How are C pointers different from Java and Python references?

A useful starting point is that a C pointer gives you more direct control over memory. You can take the address of an object with &, dereference a pointer with *, and do pointer arithmetic.

For example, if p points to an element of an int array, p + 1 points to the next element. The arithmetic accounts for the size of an int. It is not a free pass to access arbitrary memory, though. Pointer arithmetic has to stay within the relevant array, with a special allowance for forming a pointer one past its end without dereferencing it.

Java and Python references let you access objects, but the languages don't expose pointer arithmetic on those references. To move through a sequence, you use an index or an iterator instead.

Think of a Java or Python reference as a way to reach an object, not necessarily as an exposed address. The runtime decides how objects are represented and managed, so code shouldn't depend on a reference being a raw memory address.

C pointers can refer to objects with different lifetimes, including local variables and dynamically allocated memory. That flexibility comes with responsibility. Dereferencing a pointer after the object it points to has ceased to exist can cause undefined behavior.

Null is not the dividing line. Java object references can be null, and using one as if it referred to an object can cause a NullPointerException. Python has None, an object commonly used to represent the absence of a value.

One Java detail that trips people up: an object field without an explicit initializer defaults to null, but a local variable must be assigned before you can read it. Also, Java primitive values such as int are not object references.

References don't prevent dynamic allocation. Creating objects in Java or Python allocates storage for them, but their runtimes manage object lifetimes rather than requiring you to free that storage directly.

In C, you can allocate memory with malloc and release it with free. A pointer can still hold an address after free, but dereferencing it is invalid. That's one of the risks the managed languages generally hide from ordinary application code.