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.

Practice

1. Which of the following best describes the main difference between a struct and a union in C++?

easy

A. A struct stores all members separately; a union shares memory among members.

B. A struct shares memory among members; a union stores all members separately.

C. Both struct and union store members separately but differ in syntax.

D. Both struct and union share memory but differ in member access.

Solution

1. Step 1: Understand memory allocation in struct

   A struct allocates separate memory for each member, so all members can hold values simultaneously.

2. Step 2: Understand memory allocation in union

   A union shares the same memory space for all members, so only one member can hold a valid value at a time.

3. Final Answer:

   A struct stores all members separately; a union shares memory among members. -> Option A

4. Quick Check:

   Struct = separate memory, Union = shared memory [OK]

Hint: Struct = all values stored; Union = one value stored [OK]

Common Mistakes:

• Thinking union stores all members separately
• Confusing struct and union memory sharing
• Assuming both have same memory behavior

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

easy

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

B. union 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 syntax

   The correct syntax uses the keyword union, followed by the name, and braces enclosing members separated by semicolons.

2. Step 2: Check each option

   union Data { int i; float f; }; matches correct syntax. struct Data { int i; float f; }; uses struct, not union. union Data ( int i; float f; ); uses parentheses instead of braces. union Data = { int i; float f; }; uses an assignment which is invalid.

3. Final Answer:

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

4. Quick Check:

   Union syntax = keyword + braces + members [OK]

Hint: Use braces {} with union keyword for definition [OK]

Common Mistakes:

• Using struct keyword instead of union
• Using parentheses instead of braces
• Trying to assign values in definition

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

union Number {
  int i;
  float f;
};

int main() {
  Number n;
  n.i = 65;
  std::cout << n.f << std::endl;
  return 0;
}

medium

A. A floating-point number representing the bit pattern of 65

B. 0

C. 65

D. Compilation error

Solution

1. Step 1: Understand union memory sharing

   Assigning 65 to n.i stores the integer bits in the shared memory.

2. Step 2: Interpret bits as float

   Printing n.f reads the same bits as a float, resulting in a floating-point number representing the bit pattern of 65, not the integer 65.

3. Final Answer:

   A floating-point number representing the bit pattern of 65 -> Option A

4. Quick Check:

   Union shares bits; reading different type shows bit pattern [OK]

Hint: Union members share bits; reading different type shows bit pattern [OK]

Common Mistakes:

• Expecting integer 65 to print as float 65.0
• Assuming union stores separate copies
• Thinking code causes compilation error

4. Identify the error in the following code snippet:

struct Data {
  int x;
  union {
    int i;
    float f;
  } u;
};

int main() {
  Data d;
  d.x = 10;
  d.u.i = 20;
  d.u.f = 3.14;
  std::cout << d.x << ", " << d.u.i << std::endl;
  return 0;
}

medium

A. Cannot assign float to union member.

B. Accessing union member after assigning another member leads to undefined value.

C. Missing semicolon after union definition.

D. Struct cannot contain a union as a member.

Solution

1. Step 1: Analyze union member assignments

   Assigning d.u.i = 20; then d.u.f = 3.14; overwrites the same memory because union members share memory.

2. Step 2: Understand reading union members

   Reading d.u.i after assigning d.u.f gives an undefined or unexpected value because only the last assigned member is valid.

3. Final Answer:

   Accessing union member after assigning another member leads to undefined value. -> Option B

4. Quick Check:

   Union stores one value; reading old member is invalid [OK]

Hint: Only last assigned union member holds valid data [OK]

Common Mistakes:

• Thinking struct cannot have union member
• Ignoring union memory sharing rules
• Expecting both union members to hold values simultaneously

5. You want to store either an integer ID or a floating-point score in a variable but never both at the same time, and you want to save memory. Which is the best approach in C++?

hard

A. Use a class with private int and float members.

B. Use a struct with both int and float members.

C. Use two separate variables, one int and one float.

D. Use a union with int and float members.

Solution

1. Step 1: Understand memory needs

   Since only one value (int or float) is needed at a time, memory can be saved by sharing storage.

2. Step 2: Choose union for shared memory

   A union allows storing either int or float in the same memory space, saving memory compared to a struct which stores both separately.

3. Step 3: Evaluate other options

   Using two separate variables wastes memory. A class with private members behaves like struct in memory. Struct stores both values simultaneously, using more memory.

4. Final Answer:

   Use a union with int and float members. -> Option D

5. Quick Check:

   Union saves memory when only one value needed [OK]

Hint: Use union to save memory when storing one of many types [OK]

Common Mistakes:

• Choosing struct and wasting memory
• Using separate variables unnecessarily
• Confusing class with memory optimization