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

Pointer — variable with address. Reference — alias.

1int x = 10;
2int* ptr = &x;    // Pointer
3int& ref = x;     // Reference
4
5*ptr = 20;  // OK
6ref = 30;   // OK
7
8ptr = nullptr;  // OK
9// ref = nullptr;  // Error!
10
11ptr++;  // OK (pointer arithmetic)
12// ref++;  // Error!

Key differences:

• Pointer can be null, reference cannot.
• Pointer can be reassigned, reference cannot.
• Pointer has own address, reference shares.
• Pointer arithmetic allowed, reference not.