#pragma (4) 仅对其后定义的结构体有效

最新推荐文章于 2025-06-26 09:39:02 发布

原创最新推荐文章于 2025-06-26 09:39:02 发布 · 510 阅读

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#struct #c

本文探讨了C++中#pragma pack指令如何影响结构体的大小，并通过实例展示了#pragma pack(4)如何使structest1占用16个字节，而取消#pragma pack后，structest2占用24个字节。同时指出#pragma pack()作用域的限制，它只影响其后面的结构体定义。

// 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{ char c; //1,8,4 double d; int n; }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{ char c; //1,8,4 double d; int n; }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{ char c; //1,8,4 double d; int n; }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16 typedef struct structest_2{ char c; //1,8,4 double d; int n; }structest_2; structest_2 structest24; printf("%d/n",sizeof(structest24)); //输出是24， return 0; }