What does the error 'Null Pointer Assignment' mean?

Answer 1

The null pointer assignment error means your program has attempted to access a memory address that does not belong to your program. This typically occurs when accessing memory indirectly through a pointer:

int* p = nullptr; *p = 42; // Error: null pointer assignment

The above is the classic example of this type of error. The null address is typically the all-zeroes address (0x0) but, regardless of the physical address, it must never be accessed because it is a system address. We typically refer pointers to the null address when they are no longer in use or we don't have an address we can (yet) assign to them.

Passing unchecked pointers to functions is another common cause:

void f (int* p) {

*p = 42; // potential error

// ...

}

In the above example there's no guarantee p refers to a non-system address. Although we can easily test p is non-null before accessing it, that won't guarantee p refers to a non-system address. However, we can greatly reduce the risk of error by passing memory address via references instead of pointers:

void f (int& r) {

r = 42;

// ...

}

There's still potential that r refers to a system address if the address were passed via a pointer, however there is seldom any need to use unchecked pointer variables in C++. References and resource handles (or smart pointers) eliminate the need for pointers and are actually more efficient than pointers because testing for null becomes largely redundant.

The only time we really need a pointer is when "no object" is a valid argument:

void f (int* p) {

if (p == nullptr) {

// the "no object" code

} else {

// code that operates on an object

}

}