通过使用int和char数组的联合找到int值，但是这个值是怎么来的？可以告诉任何人吗？

Question

I have written c++ code for understanding union concept.In the code I have assigned some character to char array and print the int value,but I don't understand the its logic behind.need help? below code I have written.

#include <iostream>
union float_u {
    int f;
   char a[sizeof(int)];
} data1;

  using namespace std;

int main()
{


    cout << "Size of union :" << sizeof(data1)<<endl;

   data1.a[0] = '@';
   data1.a[1]= '@';


   cout << "value of f :" << data1.f << endl;

   return 0;
 }

Output coming like this:

Size of union:4

value of f:16448

Answer 1

You cannot understand C++ by writing code and studying its output. Thats because there are just too many ways to write C++ code that compiles without error, but has undefined bahvior.

I am quoting from https://en.cppreference.com/w/cpp/language/union .

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

You are writing to a . a is the active member of the union. Reading from f is undefined. You could in principle get any output.

Many compilers implement, as a non-standard language extension, the ability to read inactive members of a union.

As far as I know type punning via unions is allowed in C, so its not too difficult for compilers to also allow it in C++. Though, when you make use of a compiler extension you need to be aware that it isnt standard portable C++, and you need to read your compilers manual.