Structure vs union comparison - Visual Side-by-Side Comparison

Concept Flow - Structure vs union comparison

Declare struct with multiple members

↓

Memory allocated = sum of all members' sizes

↓

Each member has its own memory space

↓

Declare union with multiple members

↓

Memory allocated = size of largest member

↓

All members share the same memory space

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Assign value to one union member

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Reading other union members shows overlapping data

Structures allocate separate memory for each member, unions share the same memory for all members.

Execution Sample

C

struct Example {
  int a;
  float b;
};

union ExampleU {
  int a;
  float b;
};

Defines a struct with two members and a union with the same two members.

Execution Table

Step	Action	Memory Allocation	Member Values	Notes
1	Declare struct Example	Size = sizeof(int) + sizeof(float)	a = uninitialized, b = uninitialized	Struct allocates separate memory for each member
2	Assign struct.a = 10	Same as above	a = 10, b = uninitialized	Value stored in struct.a's memory
3	Assign struct.b = 3.14	Same as above	a = 10, b = 3.14	Value stored in struct.b's memory
4	Declare union ExampleU	Size = max(sizeof(int), sizeof(float))	a = uninitialized, b = uninitialized	Union allocates shared memory for all members
5	Assign union.a = 10	Same as above	a = 10, b = overlapping bits of 10	Value stored in shared memory
6	Read union.b	Same as above	a = 10, b = value from same memory bits as a	Reading b shows overlapping data, may be garbage or interpreted bits
7	Assign union.b = 3.14	Same as above	a = bits of 3.14, b = 3.14	Value stored in shared memory, overwriting a
8	Read union.a	Same as above	a = value from bits of 3.14, b = 3.14	Reading a shows overlapping data, may be garbage or interpreted bits
9	End	-	-	Execution ends

💡 Program ends after demonstrating memory differences between struct and union

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	After Step 6	After Step 7	After Step 8	Final
struct.a	uninitialized	10	10	-	-	-	-	10
struct.b	uninitialized	uninitialized	3.14	-	-	-	-	3.14
union.a	uninitialized	-	-	10	10	bits of 3.14	bits of 3.14	bits of 3.14
union.b	uninitialized	-	-	overlapping bits of 10	overlapping bits of 10	3.14	3.14	3.14

Key Moments - 3 Insights

Why does assigning to union.a affect union.b's value?

Because union members share the same memory, changing one member overwrites the shared space, affecting all members (see steps 5 and 6 in execution_table).

Why can struct.a and struct.b hold different values at the same time?

Struct allocates separate memory for each member, so each can store independent values simultaneously (see steps 2 and 3 in execution_table).

What happens when you read a union member different from the last assigned one?

You read the same memory bits interpreted as that member's type, which may produce unexpected or garbage values (see steps 6 and 8 in execution_table).

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what are the values of struct.a and struct.b?

A a = 10, b = 3.14

B a = 3.14, b = 10

C a = uninitialized, b = 3.14

D a = 10, b = uninitialized

💡 Hint

Check the 'Member Values' column at step 3 in execution_table

At which step does union.b first get a meaningful assigned value?

A Step 5

B Step 7

C Step 6

D Step 8

💡 Hint

Look for when union.b is explicitly assigned a value in execution_table

If you assign union.b = 5.5 at step 7 instead of 3.14, what happens to union.a at step 8?

A union.a holds 5.5 as int

B union.a remains unchanged

C union.a holds bits of 5.5 interpreted as int

D union.a becomes zero

💡 Hint

Recall union members share memory; see variable_tracker for union.a after step 7

Concept Snapshot

Structure vs Union in C:
- struct allocates separate memory for each member.
- union allocates shared memory equal to largest member.
- struct members hold independent values simultaneously.
- union members share memory; last write affects all.
- Reading different union member than last written may show unexpected data.

Full Transcript

This visual execution compares structures and unions in C. Structures allocate separate memory for each member, so each can hold independent values at the same time. Unions allocate shared memory equal to the largest member, so all members share the same space. Assigning a value to one union member overwrites the shared memory, affecting all members. Reading a union member different from the last assigned one may produce unexpected values because the bits are interpreted differently. The execution table shows step-by-step how values are assigned and how memory is allocated differently for struct and union. The variable tracker follows the changes in member values after each step. Key moments clarify common confusions about memory sharing and value overwriting in unions versus independent storage in structures.