Structure vs union comparison in C++

Introduction

Structures and unions help group different data types together. They let you keep related information in one place.

When you want to store multiple related values together, like a person's name and age.

When you need to save memory by sharing space for different data types, but only use one at a time.

When you want to organize data clearly for easier understanding and maintenance.

When you want to pass multiple values as one unit to functions.

When you want to model real-world objects with different properties.

Syntax

C++

struct StructName {
    type1 member1;
    type2 member2;
    // more members
};

union UnionName {
    type1 member1;
    type2 member2;
    // more members
};

A structure stores all members separately, each with its own space.

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

Examples

This structure stores a name and age separately for each person.

C++

struct Person {
    char name[20];
    int age;
};

This union can hold an int, a float, or a string, but only one at a time.

C++

union Data {
    int i;
    float f;
    char str[20];
};

Sample Program

This program shows how a structure keeps all data separately, while a union shares memory for all members. Changing one union member affects others.

C++

#include <iostream>
#include <cstring>

struct Person {
    char name[20];
    int age;
};

union Data {
    int i;
    float f;
    char str[20];
};

int main() {
    Person p1;
    strcpy(p1.name, "Alice");
    p1.age = 30;

    std::cout << "Structure Person:\n";
    std::cout << "Name: " << p1.name << "\n";
    std::cout << "Age: " << p1.age << "\n\n";

    Data d;
    d.i = 10;
    std::cout << "Union Data after setting int: " << d.i << "\n";
    d.f = 220.5f;
    std::cout << "Union Data after setting float: " << d.f << "\n";
    strcpy(d.str, "Hello");
    std::cout << "Union Data after setting string: " << d.str << "\n";

    std::cout << "Union Data int now: " << d.i << " (may be garbage)\n";

    return 0;
}

Important Notes

Structures use more memory because each member has its own space.

Unions save memory but only one member can be used at a time.

Accessing a union member different from the last one set can cause unexpected values.

Summary

Structures store all members separately and keep all values.

Unions share memory among members and only one value is valid at a time.

Use structures for grouping related data, and unions to save memory when only one value is needed at a time.

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