CPP

cpp unions

#include <iostream>
#include <cstdint>
union S
{
    std::int32_t n;     // occupies 4 bytes
    std::uint16_t s[2]; // occupies 4 bytes
    std::uint8_t c;     // occupies 1 byte
};                      // the whole union occupies 4 bytes
 
int main()
{
    S s = {0x12345678}; // initializes the first member, s.n is now the active member
    // at this point, reading from s.s or s.c is undefined behavior
    std::cout << std::hex << "s.n = " << s.n << '
';
    s.s[0] = 0x0011; // s.s is now the active member
    // at this point, reading from n or c is UB but most compilers define it
    std::cout << "s.c is now " << +s.c << '
' // 11 or 00, depending on platform
              << "s.n is now " << s.n << '
'; // 12340011 or 00115678
}

unions c++

A union is a user-defined type in which all members share the same memory location. 
This definition means that at any given time, a union can contain no more than one object from its list of members. 
It also means that no matter how many members a union has, it always uses only enough memory to store the largest member.

A union can be useful for conserving memory when you have lots of objects and limited memory. 
However, a union requires extra care to use correctly. 
You're responsible for ensuring that you always access the same member you 
assigned. If any member types have a non-trivial constructor, then you must
write additional code to explicitly construct and destroy that member. 
Before you use a union, consider whether the problem
you're trying to solve could be better expressed by using a 
base class and derived class types.