C++ Union

Introduction to C++ Union

Union is a datatype defined by the user and all the different members of the union have the same memory location. The member of the union which occupies the largest memory decides the size of the union. Union is mostly used when the user is looking to use a single memory location for different multiple members. Unions are very much similar to the structures. It is defined in the same way, as the structures are done. For defining Unions, we need to use the keyword “union” in the language C++. This article explains the concept of Union in C++. The different examples explained below will help in understanding the concept and using it according to the requirements.

Syntax:

The syntax of using Union in C++ language is written below:

union <Name of the union> {
Define the members;
} variable names ;

Explanation:

• Name of the union – One can use any name as the union’s name. After writing union, name the union according to the requirement.
• Define the members − Here, the coder has to define the member variables.
• Union objects − Here, the coder can write the objects of the union.

Working of Union in C++

Unions are used in various situations where the coder wants to use the same memory for different multiple members. For example, if we have to create a binary tree data structure where each and every leaf node will have a double data value and every internal node are with two pointers but there are no data along with them. If we create structures to implement this then each and every node will need 16 bytes where half of the bytes will be wasted for each node. If we will declare all of the nodes using Union then we can save a good amount of space. This is how we can use Union for saving space, instead of using structure. This is the main reason that even though, Unions are very much similar to Structures but Union has a good advantage.

Examples

Lets us discuss examples of C++ Union.

1. Defining a Union in C++

This is a basic example of defining a union in C++. Here we have defined the union with the union name as “EDUcba” which has two members “Me” and “No” respectively. It shows how we can set the value of “O.Me” and “O.No”, and it changes the value of “Me” and “No” respectively.

Code:

#include <iostream>
// Union is declared in the same form as structures
union EDUcba {
int Me, No;
};
int main()
{
// Union variable is defined as O
union EDUcba O;
O.Me = 8; // O.No will also get value as 8
printf("Me = %d, No = %d\n",
O.Me, O.No);
O.No = 12; // O.Me will also be updated to 12
printf("Me = %d, No = %d\n",
O.Me, O.No);
return 0;
}

Output:

2. Size of Union

This is an example that outputs the size of the Union. Here a Union is defined with Union name as “ EDUcba” with members as “M” & “N” and the objects as “Course1” & “Course2”. This program prints the size of the objects.

Code:

#include <iostream>
union EDUcba {
int M;
float N;
} Course1, Course2;
int main( ) {
printf( "Space occupied by data is : %d\t%d", sizeof(Course1), sizeof(Course2));
return 0;
}

Output:

3. Deciding of the Size of Union by the Compiler

This example explains to us how the compiler assigns the memory spaces to different unions. In a Union, space is decided by the largest member. Here we have defined three unions which have different members respectively. In the end, this programs prints the spaces allocated to the different unions.

Code:

#include <iostream>
union educba {
int M;
int N;
} EDUCBA;
union training {
int M;
char N;
} TRAINING;
union technology {
int ray[10];
char N;
} TECHNOLOGY;
int main()
{
printf("EDUCBA = \n%lu\n"
"TRAINING = \n%lu\n"
"TECHNOLOGY = \n%lu\n",
sizeof(EDUCBA) ,
sizeof(TRAINING), sizeof(TECHNOLOGY));
return 0;
}

Output:

4. Pointers to Unions

This example shows us the way to assign a pointer to a union. Here we have created a union named “ EDUCBA”. There is a pointer created to the union “EDUCBA”.

Code:

#include <iostream>
union EDUCBA {
int M;
char N;
};
int main()
{
union EDUCBA A;
A.M = 89;
// B is a pointer to union A
union EDUCBA* B = &A;
// With the help of pointer union members are accessed
printf("%d %c", B->M, B->N);
return 0;
}

Output:

5. Declaration of the Union

This example explains us the way of defining a Union where the members are defined using the scope resolution operator.

Code:

#include <iostream>
#include <cstdint>
union EDUCBA
{
std::int32_t X;     // it will occupy 4 bytes
std::uint16_t M[2]; // it will occupy 4 bytes
std::uint8_t A;     // it will occupy 1 byte
};                      // & whole union will occpy 4 bytes
int main()
{
EDUCBA M = {0x1212121}; // first member is intitalized
std::cout << std::hex << "First member is: " << M.X << '\n';
M.M[0] = 0x0088; // M.M is initialised
std::cout << "Second member is: " << +M.A << '\n' // 88
<< "Third member is: " << M.X << '\n'; // 12340088
}

 Output:

 

6. Union members with Classes as User-Defined Constructors & Destructors

This example explains to us the way of defining Unions with classes where the unions are also used as constructors and destructors.

Code:

#include <iostream>
#include <string>
#include <vector>
union EDUCBA
{
std::string ABC;
std::vector<int> XYZ;
~EDUCBA() {}
};
int main()
{
EDUCBA M = {"Heyoo! Welcome to EDUCBA"};
std::cout << "Buddy = " << M.ABC << '\n';
M.ABC.~basic_string();
new (&M.XYZ) std::vector<int>;
M.XYZ.push_back(20);
std::cout << M.XYZ.size() << '\n';
M.XYZ.~vector();
}

Output:

7. Union like Classes

This example explains the concept of Union in classes.

Code:

#include <iostream>
struct EDUCBA
{
enum{DS, DA, DE} tag;
union
{
char M;
int N;
double O;
};
};
void display(const EDUCBA& P)
{
switch(P.tag)
{
case EDUCBA::DS: std::cout << P.M << '\n'; break;
case EDUCBA::DA: std::cout << P.N << '\n'; break;
case EDUCBA::DE: std::cout << P.O << '\n'; break;
}
}
int main()
{
EDUCBA P = {EDUCBA::DS, 'Z'};
display(P);
P.tag = EDUCBA::DA;
P.N = 987;
display(P);
}

Output:

Conclusion

On the basis of this article, we understood the concept of Union in the language C++. This article explains the advantage of using Union in C++ and also explains the ways to use Union according to the different requirements of the program. This article will simplify the concept of Union for all the coders.

Recommended Articles

This is a guide to C++ Union. Here we discuss the introduction and Working of Union in C++ with Examples along with code implementation respectively. You may also have a look at the following articles to learn more –

1. C++ static_cast
2. C++ thread_local
3. C++ Static
4. Reflection in C++