c++ - union 工作不正常

Question

这是我的代码。它在 C 中运行良好，但在 C++ 中表现得像结构。

#include<iostream>
using namespace std;
int main()
{
    union myunion
    {
        int B;
        double PI;
    };

    union myunion numbers;
    numbers.PI = 3.14;
    numbers.B = 12; 

    /* now storage of PI should be over-written */

    cout<<numbers.PI<<endl;
    cout<<numbers.B<<endl;

    /* Output is  3.14 */
    /*            12   */
    /* Why it is behaving like Structures ? */
    return 0;
}

Answer 1

评论者提到，Cppreference on union states :

it's undefined behavior to read from the member of the union that wasn't most recently written.

这就是为什么您的程序会这样:这是未定义的行为。它可以显示先前的值、崩溃或介于两者之间的任何内容。

numbers.PI = 3.14;
numbers.B = 12; 

cout<<numbers.PI<<endl; // undefined behaviour