C++: What is the difference between pointer and reference?

Pointer — address variable. Reference — alias.

1int x = 10;
2int* p = &x;  // Pointer
3int& r = x;   // Reference
4
5p = nullptr;  // OK
6// r = nullptr;  // Error
7
8p++;  // OK
9// r++;  // Error

Key differences:

• Pointer can be null, reference cannot.
• Pointer can be reassigned, reference cannot.
• Pointer arithmetic allowed, reference not.