#pragma (4) 仅对其后定义的结构体有效

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// test.cpp : Defines the entry point for the console application.//#include "stdafx.h"#include <iostream>using namespace std;#pragma message("Linking...")#pragma pack(4)int _tmain(int argc, _TCHAR* argv[]){typedef struct structest{charc; //1,8,4 doubled;intn;}structest;structest structest1;printf("c=%d/n",sizeof( structest1.c));printf("d=%d/n",sizeof( structest1.d));printf("n=%d/n",sizeof( structest1.n));/*printf("d=%d",sizeof(structest.d));printf("n=%d",sizeof(structest.n));*/printf("%d/n",sizeof(structest1));#pragma pack()typedef struct structest_2{charc; //1,8,4 doubled;intn;}structest_2;structest_2 structest2;printf("%d/n",sizeof(structest2));return 0;}

输出structest1是16，structest2是24

但是如果

// test.cpp : Defines the entry point for the console application.//#include "stdafx.h"#include <iostream>using namespace std;#pragma message("Linking...")#pragma pack(4)int _tmain(int argc, _TCHAR* argv[]){typedef struct structest{charc; //1,8,4 doubled;intn;}structest;structest structest1;printf("c=%d/n",sizeof( structest1.c));printf("d=%d/n",sizeof( structest1.d));printf("n=%d/n",sizeof( structest1.n));/*printf("d=%d",sizeof(structest.d));printf("n=%d",sizeof(structest.n));*/printf("%d/n",sizeof(structest1));#pragma pack()structest structest16;printf("%d/n",sizeof(structest16)); //输出还是16typedef struct structest_2{charc; //1,8,4 doubled;intn;}structest_2;structest_2 structest24;printf("%d/n",sizeof(structest24)); //输出是24，return 0;}