union联合体(c++11非受限联合体、字节对齐）
1.非受限联合体
2.字节对齐
字节对齐原则：
字节对齐的细节和编译器实现相关，但一般而言，满足三个准则：
1) 结构体变量的首地址能够被其最宽基本类型成员的大小所整除；
2) 结构体每个成员相对于结构体首地址的偏移量（offset）都是成员大小的整数倍，如有需要编译器会在成员之间加上填充字节（internal adding）；
3) 结构体的总大小为结构体最宽基本类型成员大小的整数倍，如有需要编译器会在最末一个成员之后加上填充字节（trailing padding）。
示例：
#include <iostream>
using namespace std;
struct X
{
short s; /* 2 bytes */ // 首地址宽度要被最宽基本类型大小所整除，所以填充2个字节
/* 2 padding bytes */
int i; /* 4 bytes */ // 偏移量能整除int大小
char c; /* 1 byte */ // 结构体总大小要能够整除最宽基本类型，所以填充3个字节
/* 3 padding bytes */
};
struct Y
{
int i; /* 4 bytes */
char c; /* 1 byte */
/* 1 padding byte */
short s; /* 2 bytes */ // // 偏移量能整除short大小,所以要对上一个填充一个字节
};
struct Z
{
int i; /* 4 bytes */
short s; /* 2 bytes */
char c; /* 1 byte */ //// 结构体总大小要能够整除最宽基本类型，所以填充1个字节
/* 1 padding byte */
};
const int sizeX = sizeof(struct X); /* = 12 */
const int sizeY = sizeof(struct Y); /* = 8 */
const int sizeZ = sizeof(struct Z); /* = 8 */
int main(int argc, char const *argv[])
{
cout << "sizeX: "<< sizeX << " sizeY: "<< sizeY << " sizeZ: "<< sizeZ << endl;
return 0;
}
// sizeX: 12 sizeY: 8 sizeZ: 8
1. 字节对齐不带string类型
字节对齐示例：
#include <iostream>
using namespace std;
// 外来人口信息
struct Foreigner
{
Foreigner(double s, double ph) : addr(s), phone(ph) {}
double addr; // sizeof(double)=8
double phone;
};
// 登记人口信息
class Person
{
public:
enum class Category : char {Student, Local, Foreign};
Person(int num) : number(num), type(Category::Student) {}
Person(double id) : idNum(id), type(Category::Local) {}
Person(double addr, double phone) : foreign(addr, phone), type(Category::Foreign) {}
~Person() {}
void print()
{
cout << "Person category: " << (int)type << " sizeof:"<< sizeof(type) << endl;
// cout << "number addr:"<< &number << " idNum addr:"<< &idNum << " foreign addr:" <<&foreign << endl;
switch (type)
{
case Category::Student:
cout << "Student school number: " << number << " sizeof:" << sizeof(number) << endl;
break;
case Category::Local:
cout << "Local people ID number: " << idNum << " sizeof:" << sizeof(idNum) << endl;
break;
case Category::Foreign:
cout << "Foreigner address: " << foreign.addr
<< ", phone: " << foreign.phone << " sizeof:" << sizeof(foreign) << endl;
break;
default:
break;
}
}
private:
Category type; // 对齐最长字节8
union // 64字节
{
int number;
double idNum;
Foreigner foreign; // 元素最宽为8,长度为16
};
};
union test // typeof(test) //16
{
enum class Category : char {Student, Local, Foreign};
test(int num) : number(num) {}
~test() {}
test(double id) : idNum(id) {}
test(double addr, double phone) : foreign(addr, phone) {}
int number;
double idNum;
Foreigner foreign;
};
int main()
{
Person p1(9527);
Person p2(12.25);
Person p3(16.5, 1.256);
p1.print();
p2.print();
p3.print();
cout << "sizeof:"<< sizeof(p3) << endl; // sizeof:24
test t(16.5, 1.256);
cout << "union sizeof:"<< sizeof(t) << endl; // union sizeof:16
return 0;
}
// g++ std=c++11 test.cpp -o test
/*
Person category: 0 sizeof:1
Student school number: 9527 sizeof:4
Person category: 1 sizeof:1
Local people ID number: 12.25 sizeof:8
Person category: 2 sizeof:1
Foreigner address: 16.5, phone: 1.256 sizeof:16
sizeof:24
union sizeof:16
*/
带有string类型
How is this possible? String always size 40 and structure sizeof odd reading [duplicate]
有个疑问：Category type; // 偏移了8字节?，string占了32个字节为什么偏移了8个字节?还是string类中最宽基本类型的大小为8字节?
std::string类内部存储一些指针，这些指针在运行时分配给指向保存数据（字符）的动态分配的存储。size()您可以通过调用成员函数（或length()）来获取此数据的大小。
对于64位系统来说，指针大小占8个字节，所以对于
#include <iostream>
using namespace std;
// 外来人口信息
struct Foreigner
{
Foreigner(int s, int ph) : addr(s), phone(ph) {}
int addr;
int phone;
};
// 登记人口信息
class Person
{
public:
enum class Category : char {Student, Local, Foreign};
Person(int num) : number(num), type(Category::Student) {}
Person(char id) : idNum(id), type(Category::Local) {}
Person(int addr, int phone) : foreign(addr, phone), type(Category::Foreign) {}
~Person() {}
void print()
{
cout << "Person category: " << (int)type << " sizeof:"<< sizeof(type) << endl;
// cout << "number addr:"<< &number << " idNum addr:"<< &idNum << " foreign addr:" <<&foreign << endl;
switch (type)
{
case Category::Student:
cout << "Student school number: " << number << " sizeof:" << sizeof(number) << endl;
break;
case Category::Local:
cout << "Local people ID number: " << idNum << " sizeof:" << sizeof(idNum) << endl;
break;
case Category::Foreign:
cout << "Foreigner address: " << foreign.addr
<< ", phone: " << foreign.phone << " sizeof:" << sizeof(foreign) << endl;
break;
default:
break;
}
}
// private:
Category type; // 对齐最长字节8
union // 64字节
{
int number;
char idNum;
Foreigner foreign; // 元素最宽为8,长度为16
};
};
union test // typeof(test) //16
{
enum class Category : char {Student, Local, Foreign};
test(int num) : number(num) {}
~test() {}
test(char id) : idNum(id) {}
test(int addr, int phone) : foreign(addr, phone) {}
int number;
char idNum;
Foreigner foreign;
};
struct test2
{
test2(int num) : number(num) {}
~test2() {}
test2(string id) : idNum(id) {}
int number;
string idNum; // sizeof = 32
};
int main()
{
Person p1(9527);
Person p2('a');
Person p3(16, 1);
p1.print();
p2.print();
p3.print();
cout << "sizeof:"<< sizeof(p3) << endl; // sizeof:24
cout << " offsetof(number):" << offsetof(Person, number) << " idNum:"<< offsetof(Person, idNum) << endl;
test t(16, 1);
cout << "union sizeof:"<< sizeof(t) << endl; // union sizeof:16
test2 t2("砂隐村村北");
cout << "struct sizeof:"<< sizeof(t2) << endl;
cout << " offsetof(number):" << offsetof(test2, number) << " idNum:"<< offsetof(test2, idNum) <<" sizeof(TCHAR)"<< endl;
return 0;
}
/*
Person category: 0 sizeof:1
Student school number: 9527 sizeof:4
Person category: 1 sizeof:1
Local people ID number: a sizeof:1
Person category: 2 sizeof:1
Foreigner address: 16, phone: 1 sizeof:8
sizeof:12
offsetof(number):4 idNum:4
union sizeof:8
struct sizeof:40
offsetof(number):0 idNum:8
*/
