Union basics in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of union member assignment

What is the output of this C++ code snippet?

C++

#include <iostream>
union Data {
    int i;
    float f;
};

int main() {
    Data d;
    d.i = 65;
    std::cout << d.i << " " << d.f << std::endl;
    return 0;
}

A Compilation error

B 65 65

C 0 0

D 65 9.1e-44

💡 Hint

Remember that union members share the same memory space.

Assigning to one member of a union changes the shared memory. Reading another member interprets the same bits differently, so the float value is a strange small number.

❓ Predict Output

intermediate

Size of union vs struct

What is the output of this C++ code?

C++

#include <iostream>
union U {
    int i;
    double d;
};
struct S {
    int i;
    double d;
};
int main() {
    std::cout << sizeof(U) << " " << sizeof(S) << std::endl;
    return 0;
}

A 12 12

B 16 8

C 8 16

D 8 8

💡 Hint

Union size is the size of its largest member; struct size is sum plus padding.

Union size equals the largest member size (double = 8 bytes). Struct size is sum of members plus padding (int 4 + padding 4 + double 8 = 16).

🔧 Debug

advanced

Why does this union code cause undefined behavior?

Consider this code snippet. What is the main reason it causes undefined behavior?

C++

#include <iostream>
union U {
    int i;
    float f;
};
int main() {
    U u;
    u.i = 10;
    std::cout << u.f << std::endl;
    u.f = 3.14f;
    std::cout << u.i << std::endl;
    return 0;
}

A Reading a different union member than the one last written causes undefined behavior.

B Union members cannot be assigned values directly.

C The union must have a constructor to be used.

D The code is missing a return statement.

💡 Hint

Think about what happens when you read a union member different from the last one written.

In C++, reading a union member other than the one most recently written is undefined behavior, even if it compiles and runs.

🧠 Conceptual

advanced

Union member initialization in C++17 and later

Which statement about union member initialization in C++17 and later is correct?

A Unions must be initialized using a constructor only.

B You can initialize a union member directly using an in-class initializer.

C Only the first member of a union can be initialized at declaration.

D Unions cannot have any member initialized at declaration.

💡 Hint

C++17 introduced changes to allow in-class member initializers in unions.

Since C++17, unions can have members with in-class initializers, allowing direct initialization of union members.

❓ Predict Output

expert

Output of union with anonymous struct and bitfields

What is the output of this C++ code?

C++

#include <iostream>
union U {
    struct {
        unsigned int a : 3;
        unsigned int b : 5;
    } bits;
    unsigned char byte;
};
int main() {
    U u{};
    u.byte = 0b10101100;
    std::cout << u.bits.a << " " << u.bits.b << std::endl;
    return 0;
}

A 4 21

B 4 22

C 5 21

D 5 22

💡 Hint

Bitfields pack bits starting from least significant bit; check binary representation carefully.

The byte 0b10101100 has bits: 1 0 1 0 1 1 0 0 (MSB to LSB). bits.a uses 3 LSB bits (100 binary = 4 decimal), bits.b uses next 5 bits (10101 binary = 21 decimal).