C++PrimerPlus_4.5共用体union

共用体（union）是一种数据格式。

它能存储不同的数据类型，但只能同时存储其中一种类型

共用体的用途是当数据类型可能是1种或者是更多种格式（但不同时使用）时，可节省空间。

🌰例子：

#include <iostream>

int main() {
    using namespace std;
    
    union one4all
    {
        int int_val;
        long long_val;
        double double_val;
    };
    
    one4all pail;
    pail.int_val = 15;
    cout << pail.int_val << endl;
    pail.double_val = 12.05;
    cout << pail.double_val << endl;
    cout << pail.int_val << endl; // 输出-1717986918，代表无法输出
    return 0;    
}

🌰 例子2:

int main() {
    using namespace std;
    struct widget
    {
        char brand[20];
        int type;
        
        union id
        {
            long id_num;
            char id_char[20];
        } id_val;
    };
    
    widget prize;
    
    prize.brand[0] = 'z';
    cout << "Please enter prize type, 1 or 2 \n";
    cin >> prize.type;
    
    // prize.type = 1;
    // prize.id_val.id_num = 1;
    // prize.id_val.id_char[0] = 'h';
    
    if (prize.type == 1)
    { // 要用打括号括起来，假如if里有几行的话
        cout << "Please enter prize.id_val in int \n";
        cin >> prize.id_val.id_num;
        cout << "Prize.id_val is " << prize.id_val.id_num << endl;
    }
    else
    {
        cout << "Please enter prize.id_val in char \n";
        cin >> prize.id_val.id_char;
        cout << "Prize.id_val is " << prize.id_val.id_char << endl;
    }
    
    return 0;
}