Use a `not_null<T>` to indicate that "null" is not a valid value

Reason

Clarity. A function with a not_null<T> parameter makes it clear that the caller of the function is responsible for any nullptr checks that might be necessary. Similarly, a function with a return value of not_null<T> makes it clear that the caller of the function does not need to check for nullptr.

Example

not_null<T*> makes it obvious to a reader (human or machine) that a test for nullptr is not necessary before dereference. Additionally, when debugging, owner<T*> and not_null<T> can be instrumented to check for correctness.

Consider:

int length(Record* p);

When I call length(p) should I check if p is nullptr first? Should the implementation of length() check if p is nullptr?

// it is the caller's job to make sure p != nullptr
int length(not_null<Record*> p);

// the implementor of length() must assume that p == nullptr is possible
int length(Record* p);

Note

A not_null<T*> is assumed not to be the nullptr; a T* might be the nullptr; both can be represented in memory as a T* (so no run-time overhead is implied).

Note

not_null is not just for built-in pointers. It works for unique_ptr, shared_ptr, and other pointer-like types.

Enforcement

• (Simple) Warn if a raw pointer is dereferenced without being tested against nullptr (or equivalent) within a function, suggest it is declared not_null instead.
• (Simple) Error if a raw pointer is sometimes dereferenced after first being tested against nullptr (or equivalent) within the function and sometimes is not.
• (Simple) Warn if a not_null pointer is tested against nullptr within a function.