How to know that which variable from Union is Used?

Question

If I declare a Union as:

union TestUnion
{
    struct 
    {
      unsigned int Num;
      unsigned char Name[5];
    }TestStruct;
    unsigned char Total[7];
};         


        

Answer 1

Another example of including the union with an enum to determine what is stored. I found it much more clear and to the point.

from: https://www.cs.uic.edu/~jbell/CourseNotes/C_Programming/Structures.html

author: Dr. John T. Bell

---

In order to know which union field is actually stored, unions are often nested inside of structs, with an enumerated type indicating what is actually stored there. For example:

typedef struct Flight {
    enum { PASSENGER, CARGO } type;
    union {
        int npassengers;
        double tonnages;  // Units are not necessarily tons.
    } cargo;
} Flight;

Flight flights[ 1000 ];

flights[ 42 ].type = PASSENGER;
flights[ 42 ].cargo.npassengers = 150;

flights[ 20 ].type = CARGO;
flights[ 20 ].cargo.tonnages = 356.78;