Union basics - Step-by-Step Execution

Concept Flow - Union basics

Declare union variable

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Assign value to one member

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Access value via any member

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Value reflects last assignment

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Memory shared among members

A union lets you store different data types in the same memory space, but only one at a time. Assigning to one member changes the shared memory.

Execution Sample

C

union Data {
  int i;
  float f;
};

union Data d;
d.i = 10;

This code declares a union with int and float, creates a variable, and assigns 10 to the int member.

Execution Table

Step	Action	Member Accessed	Value Stored	Memory State (shared)
1	Declare union variable d	N/A	N/A	Uninitialized
2	Assign 10 to d.i	i	10	Memory holds int 10
3	Access d.i	i	10	Memory holds int 10
4	Access d.f	f	Bit pattern of int 10 as float	Same memory, interpreted as float

💡 Execution stops after showing union memory shared and value interpretation

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4
d.i	undefined	10	10	10
d.f	undefined	bit pattern of 10 as float	bit pattern of 10 as float	bit pattern of 10 as float

Key Moments - 2 Insights

Why does accessing d.f after assigning d.i show a strange float value?

Because union members share the same memory, the bits of the int 10 are reinterpreted as a float, not converted. See execution_table step 4.

Can we store int and float values at the same time in a union?

No, only one member's value is stored at a time since all members share the same memory space.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what value is stored in d.i?

A Undefined

B 0

C 10

D Bit pattern of float

💡 Hint

Check the 'Value Stored' column at step 2 in execution_table

At which step does the union memory hold the int value 10?

A Step 2

B Step 4

C Step 1

D Never

💡 Hint

Look at the 'Memory State' column in execution_table

If we assign a float value to d.f after step 2, what happens to d.i?

A d.i keeps the old int value

B d.i value is overwritten by float bits

C d.i becomes the float value

D d.i and d.f store values independently

💡 Hint

Recall union shares memory; assigning to one member overwrites others

Concept Snapshot

union TypeName {
  type1 member1;
  type2 member2;
};

- All members share the same memory space.
- Only one member holds a valid value at a time.
- Assigning to one member overwrites others.
- Useful to save memory or interpret data differently.

Full Transcript

This visual trace shows how a union in C works. First, we declare a union with int and float members. Then we create a variable of that union type. When we assign 10 to the int member, the union's shared memory holds that int value. Accessing the int member returns 10 as expected. However, accessing the float member reads the same memory bits but interprets them as a float, resulting in a strange float value. This happens because all union members share the same memory space, so only one value is stored at a time. Assigning to one member overwrites the memory for all members. This helps save memory and allows different views of the same data.