Union basics in C++ - Step-by-Step Execution

Concept Flow - Union basics

Declare union variable

↓

Assign value to one member

↓

Access member value

↓

Note: all members share same memory

↓

Assign value to another member

↓

Access members again

↓

Observe that last assignment overwrites previous

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

Execution Sample

C++

union Data {
  int i;
  float f;
};

Data d;
d.i = 10;
float x = d.f;

This code declares a union with int and float, assigns 10 to int member, then reads float member.

Execution Table

Step	Action	Member Assigned/Read	Value Stored	Value Read	Note
1	Declare union variable d	-	-	-	Memory allocated for largest member (float or int)
2	Assign d.i = 10	i	10 (int bits)	-	int member set, memory holds int bits for 10
3	Read d.i	i	10	10	Reading int member returns 10
4	Read d.f	f	10 (int bits)	1.4e-44 (float)	Reading float member interprets int bits as float
5	Assign d.f = 5.5	f	5.5 (float bits)	-	float member set, overwrites memory
6	Read d.f	f	5.5	5.5	Reading float member returns 5.5
7	Read d.i	i	5.5 (float bits)	1085276160 (int)	Reading int member interprets float bits as int
8	End	-	-	-	Program ends

💡 Program ends after demonstrating union member assignments and reads

Variable Tracker

Variable	Start	After Step 2	After Step 5	Final
d.i	undefined	10	bit pattern of 5.5 float	bit pattern of 5.5 float
d.f	undefined	bit pattern of 10 int	5.5	5.5

Key Moments - 3 Insights

Why does reading d.f after assigning d.i give a strange number?

Because d.i and d.f share the same memory, reading d.f interprets the int bits as a float (see execution_table step 4).

What happens when we assign d.f after assigning d.i?

The float assignment overwrites the shared memory, so the int value is lost (see execution_table step 5).

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

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

Visual Quiz - 3 Questions

Look at the execution_table, what value does d.f hold immediately after step 2?

A 5.5 as a float

B 10 as a float (1.4e-44)

C 10 as an int

D Undefined

💡 Hint

Check step 4 in execution_table where d.f reads the bits of d.i = 10

At which step does the union memory get overwritten with a float value?

A Step 3

B Step 2

C Step 5

D Step 7

💡 Hint

Look at the Action column for assignment to d.f in execution_table

If we read d.i after step 5, what happens?

A We get the int bits of 5.5 float interpreted as int

B We get the int 10

C We get 5.5 as int

D Undefined behavior

💡 Hint

See step 7 in execution_table for reading d.i after assigning d.f

Concept Snapshot

union Data {
  int i;
  float f;
};
- All members share the same memory.
- Assigning one member overwrites others.
- Reading a different member interprets the same bits differently.

Full Transcript

This visual trace shows how a union in C++ shares memory between its members. We declare a union with int and float members. Assigning to one member stores bits in shared memory. Reading another member interprets those bits differently. For example, assigning 10 to the int member and then reading the float member shows a strange float value because the bits represent an int 10, not a float 10. Assigning a float value overwrites the memory, so reading the int member then shows the int interpretation of the float bits. This demonstrates that only one member's value is valid at a time in a union.