Union, part 1

Union allows you to reuse a memory space for different-different data types, only one data can be stored at a time

Here is an example:

#include <iostream>

union U
{
    int a;
    double b;
};

void printMem(void *location)
{
    unsigned char const *pos = (unsigned char const *)location;

    for (int i = 0; i < 20; i++)
        printf("|%2.2x| ", pos[i]);
    printf("\n");
}

int main(int argc, char const *argv[])
{
    U u1;

    u1.a = 3;
    std::cout << "a: " << u1.a << ", b: " << u1.b << std::endl;
    printMem(&u1);
    
    u1.b = 50;
    std::cout << "a: " << &u1.a << ", b: " << &u1.b << std::endl;
    printMem(&u1);

    return 0;
}

Try it here:

Output:

a: 3, b: 6.95272e-310, size: 8
a: 0x7ffd9ee3ed90, b: 0x7ffd9ee3ed90, size: 8
|03| |00| |00| |00| |fd| |7f| |00| |00| |00| |3e| |6a| |2d| |2f| |ab| |1c| |8e| |00| |00| |00| |00| 
a: 0, b: 50, size: 8
a: 0x7ffd9ee3ed90, b: 0x7ffd9ee3ed90, size: 8
|00| |00| |00| |00| |00| |00| |49| |40| |00| |3e| |6a| |2d| |2f| |ab| |1c| |8e| |00| |00| |00| |00| 

Let' break down this example:

1. We create a simple union with 2 data inside, a as integer and b as double.

2. Once we store a = 3 and print it, u1's memory addess will store 3 as a integer.

3. Now when we set b = 50 and print it, u1's memory address will store 50 as a double.

4. And now if you try to read value of a, it will lose its value and will be overridden with byes of double (b).

5. Here we reused same memory location as different data types.