C++中結構體的sizeof

• 為什麼要地址對齊？
• * 我們在訪問記憶體時，如果地址是按4位元組對齊，則訪問效率會高很多。
• * 這個問題的原因在於訪問記憶體的硬體電路。一般情況下，地址匯流排總是按照對齊後的地址來訪問。
• * 例如你想得到0x00000001開始的4位元組內容，系統首先需要以0x00000000讀4位元組，
• * 然後從中取得3位元組，然後在用0x00000004作為開始地址，獲得下一個四位元組，
• * 在從中得到第一個位元組，兩次組合出你想得到的內容。但是如果地址一開始就是對齊到
• * 0x00000000，則系統只要一次讀寫即可。
• 所以，為了效能考慮，編譯器會做地址對齊。
• 地址對齊的原則：
• 每個基本型別有自己預設的對齊值。 然後可以通過#pragma pack(2)設定編譯器的預設對齊值。取兩者最小的一個作為最終對齊值。
• 若沒有#pragma pack(2)設定，那麼只看基本型別自己的對齊值。
• 總之： 在允許的情況下，儘量依次填充，但是需要保證（32位系統）
• 1 char,bool只從X地址處開始儲存；
• 2 short只從2X地址出開始儲存；
• 3 int，float，long只從4X地址處開始儲存；
• 4double只從8X地址處開始儲存；

下面是列子：

#include <iostream>
using namespace std;

//sizeof(union1) = 16
union union1 {
	long i;		//4
	int x[4];	//16
	char ch;	//1
};

//sizeof(union2) = 16
union union2{
	char a[13];
	int i;	//由於int的存在，使得union按照四個位元組來對其了
};

//sizeof(union3) = 13
union union3{
	char a[13];
	char i;
};

/////////////// 對比node1和node2 ///////////
struct node1{	//24
	int j;		//4
	double f;	//8	
	int i;		//4
};
	
struct node2{	//16
	int i;		//4
	int j;		//4
	double f;	//8	
};

/////////////// 對比node3和node4 ///////////
struct node3{	//32(注意)
	char i;		//1
	int d[5];	//20
	double f;	//8	
};

struct node4{	//40
	int d[5];	//20
	double f;	//8	
	int i;		//4
};

///////////////////////////////////////////
struct node5{	//32(注意)
	char d[10];	//10
	double f;	//8	
	int i;		//4
};

struct node6{	//20
	char d;		//1
	int f;		//4
	char i[10];	//10
};
//}__attribute__((aligned (1)));
///////////////////////////////////////
struct node7{	//12
	char x1;	//1
	short x2;	//2
	float x3;	//4
	char x4;	//4
};

#pragma pack(1) //指定按照1位元組對齊
struct node8{	//8
	char x1;	//1
	short x2;	//2
	float x3;	//4
	char x4;	//1
};
#pragma pack()	//恢復預設的位元組對齊設定
////////////////////////////////////////
struct node9{	//20
	char ch,*p;	//1,4
	union{		//4
		short a,b;	
		unsigned int c:2,d:1;
	}u;
	bool f;		//1
	struct node9 *next;	//4
};

//測試位元組對齊
void test2(){
	int size;
	struct node1 n1;
	struct node2 n2;
	struct node3 n3;
	struct node4 n4;
	struct node5 n5;
	struct node6 n6;
	struct node7 n7;
	struct node8 n8;
	struct node9 n9;
	//棧生長的方向與地址增長方向相反，所以a的地址比b的地址大四位元組
	int a = 1,b = 2,c=3;
	int *p = &b;
	printf("%d\n",*(p+1));	//1
	
	//測試union和struct的大小(後者會地址對齊，前者仍然輸出16)
	printf("union1:%d\n",sizeof(union union1));	//16
	printf("union2:%d\n",sizeof(union union2));	//16
	printf("union3:%d\n",sizeof(union union3));	//13

	//測試結構體的地址對齊
	printf("************************\n");
	printf("n1:%d\n",sizeof(n1));			
	printf("%ld\n",(unsigned long)&n1.j);	
	printf("%ld\n",(unsigned long)&n1.f);
	printf("%ld\n",(unsigned long)&n1.i);

	printf("************************\n");
	printf("n2:%d\n",sizeof(n2));		
	printf("%ld\n",(unsigned long)&n2.i);
	printf("%ld\n",(unsigned long)&n2.j);
	printf("%ld\n",(unsigned long)&n2.f);

	printf("************************\n");
	printf("n3:%d\n",sizeof(n3));		
	printf("%ld\n",(unsigned long)&n3.i);
	printf("%ld\n",(unsigned long)&n3.d);
	printf("%ld\n",(unsigned long)&n3.f);

	printf("************************\n");
	printf("n4:%d\n",sizeof(n4));		
	printf("%ld\n",(unsigned long)&n4.d);
	printf("%ld\n",(unsigned long)&n4.f);
	printf("%ld\n",(unsigned long)&n4.i);

	printf("************************\n");
	printf("n5:%d\n",sizeof(n5));		
	printf("%ld\n",(unsigned long)&n5.d);
	printf("%ld\n",(unsigned long)&n5.f);
	printf("%ld\n",(unsigned long)&n5.i);

	printf("************************\n");
	printf("n6:%d\n",sizeof(n6));		
	printf("%ld\n",(unsigned long)&n6.d);
	printf("%ld\n",(unsigned long)&n6.f);
	printf("%ld\n",(unsigned long)&n6.i);

	printf("************************\n");
	printf("n7:%d\n",sizeof(n7));		
	printf("%ld\n",(unsigned long)&n7.x1);
	printf("%ld\n",(unsigned long)&n7.x2);
	printf("%ld\n",(unsigned long)&n7.x3);
	printf("%ld\n",(unsigned long)&n7.x4);

	printf("************************\n");
	printf("n8:%d\n",sizeof(n8));
	printf("%ld\n",(unsigned long)&n8.x1);
	printf("%ld\n",(unsigned long)&n8.x2);
	printf("%ld\n",(unsigned long)&n8.x3);
	printf("%ld\n",(unsigned long)&n8.x4);

	printf("************************\n");
	printf("n9:%d\n",sizeof(n9));
	
}

int main(){
	int *ptr;
	test2();
	printf("************************\n");
	printf("char:%d\n",sizeof(char));		//1
	printf("short:%d\n",sizeof(short));		//2
	printf("int:%d\n",sizeof(int));			//4
	printf("long:%d\n",sizeof(long));		//4
	printf("float:%d\n",sizeof(float));		//4
	printf("double:%d\n",sizeof(double));	//8
	printf("pointer:%d\n",sizeof(ptr));		//4
	return 0;
}