Structure vs Union vs Class in C++

Structure vs Union vs Class in C++

Basis	Structure	Union	Class
Keyword	struct	union	class
Purpose	Groups related data and functions	Saves memory by sharing memory among members	Implements objects and OOP concepts
Default Access Specifier	public	public	private
Memory Allocation	Separate memory for each data member	Same memory is shared by all data members	Separate memory for non-static data members
Values of Members	All members can hold values simultaneously	Normally, only one member is the active member at a time	All members can hold values simultaneously
Member Functions	Yes	Yes	Yes
Encapsulation	Supported	Limited	Strongly supported
Inheritance	Supported	Not supported	Supported
Polymorphism	Supported	Not supported through inheritance	Supported
Data Hiding	Possible using private	Possible using access specifiers	Available and commonly used
Object Creation	Student s;	Data d;	Student s;
Memory Requirement	Generally greater than a union	Generally equal to the size needed for the largest member, considering alignment	Depends on data members and alignment/padding
Main Advantage	Simple grouping of related data	Efficient memory utilization	Data abstraction and OOP
Main Limitation	Public by default	Members cannot ordinarily be used simultaneously as independent stored values	Private by default may require access functions
Typical Example	Student record	Store either int, float, or char	Student/Employee/Bank Account object
Basic Syntax	struct A { int x; };	union A { int x; float y; };	class A { int x; };

Syntax Comparison

// Structure
struct Student
{
    int rollNo;
    string name;
};

// Union
union Data
{
    int i;
    float f;
};

// Class
class Student
{
private:
    int rollNo;

public:
    void display()
    {
        cout << rollNo;
    }
};

One Program Demonstrating All Three

#include <iostream>
using namespace std;

// Structure
struct StudentStruct
{
    int rollNo;
};

// Union
union Data
{
    int i;
    float f;
};

// Class
class StudentClass
{
private:
    int rollNo;

public:
    void setRollNo(int r)
    {
        rollNo = r;
    }

    void display()
    {
        cout << "Class Roll No: " << rollNo << endl;
    }
};

int main()
{
    // Structure
    StudentStruct s;
    s.rollNo = 101;

    // Union
    Data d;
    d.i = 50;

    // Class
    StudentClass c;
    c.setRollNo(102);

    cout << "Structure Roll No: " << s.rollNo << endl;
    cout << "Union Integer: " << d.i << endl;
    c.display();

    return 0;
}

Output:

Structure Roll No: 101
Union Integer: 50
Class Roll No: 102

 Most Important Exam Points

Structure	Union	Class
struct	union	class
Public by default	Public by default	Private by default
Separate memory	Shared memory	Separate memory
All members can be used together	One active member at a time	All members can be used together
Supports OOP features	Limited	Strong OOP support

One-line memory trick:
Structure = Grouping, Union = Memory Sharing, Class = Encapsulation + OOP.