What are unions in C++?

Unions are a special data type provided in C++ that allow for multiple data types to be stored in the same location.

Syntax

Unions are defined outside main the same way struct are, with the only difference being that the keyword struct is replaced by union:

To declare an instance of a union, we must start with the keyword union, followed by the name of the union we're using, which in this case is Sample. Lastly, we need a name for the union instance we are creating, which in this case is u_name.

Note: Giving a name to our instance is the same as initializing a variable with a name. Consider that we are creating a variable of type union Sample.

Once we have initialized u_name, we can then use the . operator to access the variables inside Sample in the following manner:

So far, our implementation of union seems no different from a struct in C++. However, the important difference is that all the variables we initialized inside Sample share the same memory space.

u_name.a, u_name.b, and u_name.c would all share the same memory space. This means we could efficiently only use one variable at a given time.

In the example above, when we initialize u_name.a to 'c', all the other variables will point to the same memory location and thus contain the equivalent of 'c' as well.

Similarly, in the next line, when we assign u_name.b the value 87.99, we would in reality be updating all other variables as well.

Usage

Generally, unions are useful when you want to store data that could be one of multiple data types. A union could vary the data type of given data as required.

It would be considered a bad practice to use all variables for a given union. Suppose we have some data that should be a char. In the example above, we would utilize a since it is of type char. Once a is assigned a value, b and c would essentially contain garbage values and should not be used unless the data type for our said data needs to be changed to either a float or an int.

Example

#include <iostream>
using namespace std;
// creating union
union Sample {
  char a; 
  float b;
  int c;
};
int main() {
  // initialising a union
  union Sample u_name;
  // assigning value to a
  u_name.a = 'c'; 
  // printing all values
  cout << "a " << u_name.a << endl;
  cout << "b " << u_name.b << endl;
  cout << "c " << u_name.c << endl;
  cout << endl << endl;
  // assigning value to b
  u_name.b = 87.99;
  // printing all values
  cout << "a " << u_name.a << endl;
  cout << "b " << u_name.b << endl;
  cout << "c " << u_name.c << endl;
  cout << endl << endl;
  // assigning value to c
  u_name.c = 103;
  // printing all values
  cout << "a " << u_name.a << endl;
  cout << "b " << u_name.b << endl;
  cout << "c " << u_name.c << endl;
  cout << endl << endl;
  return 0;
}