Why is using a reference safer for protection against modifications compared to a pointer?

A reference must always refer to an existing object and cannot be nullptr. A pointer can be nullptr or point to invalid memory. Dereferencing a nullptr or invalid pointer results in undefined behavior (often a segmentation fault), which is harder to detect. A reference guarantees that you are working with a valid object.

// Using a pointer
int* ptr = nullptr;
// Dereferencing attempt may lead to crash
// *ptr = 10; 

// Using a reference
int value = 5;
int& ref = value; // Reference is initialized to an existing object
// *ref = 10; // Incorrect syntax - ref is already an alias for value
ref = 10; // Changes value through the reference - safe

// A reference cannot be `nullptr`
// int& invalid_ref = nullptr; // Compilation error

A reference must be initialized upon declaration, pointing to a specific object, and cannot be reseated to refer to another object. A pointer can be changed to point to different objects or nullptr.

// Using a pointer
int a = 1;
int b = 2;
int* ptr = &a; // Pointer to a
ptr = &b;    // Pointer now points to b

Main differences in terms of safety:

Capability	Reference	Pointer
Be nullptr	No	Yes
Reseating	No	Yes
Dereferencing nullptr	Impossible	Undefined behavior
Use without initialization	Impossible (compile error)	Possible (undefined behavior when dereferencing)