内存地址对齐 sizeof

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本文详细解读内存地址对齐原则，包括每个成员起始地址与其自身大小的关系、整体对齐标准及如何使用预处理器指令进行自定义对齐。通过示例展示对齐对不同数据类型的影响，并探讨嵌套结构体、静态成员和函数的对齐特性。此外，还阐述了类层次中静态成员与函数的特殊位置。

内存地址对齐 sizeof

2010年05月11日星期二上午 08:48

内存地址的对齐主要考虑三个因素：

1：对于每个成员的起始地址是他本身所占的整数倍

2：整个所占的内存是成员中占的地址内存最多的整数倍

3：有#pragma pack(int)进行设置,如果结构体某成员的sizeof大于你设置的，则按你的设置来对齐

注意：每次用#pragma pack(int)进行设置后，要用#pragma pack()对其结束，免得造成错误

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#include<iostream>
using namespace std;
struct A
{
int a; //0 1 2 3
char b; //4 *
short c; //6 7
};
struct B
{
char a; //0 * * *
int b; //4 5 6 7
short c; //8 9 * *
};
#pragma pack(2)
struct C
{
char a; //0 *
int b; //2 3 4 5
short c; //6 7
};
#pragma pack()
#pragma pack(1)
struct D
{
char a; //0
int b; //1 2 3 4
short c; //5 6
};
#pragma pack()
int main()
{
cout<<"sizeof(A) = "<<sizeof(A)<<endl;
cout<<"sizeof(B) = "<<sizeof(B)<<endl;
cout<<"sizeof(C) = "<<sizeof(C)<<endl;
cout<<"sizeof(D) = "<<sizeof(D)<<endl;
return 0;
}

#include<iostream> using namespace std; struct A { int a; //0 1 2 3 char b; //4 * short c; //6 7 }; struct B { char a; //0 * * * int b; //4 5 6 7 short c; //8 9 * * }; #pragma pack(2) struct C { char a; //0 * int b; //2 3 4 5 short c; //6 7 }; #pragma pack() #pragma pack(1) struct D { char a; //0 int b; //1 2 3 4 short c; //5 6 }; #pragma pack() int main() { cout<<"sizeof(A) = "<<sizeof(A)<<endl; cout<<"sizeof(B) = "<<sizeof(B)<<endl; cout<<"sizeof(C) = "<<sizeof(C)<<endl; cout<<"sizeof(D) = "<<sizeof(D)<<endl; return 0; }

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sizeof(A) = 8
sizeof(B) = 12
sizeof(C) = 8
sizeof(D) = 7
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sizeof(A) = 8 sizeof(B) = 12 sizeof(C) = 8 sizeof(D) = 7 Press any key to continue

注意下面我对#pragma pack() 用法的理解，他不仅对每个成员的起始地址起作用，对整个的也起作用。请看下面的测试程序

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#include<iostream>
using namespace std;
#pragma pack(4)
struct AA
{
char a; // 0 * * * * * * *
double b; //4 5 6 7 8 9 10 11
int c; //12 13 * *
};//16
#pragma pack()
#pragma pack(2)
struct AAA
{
char a; // 0 *
double b; //2 3 4 5 6 7 8 9 起始地址也只要是2的倍数，不是8的倍数
int c; //10 11 * *
};//14 只要保证是2的倍数，不是最大double的8的倍数
#pragma pack()
int main()
{
cout<<"sizeof(AA) = "<<sizeof(AA)<<endl;
cout<<"sizeof(AAA) = "<<sizeof(AAA)<<endl;
return 0;
}

#include<iostream> using namespace std; #pragma pack(4) struct AA { char a; // 0 * * * * * * * double b; //4 5 6 7 8 9 10 11 int c; //12 13 * * };//16 #pragma pack() #pragma pack(2) struct AAA { char a; // 0 * double b; //2 3 4 5 6 7 8 9 起始地址也只要是2的倍数，不是8的倍数 int c; //10 11 * * };//14 只要保证是2的倍数，不是最大double的8的倍数 #pragma pack() int main() { cout<<"sizeof(AA) = "<<sizeof(AA)<<endl; cout<<"sizeof(AAA) = "<<sizeof(AAA)<<endl; return 0; }

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sizeof(A) = 24
sizeof(AA) = 40
sizeof(A2) = 20
sizeof(AA2) = 28
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sizeof(A) = 24 sizeof(AA) = 40 sizeof(A2) = 20 sizeof(AA2) = 28 Press any key to continue

下面接受有结构体嵌套的时候：

对于整个结构体作为成员函数时，他的起始地址不是以该结构体整体作为标准，而是以其内的成员为标准

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#include<iostream>
using namespace std;
struct A
{
int a; //0 1 2 3
char b; //4 * * *
double c; //8 9 10 11 12 13 14 15
short d; //16 17 * * * * * *
};//24
struct AA
{
char a; //0 * * * * * * *
A b; //8 .. 31
int c; //32 33 34 35 * * * *
};
#pragma pack(4)
struct A2
{
int a; //0 1 2 3
char b; //4 * * *
double c; //8 9 10 11 12 13 14 15
short d; //16 17 * *
};//20
#pragma pack()
struct AA2
{
char a; //0 * * *
A2 b; //4 .. 23
int c; //24 25 26 27
};
int main()
{
cout<<"sizeof(A) = "<<sizeof(A)<<endl;
cout<<"sizeof(AA) = "<<sizeof(AA)<<endl;
cout<<"sizeof(A2) = "<<sizeof(A2)<<endl;
cout<<"sizeof(AA2) = "<<sizeof(AA2)<<endl;
return 0;
}

#include<iostream> using namespace std; struct A { int a; //0 1 2 3 char b; //4 * * * double c; //8 9 10 11 12 13 14 15 short d; //16 17 * * * * * * };//24 struct AA { char a; //0 * * * * * * * A b; //8 .. 31 int c; //32 33 34 35 * * * * }; #pragma pack(4) struct A2 { int a; //0 1 2 3 char b; //4 * * * double c; //8 9 10 11 12 13 14 15 short d; //16 17 * * };//20 #pragma pack() struct AA2 { char a; //0 * * * A2 b; //4 .. 23 int c; //24 25 26 27 }; int main() { cout<<"sizeof(A) = "<<sizeof(A)<<endl; cout<<"sizeof(AA) = "<<sizeof(AA)<<endl; cout<<"sizeof(A2) = "<<sizeof(A2)<<endl; cout<<"sizeof(AA2) = "<<sizeof(AA2)<<endl; return 0; }

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sizeof(A) = 24
sizeof(AA) = 16
sizeof(AAA) = 14
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sizeof(A) = 24 sizeof(AA) = 16 sizeof(AAA) = 14 Press any key to continue

最后注意：static成员和函数其实是类层次的，不在对象中分配空间，而成员函数其实是被编译为全局函数了，所以也不在对象中。

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#include<iostream>
using namespace std;
struct empty{}; // 1
struct constAndStatic
{
const int i; //0 1 2 3 * * * *
const double d; //8 9 10 11 12 13 14 15
static char c;
static void TestStatic(){}
void TestNoStatic(){}
}; // 16
int main()
{
cout<<"sizeof(empty) = "<<sizeof(empty)<<endl;
cout<<"sizeof(constAndStatic) = "<<sizeof(constAndStatic)<<endl;
return 0;
}

#include<iostream> using namespace std; struct empty{}; // 1 struct constAndStatic { const int i; //0 1 2 3 * * * * const double d; //8 9 10 11 12 13 14 15 static char c; static void TestStatic(){} void TestNoStatic(){} }; // 16 int main() { cout<<"sizeof(empty) = "<<sizeof(empty)<<endl; cout<<"sizeof(constAndStatic) = "<<sizeof(constAndStatic)<<endl; return 0; }

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