A union is a special class type that can hold only one of its non-static data members at a time. The class specifier for a union declaration is similar to class or struct declaration.
A union is a user-defined type in which all members share the same memory location. This means that at any given time a union can contain no more than one object from its list of members. It also means that no matter how many members a union has,it always uses only enough memory to store the largest member.
The purpose of union is to save memory by using the same memory region for storing different objects at different times.
union(联合体、共用体)是共享内存的，以最大的结构或对象大小分配内存。union主要用来节省空间。
使用union时应注意：
(1)、同一个内存段可以用来存放几种不同类型的成员，但在每一个时刻只能存在其中一种，而不能同时存放几种，即每一瞬间只有一个成员起作用，其它的成员不起作用，不能同时都存在和起作用；
(2)、共用体变量中起作用的成员是最后一个存放的成员，在存入一个新的成员后，原有的成员就会失去作用，即所有的数据成员具有相同的起始地址。
下面是从其他文章中copy的测试代码，详细内容介绍可以参考对应的reference：
#include "union.hpp"
#include <stdint.h>
#include <assert.h>
#include <iostream>
#include <string>
#include <vector>
///////////////////////////////////////////
// reference: http://en.cppreference.com/w/cpp/language/union
union S
{
int32_t n; // occupies 4 bytes
uint16_t s[2]; // occupies 4 bytes
uint8_t c; // occupies 1 byte
};
int test_union1()
{
// The union is only as big as necessary to hold its largest data member.
// The other data members are allocated in the same bytes as part of that largest member.
S s = { 0x12345678 }; // initializes the first member, s.n is now the active member
// at this point, reading from s.s or s.c is undefined behavior
std::cout << std::hex << "s.n = " << s.n << '\n'; // s.n = 12345678
s.s[0] = 0x0011; // s.s is now the active member
// at this point, reading from n or c is UB but most compilers define it
std::cout << "s.c is now " << +s.c << '\n' // 11 or 00, depending on platform // s.c is now 11
<< "s.n is now " << s.n << '\n'; // 12340011 or 00115678 // s.n is now 12340011
return 0;
}
/////////////////////////////////////////////////////////
// reference: http://en.cppreference.com/w/cpp/language/union
// S_1 has one non-static data member (tag), three enumerator members (CHAR, INT, DOUBLE),
// and three variant members (c, i, d)
struct S_1
{
enum{ CHAR, INT, DOUBLE } tag;
union
{
char c;
int i;
double d;
};
};
void print_s(const S_1& s)
{
switch (s.tag)
{
case S_1::CHAR: std::cout << s.c << '\n'; break;
case S_1::INT: std::cout << s.i << '\n'; break;
case S_1::DOUBLE: std::cout << s.d << '\n'; break;
}
}
int test_union2()
{
S_1 s = { S_1::CHAR, 'a' };
print_s(s); // a
s.tag = S_1::INT;
s.i = 123;
print_s(s); // 123
return 0;
}
///////////////////////////////////////////////
// reference: https://msdn.microsoft.com/en-us/library/5dxy4b7b.aspx
union NumericType
{
short iValue;
long lValue;
double dValue;
};
int test_union3()
{
union NumericType Values = { 10 };
std::cout << Values.iValue << std::endl;// 10
Values.dValue = 3.1416;
std::cout << Values.dValue << std::endl; // 3.1416
return 0;
}
GitHub： https://github.com/fengbingchun/Messy_Test