Union basics

Introduction

A union lets you store different types of data in the same memory space, but only one at a time.

When you want to save memory by sharing space for different data types.

When a variable can hold different types of values but only one at a time.

When you need to interpret the same data in multiple ways.

When working with hardware or protocols that use the same bytes for different meanings.

Syntax

C

union UnionName {
    type1 member1;
    type2 member2;
    // more members
};

All members share the same memory location.

The size of the union is the size of its largest member.

Examples

This union can hold either an int or a float, but not both at the same time.

C

union Data {
    int i;
    float f;
};

This union can store a char, int, or double, sharing the same memory space.

C

union Value {
    char c;
    int i;
    double d;
};

Sample Program

This program shows how a union stores either an int or a float in the same place. When we change the float, the int value changes because they share memory.

C

#include <stdio.h>

union Number {
    int i;
    float f;
};

int main() {
    union Number num;

    num.i = 42;
    printf("Integer: %d\n", num.i);

    num.f = 3.14f;
    printf("Float: %.2f\n", num.f);

    // Note: after assigning to num.f, num.i value is overwritten
    printf("Integer after float assignment: %d\n", num.i);

    return 0;
}

Important Notes

Only one member of a union can hold a meaningful value at a time.

Changing one member overwrites the others because they share memory.

Use unions carefully to avoid confusing bugs.

Summary

Unions share memory for all members, saving space.

Only one member can be used at a time.

Useful for memory-efficient storage and interpreting data in different ways.

Practice

1. What is the main characteristic of a union in C?

easy

A. Unions can only store integers

B. Each member has its own separate memory

C. All members share the same memory location

D. Unions automatically initialize all members

Solution

1. Step 1: Understand union memory sharing

   A union allocates one shared memory block for all its members, so they overlap.

2. Step 2: Compare with struct memory allocation

   Unlike structs, unions do not allocate separate memory for each member.

3. Final Answer:

   All members share the same memory location -> Option C

4. Quick Check:

   Union memory sharing = All members share the same memory location [OK]

Hint: Remember: union members overlap in memory [OK]

Common Mistakes:

• Thinking each member has separate memory
• Assuming unions initialize all members
• Believing unions only store one data type

2. Which of the following is the correct way to define a union named Data with an int and a float member?

easy

A. union Data { int i; float f; };

B. struct Data { int i; float f; };

C. union Data ( int i; float f; );

D. union Data = { int i; float f; };

Solution

1. Step 1: Recall union syntax

   The correct syntax uses the keyword 'union', the name, and braces with members inside.

2. Step 2: Check each option

   union Data { int i; float f; }; uses correct syntax with braces and semicolons. Options A, B, and C have syntax errors or wrong keywords.

3. Final Answer:

   union Data { int i; float f; }; -> Option A

4. Quick Check:

   Correct union syntax = union Data { int i; float f; }; [OK]

Hint: Use 'union Name { members; };' syntax [OK]

Common Mistakes:

• Using struct instead of union
• Using parentheses instead of braces
• Assigning values in the definition

3. What will be the output of the following C code?

union Number {
  int i;
  float f;
};

int main() {
  union Number num;
  num.i = 65;
  printf("%c", num.f);
  return 0;
}

medium

A. A

B. 65

C. Compilation error

D. Undefined behavior or garbage character

Solution

1. Step 1: Understand union member sharing

   Setting num.i = 65 stores integer bits in union memory.

2. Step 2: Printing num.f as char

   Printing num.f with %c treats float bits as a char, causing unpredictable output.

3. Final Answer:

   Undefined behavior or garbage character -> Option D

4. Quick Check:

   Accessing different union member types = Undefined behavior or garbage character [OK]

Hint: Accessing different union members can cause garbage output [OK]

Common Mistakes:

• Expecting integer 65 to print as 'A'
• Assuming union members convert automatically
• Ignoring type mismatch in printf

4. Identify the error in this union usage:

union Data {
  int i;
  float f;
};

int main() {
  union Data d;
  d.i = 10;
  d.f = 3.14;
  printf("%d", d.i);
  return 0;
}

medium

A. Using %d to print a union member is invalid

B. Assigning to d.f overwrites d.i value

C. Cannot assign to multiple union members sequentially

D. Union must be initialized before use

Solution

1. Step 1: Analyze assignments to union members

   Assigning d.i then d.f means d.f overwrites the same memory used by d.i.

2. Step 2: Understand printf output

   Printing d.i after assigning d.f prints the overwritten value, not 10.

3. Final Answer:

   Assigning to d.f overwrites d.i value -> Option B

4. Quick Check:

   Union members share memory, last assignment wins = Assigning to d.f overwrites d.i value [OK]

Hint: Last assigned union member overwrites previous values [OK]

Common Mistakes:

• Thinking union members hold values independently
• Believing %d cannot print union members
• Assuming union needs initialization

5. You want to store either an int, a float, or a char in the same memory space and print the stored value correctly. Which approach using a union and an additional variable is best?

hard

A. Use a union for data and an enum to track the active member type

B. Use a union and print all members every time

C. Use separate variables for each type without union

D. Use a union and always print the int member

Solution

1. Step 1: Understand union limitation

   Union stores one member at a time, so we must know which member is active.

2. Step 2: Use enum to track active member

   An enum variable can track which type is stored, so printing uses correct format.

3. Final Answer:

   Use a union for data and an enum to track the active member type -> Option A

4. Quick Check:

   Union + enum for type tracking = Use a union for data and an enum to track the active member type [OK]

Hint: Track active union member with enum for correct printing [OK]

Common Mistakes:

• Printing all union members blindly
• Not tracking which member is active
• Using separate variables wastes memory