C++中有个pragma pack 会对对齐方式进行改变
// ConsoleApplication1.cpp : 定义控制台应用程序的入口点。 // #include "stdafx.h" #include <iostream> using namespace std; #pragma pack(push) //保存对齐状态 #pragma pack(4)//设定为4字节对齐 struct TEST0 { char m1; double m4; int m3; }; #pragma pack(pop)//恢复对齐状态 #pragma pack(push) #pragma pack(2)<span style="white-space:pre"> </span>//#pragma pack(1)会有不同的结果 struct TEST1 //12 { char a; int b; char c; }; #pragma pack(pop) struct TEST2 //8 { char a; char b; int c; }; struct TEST3 //8 { char a; char b; char c; int d; }; struct TEST4 //12 { char a[5]; int b; }; struct TEST5 //8 + 8 + 8 = 24 { double a; //8 char b; //4 int c; //4,location the b memery char d[5]; //5 }; int _tmain(int argc, _TCHAR* argv[]) { struct TEST0 t0; cout << sizeof(t0) << endl; struct TEST1 t1; struct TEST2 t2; struct TEST3 t3; struct TEST4 t4; struct TEST5 t5; cout << sizeof(t1) << endl << sizeof(t2) << endl; cout << sizeof(t3) << endl << sizeof(t4) << endl << sizeof(t5) << endl; return 0; } |
可以简单这样理解 要么按照默认对齐，要么按照pack里面的数值进行对齐
对TEST1 pack里面值为1与2会有不同结构
当为1时，占用6个字节
a b1 b2 b3
b4 c
当为2时， 占用8个字节
a ax b1 b2
b3 b4 c cx
