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).

Practice

1. What is the main characteristic of a union in C++?

easy

A. All members share the same memory location

B. Each member has its own separate memory

C. Unions can only store integers

D. Unions automatically initialize all members

Solution

1. Step 1: Understand union memory sharing

   A union allocates one shared memory block for all its members, so they overlap.

2. Step 2: Compare with other data structures

   Unlike structs, unions do not allocate separate memory for each member.

3. Final Answer:

   All members share the same memory location -> Option A

4. Quick Check:

   Union memory sharing = All members share the same memory location [OK]

Hint: Remember: union members overlap in memory [OK]

Common Mistakes:

• Thinking each member has separate memory
• Assuming unions initialize all members
• Believing unions only store one data type

2. Which of the following is the correct syntax to define a union named Data with an int and a float member?

easy

A. union Data = { int i; float f; };

B. struct Data { int i; float f; };

C. union Data ( int i; float f; );

D. union Data { int i; float f; };

Solution

1. Step 1: Recall union declaration syntax

   Unions are declared using the keyword union, followed by the name and braces with members inside.

2. Step 2: Check each option

   union Data { int i; float f; }; uses correct syntax with braces and semicolons. Options B, C, and D have syntax errors or wrong keywords.

3. Final Answer:

   union Data { int i; float f; }; -> Option D

4. Quick Check:

   Correct union syntax = union Data { int i; float f; }; [OK]

Hint: Use 'union Name { members; };' syntax [OK]

Common Mistakes:

• Using struct keyword instead of union
• Using parentheses instead of braces
• Assigning values in declaration

3. What will be the output of the following code?

union Number {
  int i;
  float f;
};

Number n;
n.i = 65;
std::cout << n.f << std::endl;

medium

A. A floating-point number representing the bits of 65

B. 65.0

C. 0

D. Compilation error

Solution

1. Step 1: Understand union memory sharing and type punning

   Assigning to n.i sets the bits of the union. Reading n.f interprets those bits as a float.

2. Step 2: Predict output

   The bits for integer 65 do not represent 65.0 as a float, so output is a float number corresponding to those bits, not 65.0.

3. Final Answer:

   A floating-point number representing the bits of 65 -> Option A

4. Quick Check:

   Union type punning = A floating-point number representing the bits of 65 [OK]

Hint: Union shares bits; int bits read as float show strange value [OK]

Common Mistakes:

• Expecting output 65.0
• Assuming union members hold independent values
• Thinking code causes compile error

4. Identify the error in this union usage:

union Data {
  int i;
  char c;
};

Data d;
d.i = 100;
d.c = 'A';
std::cout << d.i << std::endl;

medium

A. Output will be 100

B. Error: Cannot assign to union members separately

C. No error; output will be 65

D. Compilation error due to missing semicolon

Solution

1. Step 1: Understand union member assignment

   Assigning to d.i then to d.c overwrites the same memory because union members share memory.

2. Step 2: Determine final value of d.i

   After d.c = 'A', the first byte of d.i is set to ASCII 65, so printing d.i shows 65.

3. Final Answer:

   No error; output will be 65 -> Option C

4. Quick Check:

   Union last write wins = No error; output will be 65 [OK]

Hint: Last assigned union member overwrites memory [OK]

Common Mistakes:

• Thinking union members hold values independently
• Expecting output 100 after assigning 'A'
• Assuming syntax error

5. You want to store either an int, a float, or a char in the same memory space efficiently. Which union definition and usage correctly allows this, and how do you safely read the stored value?

hard

A. union Value { int i; float f; char c; }; Value v; v.i = 10; std::cout << v.c << std::endl;

B. union Value { int i; float f; char c; }; Value v; v.f = 3.14f; std::cout << v.f << std::endl;

C. union Value { int i; float f; char c; }; Value v; v.c = 'Z'; std::cout << v.i << std::endl;

D. union Value { int i; float f; char c; }; Value v; std::cout << v.i << std::endl;

Solution

1. Step 1: Check union definition and assignment

   union Value { int i; float f; char c; }; Value v; v.f = 3.14f; std::cout << v.f << std::endl; defines a union with int, float, char and assigns a float value correctly.

2. Step 2: Verify safe reading of stored value

   union Value { int i; float f; char c; }; Value v; v.f = 3.14f; std::cout << v.f << std::endl; reads the same member v.f that was assigned, ensuring correct interpretation.

3. Step 3: Analyze other options

   Other options either assign to one member and read a different one, leading to unexpected values, or read uninitialized memory.

4. Final Answer:

   union Value { int i; float f; char c; }; Value v; v.f = 3.14f; std::cout << v.f << std::endl; -> Option B

5. Quick Check:

   Read the same union member you wrote [OK]

Hint: Always read the union member you last wrote [OK]

Common Mistakes:

• Reading a different member than assigned
• Using uninitialized union members
• Assuming union members hold independent values