What is a reference in C++?

A reference is an alias (alternative name) for an existing object.

Main characteristics:

• Initialization: Must be initialized at the time of declaration.
• Immutability: After initialization, the reference cannot be changed to refer to another object.
• No null: Cannot be "null," always refers to a valid object.
• Cannot be reseated: Assigning a new value through the reference changes the object it refers to, not the reference itself.
• Used in parameter passing: Often used to pass objects by reference in functions to avoid copying and to modify the original object.

Example:

int x = 10;
int& ref = x; // ref is a reference to x

ref = 20;     // Changes the value of x
// x is now 20

Comparison with pointers:

Characteristic	Reference	Pointer
Initialization	Mandatory	Not mandatory
Null value	Impossible	Possible (nullptr)
Reseating	Impossible	Possible
Access syntax	Like a normal object	Dereferencing (*)
Memory allocation	Does not occupy its own memory (alias)	Occupies memory to store address

References provide a safer and more convenient alternative to pointers in many cases, especially when passing arguments by reference or returning references from functions.