pack的讨论

 

#include <iostream> using namespace std; #pragma pack(2) struct S0 { char a; }; struct S1 { char a; long b; }; struct S2 { char c; struct S1 d; long long e; }; int main() { std::cout << "size of char: " << sizeof(char) << endl; std::cout << "size of long: " << sizeof(long) << endl; std::cout << "size of long long: " << sizeof(long long) << endl; std::cout << "size of s0: " << sizeof(S0) << endl; std::cout << "size of s1: " << sizeof(S1) << endl; std::cout << "size of s2: " << sizeof(S2) << endl; return 1; } #pragma pack()

 

当指定pack是1的时候，输出结果是：

size of char: 1
size of long: 4
size of long long: 8
size of s0: 1
size of s1: 5
size of s2: 14

 

当指定pack是2的时候，输出结果是：

size of char: 1
size of long: 4
size of long long: 8
size of s0: 1
size of s1: 6
size of s2: 16

 

当指定pack是4的时候，输出结果是：

size of char: 1
size of long: 4
size of long long: 8
size of s0: 1
size of s1: 8
size of s2: 20

 

当指定pack是8的时候，输出结果是：

size of char: 1
size of long: 4
size of long long: 8
size of s0: 1
size of s1: 8
size of s2: 20

 

分析：

感觉没有太多的意义，但是，对数据结构在内存中的分布于非常大的影响。

有兴趣的话，倒是可以研究一下。

pack默认值应该是4.

 

更加多的，请参阅：

http://blog.youkuaiyun.com/yc0188/archive/2005/02/26/303212.aspx