HEAD_ADDRESS(ClassName,MemberName,Addre) / Addre – offsetof(ClassName, MemberName)

struct S { int ID; char *addr; char name[20]; };

struct S1 { S employee; char *title; };

struct S2 { char singlechar; int arraymember[10]; char anotherchar; };

int main(int argc, char* argv[]) { /* Example 1 S2 s2 = {‘a’, 10, ‘b’}; int head = int(&(s2.singlechar)); int memb = int(&(s2.anotherchar));

int i = OBJECT_HEAD_ADDRESS(S2, anotherchar, memb);

printf (“offsetof(S2, singlechar) is %d/n”, offsetof(S2, singlechar)); printf (“offsetof(S2, arraymember) is %d/n”, offsetof(S2, arraymember)); printf (“offsetof(S2, anotherchar) is %d/n”, offsetof(S2, anotherchar)); printf(“s2.anotherchar’s address is %x ==> s2’s address is %x/n”, memb, i); */ /* Example 2 S s = {100, “Nanjing”, “Thomas”}; int id = int(&(s.ID)); int ad = int(&(s.addr)); int nm = int(&(s.name));

int j = OBJECT_HEAD_ADDRESS(S, addr, ad);

printf (“offsetof(S, ID) is %d/n”, offsetof(S, ID)); printf (“offsetof(S, addr) is %d/n”, offsetof(S, addr)); printf (“offsetof(S, name) is %d/n”, offsetof(S, name)); printf (“s.ID’s address : %x/ns.addr’s address : %x/ns.name’s address : %x/n”, id, ad, nm); printf(“s.addr’s address is %x ==> s’s address is %x/n”, ad, j); */ /* Example 3 */ S1 s1 = { {100, “Nanjing”, “Thomas Chen”},”Thomas Chen”}; int td = int(&(s1.title)); int k = OBJECT_HEAD_ADDRESS(S1, title, td);

printf (“offsetof(S1, employee) is %d/n”, offsetof(S1, employee)); printf (“offsetof(S1, title) is %d/n”, offsetof(S1, title)); printf(“s1.title’s address is %x ==> s1’s address is %x/n”, td, k);

return 0; } /* Example 1 Output */ offsetof(S2, singlechar) is 0 offsetof(S2, arraymember) is 4 offsetof(S2, anotherchar) is 44 s2.anotherchar’s address is 12ff7c ==> s2’s address is 12ff50

/* Example 2 Output */ offsetof(S, ID) is 0 offsetof(S, addr) is 4 offsetof(S, name) is 8 s.ID’s address : 12ff64 s.addr’s address : 12ff68 s.name’s address : 12ff6c s.addr’s address is 12ff68 ==> s’s address is 12ff64

/* Example 3 Output */ offsetof(S1, employee) is 0 offsetof(S1, title) is 28 s1.title’s address is 12ff7c ==> s1’s address is 12ff60

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