C Union

Summary: in this tutorial, you’ll learn about a new type called C union and how to use it effectively in your program.

What is a C union? #

A structure allows you to define a new data type with multiple related fields. Each field takes up a separate storage location. For example:

struct point
{
    int x;
    int y;
};Code language: C++ (cpp)

The point structure has two fields x-coordinate and y-coordinate. Each takes up a separate space in the memory.

A union is similar to a structure. However, it defines a single location to store values of different fields at a single point in time.

union quantity {
    int i_value;
    float f_value;
}Code language: C++ (cpp)

In this quantity union, the i_value and f_value fields share the same memory location.

The following picture illustrates the difference between a structure and a union:

By definition, a union is a type that stores different values in the same memory location but not simultaneously. It is a group of data objects that share a single block of memory.

C union syntax #

The syntax of defining a union is similar to the syntax of defining a structure type. The following illustrates the syntax for defining a union:

union union_name
{
    type field_name;
    type field_name;
    //...
};Code language: C++ (cpp)

In this syntax:

• First, start with the union keyword followed by the union name.
• Second, specify the fields with types.

To access a member of a union, you use the ( .) operator like this:

union_name.fieldCode language: C++ (cpp)

Union vs. Structure #

In a structure, each field stores data separately. If you change the value of one field of a structure, the values of the other fields do not change.

However, all the fields share the same memory block in a union. This memory block is big enough to store the value of the largest field. Smaller members use as much memory as necessary. If you change a field’s value, other fields’ values also change.

If you need to store data in members simultaneously, use a structure.

Initialize a union #

C allows you to initialize a union in two ways:

1. Initialize a union by initializing the first member of a union.
2. Or initialize a union by assigning it to another union with the same type.

The following program demonstrates how to initialize a union in both ways.

#include <stdio.h>
#include <stdlib.h>

int main()
{
    typedef union
    {
        char ch;
        int flag;
        float f;
    } data;

    data d;
    d.ch = 'A';

    // initialize one union to another
    data d2 = d;

    // initialize first member of union
    data d3 = {'B'};

    return 0;
}Code language: C++ (cpp)

C Union Example #

In this example, we have an account structure that could be personal or business account based on the account_type enumeration. If it is a personal account the info member is associated with the person structure, otherwise, it is associated with the  company structure:

#include <stdio.h>
#include <stdlib.h>

// define account type: personal or business
enum account_type
{
    personal = 1,
    business = 2
};

// person name
struct person
{
    char *name;
};

// company with name and tax no
struct company
{
    char *name;
    char *tax_no;
};

// profile
typedef union
{
    struct person individual;
    struct company biz;
} profile;

// account
typedef struct
{
    char *username;
    char *password;
    enum account_type type;
    profile info;
} account;

void display(account acc);

int main()
{

    printf("Union Demo\n");

    account acc1, acc2;

    acc1.type = personal;
    acc1.username = "acc1";
    acc1.password = "secret";
    acc1.info.individual.name = "John Doe";

    display(acc1);

    acc2.type = business;
    acc2.username = "acc2";
    acc2.password = "secret2";
    acc2.info.biz.name = "My Company";
    acc2.info.biz.tax_no = "112121";

    display(acc2);

    return 0;
}

/*
   displays account on the screen
*/
void display(account acc)
{

    switch (acc.type)
    {
    case personal:
        printf("Personal Account\n");
        printf("Username:%s\nName:%s\n", acc.username,
               acc.info.individual.name);
        break;
    case business:
        printf("Business Account\n");
        printf("Username:%s\nCompany:%s\nTax no.:%s\n", acc.username,
               acc.info.biz.name,
               acc.info.biz.tax_no);
        break;
    }
    printf("-------------------------------\n");
}Code language: C++ (cpp)

In this tutorial, you have learned how to use a C union and understand the differences between a union and a structure.