Structure vs union comparison in C++ - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of structure size and member values

What is the output of this C++ code?

C++

#include <iostream>
using namespace std;

struct Data {
    int i;
    char c;
    double d;
};

int main() {
    Data data = {10, 'A', 3.14};
    cout << sizeof(data) << " ";
    cout << data.i << " " << data.c << " " << data.d << endl;
    return 0;
}

A 41.3 A 01 42

B 24 10 A 3.14

C 24 0 A 3.14

D 16 10 A 0

💡 Hint

Remember that structure size includes padding for alignment.

The structure has padding bytes to align members properly. Typically, the size is 24 bytes on a 64-bit system. The values printed are exactly those initialized.

❓ Predict Output

intermediate

Output of union member values after assignment

What is the output of this C++ code?

C++

#include <iostream>
using namespace std;

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

int main() {
    Data data;
    data.i = 10;
    data.c = 'A';
    cout << data.i << " " << data.c << endl;
    return 0;
}

A 65 10

B 10 A

C 10 10

D 65 A

💡 Hint

In a union, all members share the same memory location.

Assigning to data.c overwrites the first byte of data.i. The ASCII value of 'A' is 65, so data.i now starts with 65 in its lowest byte, resulting in 65 when printed as int.

🧠 Conceptual

advanced

Difference in memory allocation between struct and union

Which statement correctly describes the difference in memory allocation between a structure and a union in C++?

A A structure allocates memory for all members separately; a union allocates memory equal to its largest member only.

B A union allocates memory for all members separately; a structure allocates memory equal to its largest member only.

C Both structure and union allocate memory equal to the sum of all members' sizes.

D Both structure and union allocate memory equal to the size of their smallest member.

💡 Hint

Think about how members share memory in a union.

Structures allocate memory for each member separately, so total size is sum plus padding. Unions allocate memory only once, equal to the size of the largest member, since all members share the same memory space.

❓ Predict Output

advanced

Output of modifying union members

What is the output of this C++ code?

C++

#include <iostream>
using namespace std;

union Data {
    int i;
    float f;
};

int main() {
    Data data;
    data.i = 1065353216; // bit pattern for float 1.0
    cout << data.f << endl;
    return 0;
}

A 1

B 0

C 1065353216

D Undefined behavior

💡 Hint

The int value corresponds to the bit pattern of float 1.0.

The integer 1065353216 has the same bit pattern as the float 1.0 in IEEE 754 format. Accessing the float member after setting the int member shows 1.0.

🧠 Conceptual

expert

Why use union over struct in embedded systems?

Why might a programmer prefer using a union instead of a structure in embedded systems programming?

A To ensure each member has its own separate memory for thread safety.

B To allow simultaneous storage of multiple data types for faster access.

C To save memory by overlapping storage of different data types that are never used simultaneously.

D To automatically initialize all members to zero.

💡 Hint

Think about limited memory in embedded devices.

Unions save memory by sharing the same memory space for different members, which is useful when only one member is used at a time. Structures allocate separate memory for each member, which uses more memory.