What are reference data types?

Reference data types store not the actual value, but a reference (memory address) to the location where the value is stored.

Features:

• Variables of reference type can hold a null reference.
• When assigning the value of one reference type variable to another, the reference is copied, not the actual value. Both variables then point to the same memory location.
• Changes made through one reference will be visible through the other if they point to the same object.
• Memory for reference type objects is managed by the garbage collector.

Examples of reference data types:

• Strings (String)
• Arrays
• Objects of user-defined classes
• Delegates (in some languages)

Difference from value types:

Characteristic	Value types	Reference types
Storage	The actual value	A reference to the value
Copying	The actual value is copied	The reference is copied
Modification	Changes in the copy do not affect the original	Changes through one reference are visible through another (if they point to the same object)
Memory	Managed by the stack (usually)	Managed by the heap, cleaned up by the garbage collector
Examples	int, float, bool, struct (in some languages)	String, Array, Object

// Example in C#
string s1 = "Hello"; // s1 holds a reference to the string "Hello"
string s2 = s1;      // s2 now holds the same reference as s1

// Changes will not be visible through the other reference,
// because strings in C# are immutable.
// When changed, a new string is created and the reference is reassigned.
s1 = "World"; 

Console.WriteLine(s2); // Outputs "Hello"

int[] arr1 = { 1, 2, 3 }; // arr1 holds a reference to the array {1, 2, 3}
int[] arr2 = arr1;     // arr2 now holds the same reference

arr2[0] = 10;          // Change through arr2

Console.WriteLine(arr1[0]); // Outputs 10, change is visible through arr1

// Example in Java
String s1 = "Hello"; // s1 holds a reference to the string "Hello"
String s2 = s1;      // s2 now holds the same reference as s1

// Strings in Java are immutable.
// When changed, a new string is created and the reference is reassigned.
s1 = "World"; 

System.out.println(s2); // Outputs "Hello"

int[] arr1 = { 1, 2, 3 }; // arr1 holds a reference to the array {1, 2, 3}
int[] arr2 = arr1;     // arr2 now holds the same reference

arr2[0] = 10;          // Change through arr2

System.out.println(arr1[0]); // Outputs 10, change is visible through arr1